Aaron james s - PLACEMENT
SATHYABAMA UNIVERSITY
DEPARTMENT OF ELECTRONICS AND INSTRUMENTATION
ENGINEERING
ELECTRONIC DEVICES SECX1001
PREPARED BY:
S.AARON JAMES me., mba .,
(EIE DEPT)
TEXT BOOKS:
1. Millman and Halkias, Electronic devices and circuits, 2nd Edition, McGraw Hill Publication, 2007.
2. G.K.Mithal, Basic Electronic Devices and circuits, 2nd Edition, G.K.Publishers Pvt. Ltd., 1998.
REFERENCE BOOKS:
1. David Bell,Fundamentals of Electronic Devices and Circuits, 5th Edition, Oxford University Press 2008.
2. Robert L. Boylestad, Electronic Devices and Circuit Theory, 6th Edition, PHI, 1998.
3. Ben G Streetman and Sanjay Banerjee, Solid State Electronic Devices, 6th Edition, Pearson Education, 2005.
4. Roody and Coolen, Electronic Communications, 4th Edition, Pearson Education, Reprint 2007.
UNIVERSITY EXAM QUESTION PAPER PATTERN:
Max Marks : 80
Exam Duration : 3 hrs
PART A : 2 Questions from each unit, each carrying 2 marks 20 marks
PART B : 2 Questions from each unit with internal choice, each carrying 12 marks 60 marks
SYLLABUS:
UNIT I SEMICONDUCTOR DIODE
Intrinsic and Extrinsic semiconductor - Charge density, Mobility and Conductivity in Semiconductor, Drift and diffusion current, Continuity equation, Hall effect - PN junction - Energy band diagram of PN junction, Current components in PN junction, Junction capacitance - Application of diode - Diode switch, Clipper, Clamper and Voltage multipliers - Zener diode - Zener voltage regulators.
UNIT II BIPOLAR JUNCTION TRANSISTOR Construction and Operation of NPN and PNP transistor - Current components in a transistor, Eber moll’s Equation-Characteristics of CE,CB,CC configuration - Base width modulation, Transistor breakdown, Transistor biasing – Bias Stabilization and Compensation, Thermal runway problems, Heat sinks, Switching characteristics.
UNIT III FIELD EFFECT TRANSISTOR
JFET- Construction, Operation and Characteristics, Expression for pinch off voltage and drain current - MOSFET- Enhancement and Depletion mode operation and characteristics, Handling precautions of MOSFET, Gate capacitance- FET as VVR - Comparison of MOSFET and JFET - Comparison of BJT and JFET.
UNIT IV SPECIAL SEMICONDUCTOR DEVICES
SCR- UJT- Diac- Triac - Varactor diode - PIN diode - Tunnel diode - Gunn diode - Principle of photo electronic devices - Solar cell, Photo diode and Photo transistor - LED, LCD, LASER diode, CCD - Operation, Characteristics and Applications.
UNIT V PRINCIPLES OF CRT
Force on charged particle in electric field and magnetic field - Motion of charged particle in electric and magnetic field - Principles of CRT - Deflection and focusing of electron beam in CRT and TV picture tube-Orientation of electric and magnetic field in CRT - Applications of CRO.
PREVIOUS YEAR QUESTION PAPERS:
SATHYABAMA UNIVERSITY
(Established under section 3 of UGC Act,1956)
Course & Branch :B.E - E&C/ECE/EEE/EIE/ETCE
Title of the Paper : Electronic Devices Max. Marks:80
Sub. Code : SECX1001 (2010) Time : 3 Hours
Date : 07/12/2011 Session :FN
______________________________________________________________________________________________________________________
PART - A (10 x 2 = 20)
Answer ALL the Questions
1. Define following the terms. (a) Insulator (b) Conductor
2. What is diffusion current?
3. Which is most commonly used transistor configuration? Why?
4. What is current amplification factor for CB configuration?
5. Define pinch off Voltage.
6. Compare BJT and JFET.
7. What are the advantages of LCD?
8. What is DIAC?
9. What are the applications of CRO?
10. Define field intensity.
PART – B (5 x 12 = 60)
Answer ALL the Questions
11. Describe the operation of Zener diode and explain its characteristics.
(or)
12. Write short notes on:
(a) Intrinsic Semiconductor.
(b) Extrinsic Semiconductor.
13. Explain the operation of NPN transistor.
(or)
14. Explain the common emitter configuration and its input output characteristics.
15. Explain a construction and operation of MOSFET.
(or)
16. Explain characteristics of JFET with help of neat sketches.
17. Explain the operations and characteristics of SCR.
(or)
18. Write short notes on:
(a) Photo Diode (b) Photo Transistor
19. Explain the principles of CRT in detail.
(or)
20. Explain the force on charged, practices in an electric field.
SATHYABAMA UNIVERSITY
(Established under section 3 of UGC Act,1956)
Course & Branch :B.E - E&C/ECE/EEE/EIE/ETCE/P-EEE
Title of the Paper :Electronic Devices Max. Marks :80
Sub. Code :SECX1001 Time : 3 Hours
Date :06/06/2011 Session :FN
______________________________________________________________________________________________________________________
PART - A (10 x 2 = 20)
Answer ALL the Questions
1. Write the applications of PN diode.
2. What is diffusion current?
3. Define early effect.
4. What is thermal runaway?
5. Compare N channel FET with P channel FET.
6. Define Pinch off voltage.
7. Compare LED and LCD.
8. Give the structure of a DIAC.
9. Write the applications of CRO.
10. What is meant by deflection sensitivity of a CRT?
PART – B (5 x 12 = 60)
Answer All the Questions
11. Explain the Energy Bands and give its types.
(or)
12. Describe the operation of Zener diode and explain its characteristics.
13. Discuss in detail the input and output characteristics of Common emitter configuration and explain how the h-parameters can be derived from the same.
(or)
14. Explain the following
(a) Thermal run away
(b) Switching characteristics of BJT.
15. Discuss in detail about the construction and working of Depletion MOSFET and Enhancement MOSFET. (or)
16. Derive an expression for voltage gain for a FET amplifier with CS and CD configuration.
17. Discuss in detail the construction, principle of operation and any one application of UJT. (or)
18. Explain the principle of photo electronic devices and explain the operation of anyone of them.
19. Derive the expression for force on charged particle in electric and magnetic field.
(or)
20. Explain the deflection and focusing of electron beam in CRT.
TWELVE MARK QUESTIONS-ANSWERS
12 MARKS:
UNIT I SEMICONDUCTOR DIODE
1)INTRINSIC AND EXTRINSIC SEMICONDUCTORS:
INTRINSIC SEMICONDUCTOR is an un-doped semiconductor, in which there is no impurities added where as extrinsic semiconductor is a doped semiconductor, which has impurities in it.
Doping is a process, involving adding dopant atoms to the intrinsic semiconductor
An intrinsic semiconductor is the purest form of semiconductor. Group 14 elements like Germanium and Silicon are typical examples of intrinsic semiconductors. These are free from the presence of any dopingagents.
An EXTRINSIC SEMICONDUCTOR is obtained by doping an intrinsic semiconductor with otherelements.
When we dope group 14 element with a group 13 element, we get a p type semiconductor where the majority charge carriers are holes. When the group 14 element, that is , the intrinsic semi conductor is doped with a group 15 element we get a n type semiconductor. Here the majority charge carriers are electrons.
Note: THERMAL ENERGY: The power that is created by heat, or the increase in temperature.
Intrinsicsemiconductor
A pure semiconductor free from any impurity is called intrinsic semiconductor. Here charge carriers (electrons and holes) are created by thermal excitation. Si and Ge are examples. Both Si and Ge are tetravalent. I.e. each has four valence electrons in the outermost shell. Consider the case of Ge. It has a total of 32 electrons. Out of these 32 electrons, 28 are tightly bound to the nucleus, while the remaining 4 electrons (valence electrons) revolve in the outermost orbit. In a solid, each atom shares its 4 valence electrons with its nearest neighbors to form covalent bonds.The energy needed to liberate an electron from Ge atom is very small, of the order of 0.7 eV. Thus even at room temperature, a few electrons can detach from its bonds by thermal excitation. When the electron escapes from the covalent bond, an empty space or a hole is created. The number of free electrons is always equal to the number of holes.
[pic]
Extrinsicsemiconductor:
Extrinsic semiconductors are formed by adding suitable impurities to the intrinsic semiconductor. This process of adding impurities is called doping. Doping increases the electrical conductivity in semiconductors. The added impurity is very small, of the order of one atom per million atoms of the pure semiconductor. The added impurity may be pentavalent or trivalent. Depending on the type of impurity added,
Extrinsic semiconductors can be divided into two classes: n-type andp-type.
n-type semiconductor
When pentavalent impurity is added to pure semiconductor, it results in n-type semiconducutor. Consider the case when pentavalent Arsenic is added to pure Silicon(Si) crystal. As shown in the figure, four electrons of Arsenic atom form covalent bonds with the four valence electrons of neighbouring Si atoms. The fifth electron of Arsenic atom is not covalently bonded, but it is loosely bound to the Arsenic atom. Now by increasing the thermal energy or by applying electric field, this electron can be easily excited from the valence band to the conduction band. Thus every Arsenic atom contributes one conduction electron without creating a positive hole. Hence Arsenic is called donor element since it donates free electrons. Since current carriers are negatively charged particles, this type of semiconductor is called n-type semiconductor.
[pic]
p-typesemiconductor
When trivalent impurity is added to pure semiconductor, it results in p-type semiconducutor. Consider the case when trivalent gallium is added to pure silicon(si) crystal. As shown in the figure, three valence electrons of Gallium atom form covalent bonds with the three neighbouring Si atoms. There is a deficiency of one electron (hole) in the bonding with the fourth Si atom. The Si atom will steal an electron from the neighbouring Si atom to form a covalent bond. Due to this stealing action, a hole is created in the adjascent atom.
[pic]
2)ZENER DIODE:
A zener diode is a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "zener knee voltage" or "zener voltage."
[pic]Fig)Symbol of zener diode Reverse current [pic] is shown.
[pic]
VI(VOLTAGE CURRENT CHARACTERISTICS) OF ZENER DIODE:
[pic]
The illustration above shows this phenomenon in a Current vs. Voltage graph. With a zener diode connected in the forward direction, it behaves exactly the same as a standard diode. In the reverse direction however there is a very small leakage current - i.e. just a tiny amount of current is able to flow. Then, when the voltage reaches the breakdown voltage (Vz), suddenly current can flow freely through it. In reverse-bias mode, they do not conduct until the applied voltage reaches or exceeds the so-called zener voltage,
Zener breakdown & Avalanche breakdown
Zener breakdown occur when the doping level in a semiconductor is high and the band gap is narrow. when these conditions are satisfied tunneling of electrons from one level to other takes place and conductivity increases. when the conditions of zener breakdown are not satisfied avalanche breakdown takes place at higher temp.
in avalanche breakdown electrons collide with the particles in the depletion region. this result in ionisation and result in the formation of a electron hole pair.this extra electron may result in further multiplication called avalanche multiplication
ZENER VOLTAGE REGULATORS.(Zener Diode Voltage Regulator Circuit)
• A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level.
[pic]Fig 1
[pic]Fig 2
Resistor value (ohms) = (VIN - VOUT) / (Zener current + Load current)
Where: VIN is the input voltage
VOUT is the Zener diode voltage
IL IS THE LOAD CURRENT
VZ IS THE ZENER BREAKDOWN VOLTAGE
In fig1) A zener diode can be used to make a simple voltage regulation circuit as pictured above. The output voltage is fixed at the zener voltage of the zener diode used and so can be used to power devices requiring a fixed voltage.
In fig2)
Input voltage is varying
Zener diode is reverse biased and as long as the input voltage does not fall below vz ( zener breakdown voltage),the voltage across the diode will be constant and hence the load voltage wil also be constant.
Applications of Zener Diodes are as follows:
1. Voltage Regulators ( overvoltage protection)
2. Surge Suppressors .i.e. for device protection
3) HALL EFFECT
[pic]
[pic]
4) P–N JUNCTION DIODE :
[pic]
PN junction is a practical device or as a rectifying device we need to firstly bias the junction, ie connect a voltage potential across it. On the voltage axis above, "Reverse Bias" refers to an external voltage potential which increases the potential barrier. An external voltage which decreases the potential barrier is said to act in the "Forward Bias" direction.
Reverse Bias:
When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material. The positive voltage applied to the N-type material attracts electrons towards the positive electrode and away from the junction, while the holes in the P-type end are also attracted away from the junction towards the negative electrode.
The net result is that the depletion layer grows wider due to a lack of electrons and holes and presents a high impedance path, almost an insulator. The result is that a high potential barrier is created thus preventing current from flowing through the semiconductor material.
Reverse Biased Junction Diode showing an Increase in the Depletion Layer
[pic]
Forward Biased Junction Diode showing a Reduction in the Depletion Layer
[pic]
This condition represents the low resistance path through the PN junction allowing very large currents to flow through the diode with only a small increase in bias voltage. The actual potential difference across the junction or diode is kept constant by the action of the depletion layer at approximately 0.3v for germanium and approximately 0.7v for silicon junction diodes.
Since the diode can conduct "infinite" current above this knee point as it effectively becomes a short circuit, therefore resistors are used in series with the diode to limit its current flow. Exceeding its maximum forward current specification causes the device to dissipate more power in the form of heat than it was designed for resulting in a very quick failure of the device.
There are two operating regions and three possible "biasing" conditions for the standard Junction Diode and these are:
1. Zero Bias - No external voltage potential is applied to the PN-junction.
2. Reverse Bias - The voltage potential is connected negative, (-ve) to the P-type material
and positive, (+ve) to the N-type material across the diode which has the effect of
Increasing the PN-junction width.
3 . Forward Bias - The voltage potential is connected positive, (+ve) to the P-type material and negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the PN-junction width.
ENERGY BAND DIAGRAM &CURRENT COMPONENTS OF PN JUNCTION DIODE:
[pic]
Current components in PN junction
1.UNDER FORWARD BIAS,FIG1
2.UNDER REVERSE BIAS,FIG2
3.UNDER EQUILIBRIUM,FIG3
[pic]
FIG1)UNDER FORWARD BIAS CONDUCTION TAKES PLACE,NO DEPLETON REGION
[pic]
FIG2)UNDER REVERSED BIAS CONDUCTION DOES NOT TAKES PLACE,DEPLETION REGION FORMS.
[pic]
FIG3)UNDER EQUILIBRIUM CONDITION ,ZERO VOLTAGE ACROSS DIODE.NO CURRENT FLOW
5) PN DIODE JUNCTION CAPACITACE:
[pic]
[pic]
[pic]
[pic]
[pic][pic][pic]
6)APPLICATION OF DIODE-
1)DIODE SWITCH,
2)CLIPPER,
3)CLAMPER AND
4)VOLTAGEMULTIPLIERS
1)DIODE SWITCH
Diode Switch
[pic]
Figure 1: Basic diode switch
In addition to their use as simple rectifiers, diodes are also used in circuits that mix signals together (mixers), detect the presence of a signal (detector), and act as a switch “to open or close a circuit”. Diodes used in these applications are commonly referred to as “signal diodes”. The simplest application of a signal diode is the basic diode switch shown in figure 1.
When the input to this circuit is at zero potential, the diode is forward biased because of the zero potential on the cathode and the positive voltage on the anode. In this condition, the diode conducts and acts as a straight piece of wire because of its very low forward resistance. In effect, the input is directly coupled to the output resulting in zero volts across the output terminals. Therefore, the diode, acts as a closed switch when its anode is positive with respect to its cathode.
If we apply a positive input voltage (equal to or greater than the positive voltage supplied to the anode) to the diode's cathode, the diode will be reverse biased. In this situation, the diode is cut off and acts as an open switch between the input and output terminals. Consequently, with no current flow in the circuit, the positive voltage on the diode's anode will be felt at the output terminal. Therefore, the diode acts as an open switch when it is reverse biased.
2)CLIPPER
A circuit that prevents the amplitude of a waveform from exceeding a specified value. Also called LIMITER. a circuit designed to limit the amplitude of an output signal to a preset level
The basic components required for a clipping circuit are – an ideal diode and a resistor. In order to fix the clipping level to the desired amount, a dc battery must also be included. When the diode is forward biased, it acts as a closed switch, and when it is reverse biased, it acts as an open switch. Different levels of clipping can be obtained by varying the amount of voltage of the battery and also interchanging the positions of the diode and resistor.
Depending on the features of the diode, the positive or negative region of the input signal is “clipped” off and accordingly the diode clippers may be positive or negative clippers.
There are two general categories of clippers: series and parallel (or shunt). The series configuration is defined as one where diode is in series with the load, while the shunt clipper has the diode in a branch parallel to the load.
1. Positive Clipper and Negative Clipper
Positive Diode Clipper
In a positive clipper, the positive half cycles of the input voltage will be removed. The circuit arrangements for a positive clipper are illustrated in the figure given below.
[pic]
As shown in the figure, the diode is kept in series with the load. During the positive half cycle of the input waveform, the diode ‘D’ is reverse biased, which maintains the output voltage at 0 Volts. Thus causes the positive half cycle to be clipped off. During the negative half cycle of the input, the diode is forward biased and so the negative half cycle appears across the output.
In Figure (b), the diode is kept in parallel with the load. This is the diagram of a positive shunt clipper circuit. During the positive half cycle, the diode ‘D’ is forward biased and the diode acts as a closed switch. This causes the diode to conduct heavily. This causes the voltage drop across the diode or across the load resistance RL to be zero. Thus output voltage during the positive half cycles is zero, as shown in the output waveform. During the negative half cycles of the input signal voltage, the diode D is reverse biased and behaves as an open switch. Consequently the entire input voltage appears across the diode or across the load resistance RL if R is much smaller than RL. Actually the circuit behaves as a voltage divider with an output voltage of [RL / R+ RL] Vmax = -Vmax when RL >> R
Negative Diode Clipper
The negative clipping circuit is almost same as the positive clipping circuit, with only one difference. If the diode in figures (a) and (b) is reconnected with reversed polarity, the circuits will become for a negative series clipper and negative shunt clipper respectively. The negative series and negative shunt clippers are shown in figures (a) and (b) as given below.
[pic]
In all the above discussions, the diode is considered to be ideal one. In a practical diode, the breakdown voltage will exist (0.7 V for silicon and 0.3 V for Germanium). When this is taken into account, the output waveforms for positive and negative clippers will be of the shape shown in the figure below.
[pic]
Negative and Positive Clipping Waveforms
APPLICATION:
The diode clipper can be used for the protection of different types of circuits
3)CLAMPER
A clamping circuit is used to place either the positive or negative peak of a signal at a desired level. For a clamping circuit at least three components — a diode, a capacitor and a resistor are required. The dc component is simply added or subtracted to/from the input signal. The clamper is also referred to as an IC restorer and ac signal level shifter.
In some cases, like a TV receiver, when the signal passes through the capacitive coupling network, it loses its dc component. This is when the clamper circuit is used so as to re-establish the the dc component into the signal input. Though the dc component that is lost in transmission is not the same as that introduced through a clamping circuit, the necessity to establish the extremity of the positive or negative signal excursion at some reference level is important.
A clamp circuit adds the positive or negative dc component to the input signal so as to push it either on the positive side, as illustrated in figure (a) or on the negative side, as illustrated in figure (b).
The circuit will be called a positive clamper , when the signal is pushed upward by the circuit. When the signal moves upward, as shown in figure (a), the negative peak of the signal coincides with the zero level.
The circuit will be called a negative clamper, when the signal is pushed downward by the circuit. When the signal is pushed on the negative side, as shown in figure (b), the positive peak of the input signal coincides with the zero level.
[pic]
4) VOLTAGE MULTIPLIER:
A Voltage Multiplier Circuit, is a special type of rectifier circuit which produces a DC output voltage which is many times greater than its AC input voltage. Although it is usual to use a transformer to increase the voltage, sometimes a suitable step-up transformer or a specially insulated transformer required for high voltage applications may not be available. One alternative approach is to use a voltage multiplier circuit.
Voltage multipliers are AC-to-DC voltage converters used in electrical and electronic circuit applications such as in microwave ovens, cathode-ray tube (CRT) field coils, electrostatic and high voltage test equipment, etc, where it is necessary to have a very high DC voltage generated from a relatively low AC voltage. The high voltage produced by a voltage multiplier circuit is in theory unlimited, but due to their relatively poor voltage regulation and low current capability most multiplier circuits produce voltages in the range of about 10kV to 30kV with a low current of less than ten milliamperes.
Voltage multiplier circuits are constructed from series combinations of rectifier diodes and capacitors that give a DC output equal to some multiple of the peak voltage value of the AC input voltage. By adding a second diode and capacitor to the output of the simple half-wave rectifier, we can increase its output voltage. The most commonly used type of voltage multiplier circuit is the Half Wave Series Multiplier also known as a “cascade voltage doubler”.
Voltage Multipliers
[pic]
FIGURE 4-5 Voltage doublers.
A voltage multiplier provides a dc output voltage that is a multiple of the circuit’s peak input voltage. For example, a voltage doubler with a peak input of 10 V provides a dc output that is approximately 20 V. Two voltage doublers are shown in Figure 4-5.
Each of the circuits in Figure 4-5 provides a dc load voltage that is approximately twice the value of the peak source voltage. The half-wave doubler gets its name from the fact that the output capacitor ([pic]) is charged during the positive half-cycle of the input signal. In contrast, the output capacitor in the full-wave doubler ([pic]) is charged during both alternations of the input cycle. Note that the output from a full-wave doubler has less ripple than the output from a comparable half-wave doubler.
7) CONTINUITY EQUATION:(REFER TEXT BOOK)
UNIT II BIPOLAR JUNCTION TRANSISTOR
1) CONSTRUCTION AND OPERATION OF NPN AND PNP TRANSISTOR
Transistor is a solid state devices. It consists of Si or Ge Crystal in which a layer of n-type (or p-type) is sandwiched between two layers of p-type (or n-type) material. It is pnp (or npn) transistor. Each of this transistor has two p-n junctions. Transistor has three terminals one side section supplying free charges is called emitter other side section collecting these charges is called collector. Middle section which is formed between emitter and collector is called base.
[pic]
working of transistor (pnp and npn).
NOTE:
[pic]-(ELECTRONS
Ans. Transistors of both types (pnp and npn) behave exactly in the same way except change in biasing and majority carriers. When no battery is connected between the different terminals of a transistor, the transistor is said to be unbiased or in an open circuit state. The doping of semiconductor into n and p type, creates excess holes and [pic]. A layer of positive ions on n side and layer of negative ions on p side creates potential barrier.
Operation of npn transistor The process of applying dc voltages across different terminals of a transistor is called biasing. For normal operation emitter-base junction ‘is forward biased and collector-base junction always reverse biased as shown in diagram.
The forward bias reduces barrier potential at E-B junction. [pic] are emitted on injected into the emitter by the emitter bias supply [pic]. These conduction band [pic] have enough energy to overcome the E-U barrier potential. The injected [pic] enter very thin, lightly doped base region but only few [pic] recombine with the holes doped into the base.
Injection of [pic] makes the [pic] concentration on the emitter very large and on the collector the concentration of [pic] is very small and it is large in base region. Injected [pic] diffuse into collector region due to extremely small thickness of base which is much less than the diffusion length. Most of the [pic] cross into collector region. It is reverse biased and creates a strong electrostatic field between base and collector. The field immediately collects the diffused [pic] which enter the collection junction. In base region, a few [pic] combine and neutralize and rest of [pic] are collected by the collector. To maintain base neutrality base electrode provides equal number of [pic] which have combined with the holes and results in a base current. Thus emitter current ‘F is equal to the sum of collector current [pic] and base current [pic].
Operation of PNP transistor: PNP transistor behaves exactly the same way as npn transistor, with the exception that majority charge carriers are holes. Holes are emitted from p-type emitter across the forward biased emitter-base junction into the base. In lightly doped n-type base, holes find few-[pic] to recombine. Some holes flow out of via base terminal but most are drawn by the collector by positive-negative electric field at the reverse biased collector base junction.
[pic]
[pic]
Different operating conditions for a transistor:
Ans. A transistor has two junctions and each of these two junctions may be forward or reverse biased. Therefore there are four possible ways of biasing.
[pic]
The diagram for the same is shown below:
[pic]
[pic]
[pic]
2A) DRAW EBERS-MOLL MODEL AND HENCE EXPLAIN TRANSISTOR ACTION.
Ans. The dependence of current in transistor upon the junction voltages or vice versa may be obtained by,
[pic]
Subscript n to [pic] has been added in order to indicate normal operation of transistor with inverted mode of operations equation can be written as,
[pic]
[pic]is emitter junction reverse saturation current. [pic] is voltage drop from p-side to n-side at emitter junction.
Ebers-Moll model for both of their equations for a pnp transistor is shown.
[pic]
PN+PN GIVES PNP TRANSISTOR
[pic]
where
▪ [pic] is the collector current
▪ [pic] is the base current
▪ [pic] is the emitter current
▪ [pic] is the reverse saturation current
[pic] is the base–emitter voltage
▪ [pic] is the base–collector voltage
▪ [pic] is emitter junction reverse saturation current
It involve two diodes placed back to back with reverse saturation current, [pic] and [pic] and two dependant current controlled current sources shunting the ideal diodes. The model is valid for both forward and reverse biased static voltages applied across transistor junctions.
It is evident that dependent current sources can be eliminated from the figure provided [pic]. By making the width of base much larger than the diffusion length of minority carriers in the base, all the minority carriers will recombine in the base and none will survive to reach the collector. In such a condition [pic] and hence [pic] will be zero. The transistor action thus ceases and simply have two diodes back to back.So a transistor cannot be constructed by simply connecting the separate diodes (isolated) back to back.
2B) CE, CB, CC Configurations:
Transistor Configuration:
➢ We know that transistor has three terminals namely emitter(E), base(B), collector(C).
➢ However, when a transistor is connected in a circuit, we require four terminals (ie) two terminals for input and two terminals for output.
➢ This difficulty is overcome by using one of the terminals as common terminal.
➢ Depending upon the terminals which are used as a common terminal to the input and output terminals, the transistors can be connected in the following three different configuration.
1. Common base configuration
2. Common emitter configuration
3. Common collector configuration
1. Common base configuration:
❖ In this configuration base terminal is conncted as a common terminal.
❖ The input is applied between the emitter and base terminals.The output is taken between the collector and base terminals.
[pic].
2. Common emitter configuration:
❖ In this configuration emitter terminal is conncted as a common terminal.
❖ The input is applied between the base and emitter terminals.The output is taken between the collector and base terminals.
[pic]
3. Common collector configuration:
❖ In this configuration collector terminal is conncted as a common terminal.
❖ The input is applied between the base and collector terminals.The output is taken between the emitter and collector terminals
[pic]
CB, CE,CC Characteristics :
1. Common base Characteristics :
Input characteristics:
❖ The output(CB) voltage is maintained constant and the input voltage (EB) is set at several convenient levels.For each level of input voltage, the input current IE is recorded.
❖ IE is then plotted versus VEB to give the common-base input characteristics.
Output characteristics:
❖ The emitter current IE is held constant at each of several fixed levels. For each fixed value of IE , the output voltage VCB is adjusted in convenient steps and the corresponding levels of collector current IC are recorded
❖ .For each fixed value of IE, IC is almost equal to IE and appears to remain constant when VCB is increased.
2. Common-Emitter Characteristics :
Input characteristics
❖ The output voltage VCE is maintained constant and the input voltage VBE is set at several convenient levels.For each level of input voltage, the input current IB is recorded.
❖ IB is then plotted versus VBE to give the common-base input characteristics.
Output characteristics:
❖ The Base current IB is held constant at each of several fixed levels. For each fixed value of IB , the output voltage VCE is adjusted in convenient steps and the corresponding levels of collector current IC are recorded
❖ .For each fixed value of IB, IC level is Recorded at each VCE step.For each IB level, IC is plotted versus VCE to give a family of characteristics.
3. Common-Collector Characteristics :
Input characteristics:
❖ The common-collector input characteristics are quite different from either common base or common-emitter input characteristics.
❖ The difference is due to the fact that the input voltage (VBC) is largely determined by (VEC) level .
VEC = VEB + VBC
VEB = VEC - VBC
Output characteristics:
❖ The operation is much similar to that of C-E configuration.When the base current is ICO, the emitter current will be zero and consequently no current will flow in the load.
❖ When the base current is increased, the transistor passes through active region and eventually reaches saturation. Under the saturation conditions all the supply voltage, except for a very small drop across the transistor will appear across the load resistor.
3)DRAW AND EXPLAIN THE INPUT/OUTPUT CHARACTERISTICS OF A COMMON BASE TRANSISTOR CONFIGURATION?
Ans. In the common base configuration (CB), input is connected between emitter and base and output is taken across collector and base. The figure is shown for npn transistor. The base is common between input and output.
[pic]
The EB junction is Forward biased and CB junction is reverse biased. [pic] flow in the input circuit, [pic] is the output current.
[pic]
Its value ranges from 0.95 to 0.99.
Collector current consists of the current produced by normal transistor action and due to leakage current.
[pic]
[pic]
Characteristics of CB configuration
(a) Input characteristics : The curve drawn between emitter current [pic] and emitter base voltage [pic] for a constant value of [pic] is known as input characteristic of CB mode. The curve is as shown.
[pic]
Increase in [pic], it conducts better, although effect is not significant. This is because large collector base (reverse bias) voltages cause the depletion layer at the collector base junction to penetrate deeper into the base of transistor, thus reducing the distance and resistance between emitter base and collector base regions.
Thus exists a cut in, offset on Threshold voltage [pic] below which the emitter current is very small. Sufficient [pic] flows only when emitter base junction is forward biased beyond the knee of the characteristic.
[pic]increase rapidly with small increase in [pic] which means low dynamic input resistance
[pic]
Since small [pic] causes large change in [pic] [pic] is quite low. This dependence of [pic] on [pic] causes the distortion of signals.
(b) Output Characteristics : The curve drawn between collector current [pic] and collector base voltages [pic] for a given emitter current [pic] is output characteristics. The [pic] varies with [pic] only for a very low voltage but transistor is never operated in this region.
In active region, [pic] is almost equal to [pic] and appears constant with increase in [pic]. This is because the increase in [pic] expands the collector base depletion region and thus shortens the distance between the two depletion regions. When [pic] is constant, [pic] is so small that it is noticed only for large variation in [pic]. Transistor is normally operated in this region.
Although [pic] is practically independent of [pic] over the transistor operating range. However, if [pic] is increased beyond a certain value, [pic] increases rapidly because of avalanche or zener or both effects. It is known as punch through or reach through, thus destroying the device. It is necessary to maintain [pic].
A very large change in [pic] causes very small change in [pic]. hence output dynamic resistance is very high.
[pic]
In cut off small current occurs i.e. [pic]
In saturation region, [pic] flows even when [pic] = 0.
CB configuration is rarely used in audio frequency (AF) circuits because its current given is less than unity and its input/output resistance are quite different.
4) DRAW AND EXPLAIN THE COMMON EMITTER CHARACTERISTICS.
Ans. In CE configuration input is connected between the base and emitter while output is taken between collector and emitter. This emitter is the common terminal.
[pic]
[pic]is the input voltage and [pic] is output supply at collector. [pic] flows in the circuit and then the output resistance, There will be high power and voltage gains. EC is commonly used because its current, voltage and power gains are quite high and output to input impedance rate is moderate.
The ratio of change in [pic] to change in [pic] is [pic] .
[pic]
In CE, small collector current flows even when [pic] = 0. This is cut off current denoted by [pic] and is larger than [pic]
[pic]
Characteristics of CE configuration
(a) Input Characteristics: The curve drawn between [pic] and [pic] voltage at constant [pic] gives the input characteristics as shown.
[pic]
[pic]
(i) In comparison to CB arrangement, base current increases less rapidly with increase in [pic] voltage. This shows that input resistance is larger in CE configuration.
(ii) Increasing [pic] causes [pic] to be lower for a given beam of [pic]. This is so, higher levels of [pic] provide greater collector -base junction reverse bias, causing greater depletion region penetration into the base, thus reducing distance between collector-base and emitter-base regions. As a result more charge carriers from the emitter flows across the collector base junctions and few flow out through the base load.
[pic]
(b) Output Characteristics. These are drawn between [pic] and [pic] at [pic] constant.
(i) [pic] varies with [pic] for [pic] between 0 to 1 V and then becomes constant and independent of [pic]. This transistor is always operated above 1 V.
(ii) In active region, for small values of [pic] the effect of collector voltage [pic] over [pic] is small but for large values of [pic] this effect increases.
(iii) With lower [pic] the transistor is in saturation region and [pic] here does not cause a corresponding change in [pic].
(iv) With very high [pic], collector-base junction completely breakdown and hence [pic] increases rapidly transistor is destroyed.
(v) In cut off, small [pic] flows even when [pic] = 0. This is [pic].
Moderate output to input impedance ratio makes this configuration an ideal one for coupling between various transistor stages.
[pic]
5)DRAW AND EXPLAIN COMMON COLLECTOR CHARACTERISTICS (CC)
Ans. In this configuration, input is applied between base and collector and emitter-collector is output. Collector is thus the common terminal. The circuit is same as that of CE but a load resistor [pic] is placed in emitter circuit instead of collector circuit. [pic] flows in and [pic] flow out. So the change in [pic] to that of change in [pic] gives. The current amplification factor [pic].
[pic]
[pic]
When base current [pic] equal to [pic] is zero or no current flow in [pic]. With increase in [pic] the transistor passes through the active region and finally reaches saturation. Almost the complete supply voltage, except for a small voltage drop across the transistor appears across the [pic] resistor.
[pic]
The collector common (CC) arrangement gives very high input impedance and very low output impedance and hence voltage given is less than unity. This configuration is seldom used for amplification. Owing to relatively high input impedance and low output independence, this configuration is used for impedance matching is for driving low impedance from a high impedance sources.
6) BJT(BIPOLAR JUNCTION TRANSISTOR)TRANSISTOR BREAKDOWN
Usage of BJTs
BJTs are a kind of transistors
Used for current amplification
Easily built on semiconductor wafers
Cheap and small
Good electrical properties and speed
Structure of BJTs
Two diodes with one joined electrode
NPN-type and PNP-type, respectively
Potential between C-E: C-B reverse-bias
Potential between B-E: Current flow
Very thin base
Most electrons get into C region
Amplified current flow between E and C
Reason of Breakdown:
[pic]
Too high potential on reverse-bias diode
Electrons highly accelerated
Impact-ionization creates new electron/hole
Pairs
Avalanche-effect leads to breakdown current
7)THERMAL RUNAWAY PROBLEM & HEAT SINK
Fixing of a suitable operating point is not sufficient in transistors. It is also to be ensured that it remains fixed also. The transistor parameters are temperature dependent and parameter such as [pic] change from unit to unit are responsible for Q point to shift.
Flow of current in collector circuit produces heat at the collector junction. This increases the temperature. More minority carriers are generated in base collection region (since more bonds are broken). The leakage current [pic] increases. Since
[pic]
The increases in [pic] will increase [pic] to increase, which in turn increases the [pic]. This further raises the temperature of the collector-base junction and whole cycle repeats again. Such cumulative increase in [pic] will ultimately shift the operating point into the saturation region. This is very dangerous. The excess heat at junction may even burn the transistor. This is known as thermal runaway.
[pic]
[pic]
8) VARIOUS METHODS FOR TRANSISTOR BIASING
Various methods used for biasing of transistors are
1. Base Resistor Method. Biasing circuit is very simple, biasing conditions can be easily set, calculations are very simple and there is no loading of source by the biasing circuit but this method is rarely used because of very poor stabilization and strong chances of thermal runway.
2. Collector to Base Bias Method. This method is simple and provides better bias stability but the circuit provides a negative feedback resulting in reduced amplifier gain. Stability factor is also fairly high.
3. Self Bias or Emitter Bias (Voltage Divider Bias) Method. This is the most commonly used biasing arrangement as it provides good bias stability. The emitter resistance RE provides stabilization.
[pic]
9)SWITCHING CHARACTERISTICS OF TRANSISTOR
Switching Characteristics: Gives you and idea of how fast the transistor can turn on and off when used as a switch. Note: The switching time depends on the base current. The larger the base current the faster the transistor can switch on.
Transistor Switching Example
When VBE is less than 0.7V the transistor is off and the lamp does not light.
[pic]
When VBE is greater than 0.7V the transistor is on
and the lamp lights.
APPLICATIONS:
transistor switches can be used for controlling high power devices such as motors, solenoids or lamps, but they can also used in digital electronics and logic gate circuits.
10)HANDLING PRECAUTIONS OF MOSFET:
1)Mosfets should be protected from static electricity. Handle with anti static protection. Use solder-wicks when desoldering as solder-suckers create enough static electricity to damage mosfets. Never insert a mosfet into a circuit or remove it without shutting the power off.
2)Also susceptible to over voltage. A mosfet will break if a short circuit happens thru it and blow other paralleled mosfets too. . Although silicon dioxide is an excellent insulator, the layer used on a MOSFET is extremely thin, and therefore can be permanently damaged if a high voltage is applied across it. It will break down just as any other insulator will. Because it is so very thin, it does not need very high voltages to cause total breakdown, and as the gate has such a very high resistance, any voltage present will not be reduced by current flow.
3)Once the transistor is connected into a circuit, the components of the circuit should afford sufficient protection by forming conducting paths around the device, so preventing the build up of high static voltages. In most modern devices special protection diodes are built in to the device to give some protection against static damage. This protection is limited however, and manufacturers handling instructions should be studied before handling any MOS device.
Diagram:refer class notes
UNIT III FIELD EFFECT TRANSISTOR
1)EXPLAIN FET AND ITS ADVANTAGES
Ans. The field effect transistor is a semiconductor device which depends for its operation on the control of current by an electric field. There are two types of FETs: Junction FETs or JFETs and metal oxide semiconductor FETs on MOSFETs.
FET enjoys many advantages or a conventional transistor.
(1) Its operation depends upon the flow of majority carriers only. It is therefore a unipolar device. Conventional transistor is bipolar device.
(2) It is relatively immune to radiation.
(3) It exhibits high input resistance, typically many mega ohms.
(4) It is less noisy than a tube and a bipolar transistor.
(5) It exhibits no offset voltage at zero drain current, and hence makes an excellent signal chapper.
(6) It has Thermal Stability.
The main disadvantage of FET is its relatively small gain bandwidth product in comparison with that which can be obtained with a conventional transistor.
2)JFET- CONSTRUCTION, OPERATION AND CHARACTERISTICS, EXPRESSION FOR PINCH OFF VOLTAGE AND DRAIN CURRENT
[pic]
The structure of n-channel FET is shown. But it can be p-channel type also. Only difference is p-type is replaced by n-type and vice versa in n-channel type.
For n-channel JFET, there is a n-type Si-bar. This bar behaves like a resistor between its two terminals called source and drain. Then a p-type material is heavily doped on either side of the bar. These p-regions are called gates. Usually the two are connected together.
he gate terminal is analogous to the base of BJT. This is used to control the current flow from source to drain. Thus source and drain are analogous to emitter and collector terminals respectively. The circuit symbol is also shown above. The arrow is put in the gate terminal and it points into the JFET for n-channel and opposite for p-channel.
Operation : Normally to operate an n-channel JFET a positive voltage to the drain with respect to the source is applied. Due to this voltage, the majority carriers in the bar start flowing from source to drain. It makes the drain current [pic]. It is analogous to [pic] in BJT. The [pic] in the bar have to pass through the space between two p-regions. The width of this space can be controlled by varying the gate voltage. That is why this space is called channel.
Source, Drain, Gate and Channel in JFET.
Ans. Source : The source is a terminal through which the majority carriers enter the bar.
Drain: The drain is the terminal through which the majority carriers leave the bar.
Gate : On both sides of n-type bar, heavily doped p regions are formed. These regions are called gates. The two gates are joined together to form a single gate.
Channel : The region between source and drain sandwiched between the two gates are joined together to form single gate.
expression for pinch off voltage in JFET?
Ans. Space charge width W (x) = a — b(x)
[pic]
where [pic]= dielectric constant of channel material
e = magnitude of electronic charge
[pic]= junction contact potential at x
V(x) = applied potential across space charge region at x and is a negative number for an applied reverse bias.
a — b (x) = penetration W(x) of depletion region into channel at a point x along channel.
3)WHAT IS MOSFET? EXPLAIN ITS CONSTRUCTION&TYPES.
Ans. MOSFET is metal oxide semiconductor field effect transistor also known as insulated gate FET.
The n channel MOSFET consists of a lightly doped p-type substrate into which two highly doped [pic] regions. are diffused as shown.
These [pic] sections, which will act as the source and drain are separated by about 1 mile distance. A thin layer of insulating silicon diode [pic] is grown over the surface of structure and holes are cut into the oxide layer, allowing contact with the source and drain. Then the gate metal area is overlaid on the oxide, covering the entire channel region. Simultaneously, metal contacts are made to the drain and source. The contact to the metal over the channel area is the gate terminal.
[pic]
The metal area of the gate, in conjunction with the insulating dielectric oxide layer and the semiconductor channel, forms a parallel plate capacitor. The insulating layer of [pic] is the reason why this device is called insulating gate FET. This layer results in an extremely high input resistance for MOSFET.
MOSFET is metal oxide semiconductor field effect transistor. MOSFET has N substrate which is lightly doped. Two contacts drain and source are brought out from substrate. It has a p - type channel. There is a insulating layer of silicon oxide between metal and N channel. Gate is taken from this metal plate. This insulating layer gives a very high input impedance. MOSFET can operate in two modes:
Depletion mode : The gate is mode -ve and drain side is connected to positive of battery charge in gate Voltage changes the conductivity of N-channel and controls drain current.
[pic]
(b) Enhancement mode : Here gate is connected to +ve voltage. The drains and transfer characteristic of MOSFET are as shown:
[pic]
4. EXPLAIN ENHANCEMENT TYPE MOSFET AND ITS CHARACTERISTICS.
Ans. The diagram of enhancement type MOSFET is same as drawn in previous question.
If we ground the substrate for the structure and apply a positive voltage at the gate, an electric field will be directed perpendicularly through the oxide. This field will end on “induced” negative charges on the semiconductor increases. The region beneath the oxide now has n-type carriers, the conductivity increases and current flows from source to drain through the induced channel. Thus the drain current is ‘enhanced’ by the positive gate voltage and that’s why called enhancement type MOS.
The V-I characteristics of n-channel enchancement mode MOSFET are shown and its transfer curve also.
[pic]
The current [pic] at [pic] is very small, being of the order of few nAs. As [pic] is made positive, the current [pic]increases slowly at first and then much more rapidly with an increase in [pic]. A current [pic] (N), corresponding approximately to the maximum value given on drain characteristics and [pic] for this current is also promoted.
[pic]
5. EXPLAIN DEPLETION TYPE MOSFET AND ITS CHARACTERISTICS.
Ans. An n channel is diffused between the source and the drain as shown in the figure.
[pic]
With this device an appreciable drain current [pic] flows for [pic] = 0. If gate voltage is made negative positive charges are induced in the channel through the [pic] of the gate capacitor. Since current in FET is due to majority carriers, the induced positive charges make the channel less conductive, and the drain current drops as [pic] is made more negative. The redistribution of charge in channel causes an effective depletion of majority carriers, which accounts for the designation depletion MOSFET.
The volt ampere characteristics of this device and transfer curve are drawn, The depletion and enhancement regions, corresponding to [pic] negative and positive respectively. The gate source cut off voltage [pic] (OFF) at which is reduced to some specified negligible value at a recommended [pic]. This gate voltage corresponds to pinch off voltage up of JFET.
6. COMPARE JFET AND MOSFET?
Ans. JFETs and MOSFETs are quite similar in their operating principles and electrical characteristics. They still differ in some way.
(1) JFETs can be operated in depletion mode only whereas MOSFETs can be operated m either depletion mode or in enhancement mode. In JFET, if the gate is forward biased, excess carrier in junction occurs and the gate current is substantial. Thus channel conductance is enhanced to some degree due to excess carriers but the device is never operated with gate forward biased because gate current is undesirable.
(2) MOSFETs have input impedance much higher than that of JFETs. This is due to negligibly small leakage current.
(3) JFET is operated with a reverse bias on the junction, the gate current L is larger than it would be in a comparable MOSFETs. The current caused by minority carrier extraction across a reverse biased junction is greater, per unit area, than the leakage current that is supported by the oxide layer in MOSFET. Thus MOSFET devices are more useful in electrometer applications than one the JFETs.
(4) JFETs have characteristic curves more flatter than those of MOSFETs indicating a higher drain resistance.
7)COMPARE BJT AND JFET
Ans. (i) JFET’s operation depends upon the flow of majority carriers only, it is therefore a unipolar device. But BJT, has both minority and majority carriers and hence named bipolar.
(ii) JFET is simpler to fabricate, smaller in size, rugged in construction and has higher efficiency.
(iii) JFET has high input impedance because its input circuit is reverse biased and permits high degree of isolation between input and output circuits. However, BJT is forward biased and hence has low input impedance.
(iv) JFET carries very small current because of reverse biased gate and therefore, it carries extremely small current and input voltage controls the output current. So JFET is voltage driven while BJT is current driven.
(v) BJT uses current into its base for controlling a large current between collector and emitter whereas JFET, voltage on gate is used for controlling drain current.
(vi) JFET has no junction like an ordinary BJT and the conduction is through bulk material current carriers that do not cross junctions.
(vii) JFET is more immune to radiation than BIT.
(viii) JFET has negative temperature coefficient of resistance and hence has better thermal stability.
The main drawback of JFET is its relative small gain bandwidth product in comparison to conventional BJT.
Bipolar Transistor Construction
|[pic] |
PROBLEM8. For an n-channel JFET, [pic]= 8.7mA, [pic] = -3V, [pic] = - 1V. Find values of [pic]
Ans. Drain-source saturation current,
[pic]
9)EXPLAIN HOW AN FET IS USED AS VVR?
Ans. FET is a device that is usually operated in the constant current portion of its output characteristic. But if it is operated on the region prior to pinch off, it will behave as voltage variable resistor (VVR). It is due to fact that in this region drain to source resistance [pic] can be controlled by varying the bias voltage [pic]. In such applications the FET is referred to voltage variable resistor or voltage dependent resistor. It can be employed as VVR for small ac signals when it is employed in this way, it does not require a dc drain voltage from the supply.
[pic]
UNIT IV SPECIAL SEMICONDUCTOR DEVICES
1)SCR(SILICON CONTROLLED RECTIFIER)
A Silicon-Controlled Rectifier (SCR) is a four-layer (p-n-p-n) semiconductor device that doesn't allow current to flow until it is triggered and, once triggered, will only allow the flow of current in one direction. It has three terminals: 1) an input control terminal referred to as a 'gate'; 2) an output terminal known as the 'anode'; and 3) a terminal known as a 'cathode', which is common to both the gate and the anode.
The SCR is made up of four layers of semiconductor material arranged PNPN. The construction is shown in view A of figure . In function, the SCR has much in common with a diode
[pic]
FIG)The Silicon-Controlled Rectifier (SCR)
[pic]
[pic]
[pic]
Consider the SCR as a transistor pair, one PNP and the other NPN, connected as shown in views B and C. The anode is attached to the upper P-layer; the cathode, C, is part of the lower N-layer; and the gate terminal, G, goes to the P-layer of the NPN triode.
[pic]
[pic]
SCR is a four-layer device with three terminals, namely, the anode, the cathode and the gate. When the anode is made positive with respect to the cathode, junctions J1 and J3 are forward biased and junction J2 is reverse-biased and only the leakage current will flow through the device. The SCR is then said to be in the forward blocking state or in the forward mode or off state. But when the cathode is made positive with respect to the anode, junctions J1 and J3 are reverse-biased, a small reverse leakage current will flow through the SCR and the SGR is said to be in the reverse blocking state or in reverse mode.
When the anode is positive with respect to cathode i.e. when the SCR is in forward mode, the SCR does not conduct unless the forward voltage exceeds certain value, called the forward breakover voltage, VFB0. In non-conducting state, the current through the SCR is the leakage current which is very small and is negligible. If a positive gate current is supplied, the SCR can become conducting at a voltage much lesser than forward break-over voltage. The larger the gate current, lower the break-over voltage. With sufficiently large gate current, the SCR behaves identical to PN rectifier. Once the SCR is switched on, the forward voltage drop across it is suddenly reduced to very small value, say about 1 volt. In the conducting or on-state, the current through the SCR is limited by the external impedance.
When the anode is negative with respect to cathode, that is when the SCR is in reverse mode or in blocking state no current flows through the SCR except very small leakage current of the order of few micro-amperes. But if the reverse voltage is increased beyond a certain value, called the reverse break-over voltage, VRB0avalanche break down takes place. Forward break-over voltage VFB0 is usually higher than reverse breakover voltage,VRBO.
2) WHAT IS UJT ? DRAW ITS CHARACTERISTICS AND EXPLAIN ITS OPERATION.
Ans. Uni Junction Transistor (UJT) is also called double base diode. It is a 2 layer, 3 terminal solid state switching device. The device has a unique characteristics that when it is triggered, its emitter current increases regeneratively until it is restricted by emitter power supply. Few applications include oscillators, pulse generators, saw tooth generators, triggering circuits, phase control etc.
Construction: Basic structure of UJT is shown. It essentially consists of a lightly dopped N-type Si bar with small piece of heavily dopped P-type material alloyed to its one side to produce single p-n junction. The single pn junction accounts for uni junction transistor.
[pic]
The Si bar at its ends has true ohmic contacts designated as base-I ([pic]) and base 2 ([pic]) and p-type region is termed as emitter E. Emitter junction is usually located closer to [pic] than [pic], so device is not symmetrical.
The equivalent circuit and static emitter characteristics are shown below.
[pic]
Imagine emitter supply voltage is turned to zero. Then intrinsic stand off voltage reverse biases the diode. If [pic] is barrier voltage of emitter diode then total reverse bias is [pic]
Let [pic] is slowly increased . When [pic] becomes equal to [pic] will be reduced to zero. With equal voltage levels on each ‘side of the diode neither reverse nor forward current will flow. When emitter supply voltage is still increased diode becomes forward biased as soon as it exceeds [pic]. This [pic] is peak point voltage and is denoted as [pic]
When [pic] starts to flow through [pic] to ground i.e. [pic]. This is minimum current that is required to trigger the UJT. This is peak point emitter current denoted by [pic] is inversely proportional to interbase voltage [pic]. Emitter diode starts conducting now, charge carriers are injected in [pic] region of bar. Since resistance of semiconductor material depends upon doping, the resistance of region [pic] decreases rapidly due to additional charge carriers with the decrease in resistance, the voltage drop across [pic] also decrease, cause the emitter diode to be more heavily forward biased. This in turn, results in large forward current and consequently more charge carriers are injected causing still further reduction in resistance of [pic] region. This emitter current goes on increasing until it is limited by the emitter power supply. Since [pic] decreases with increase in emitter current, UJT has negative resistance characteristics.
[pic]is the emitter voltage at the valley point known as valley point voltage and [pic] is the valley point current.
3)DIAC
Diac-Operation and Construction
A diac is an important member of the thyristor family and is usually employed for triggering triacs. A diac is a two-electrode bidirectional avalanche diode which can be switched from off-state to the on-state for either polarity of the applied voltage. This is just like a triac without gate terminal, as shown in figure. Its equivalent circuit is a pair of inverted four layer diodes. Two schematic symbols are shown in figure. Again the terminal designations are arbitrary since the diac, like triac, is also a bilateral device. The switching from off-state to on-state is achieved by simply exceeding the avalanche break down voltage in either direction.
Construction of a Diac.
A diac is a P-N-P-N structured four-layer, two-terminal semiconductor device, as shown in figure.A. MT2 and MTX are the two main terminals of the device. There is no control terminal in this device. From the diagram, a diac unlike a diode, resembles a bipolar junction transistor (BJT) but with the following exceptions.
▪ there is no terminal attached to the middle layer (base),
▪ the three regions are nearly identical in size,
▪ the doping level at the two end P-layers is the same so that the device gives symmetrical switching characteristics for either polarity of the applied voltage.
[pic]
DIAC Circuit Symbol
Operation of a Diac.
When the terminal MT2 is positive, the current flow path is P1-N2-P2-N3 while for positive polarity of terminal MT1 the current flow path is P2-N2-P1-N1. The operation of the diac can be explained by imagining it as two diodes connected in series. When applied voltage in either polarity is small (less than breakover voltage) a very small amount of current, called the leakage current, flows through the device. Leakage current caused due to the drift of electrons and holes in the depletion region, is not sufficient to cause conduction in the device. The device remains in non-conducting mode. However, when the magnitude of the applied voltage exeeds the avalanche breakdown voltage, breakdown takes place and the diac current rises sharply, as shown in the characteristics shown in figure.
[pic]
Characteristics of a Diac
Volt-ampere characteristic of a diac is shown in figure. It resembles the English letter Z because of the symmetrical switching characteristics for either polarity of the applied voltage.
The diac acts like an open-circuit until its switching or breakover voltage is exceeded. At that point the diac conducts until its current reduces toward zero (below the level of the holding current of the device). The diac, because of its peculiar construction, does not switch sharply into a low voltage condition at a low current level like the SCR or triac. Instead, once it goes into conduction, the diac maintains an almost continuous negative resistance characteristic, that is, voltage decreases with the increase in current. This means that, unlike the SCR and the triac, the diac cannot be expected to maintain a low (on) voltage drop until its current falls below a holding current level.
4)TRIAC
The TRIAC SCR equivalent and, TRIAC schematic symbol
[pic]
Applications of Triac
Next to SCR, the triac is the most widely used member of the thyristor family. In fact, in many of control applications, it has replaced SCR by virtue of its bidirectional conductivity. Motor speed regulation, temperature control, illumination control, liquid level control, phase control circuits, power switches etc. are some of its main applications.
1. High Power Lamp Switching.
Use of the triac as an ac on/off switch is shown in figure. When the switch S is in position 1, the triac is cut-off and so the lamp-is’dark. When the switch is put in position 2, a small gate current flowing through the gate turns the triac on and so the lamp is switched on to give rated output.
[pic]
Triac Application
[pic]
Typical V-I characteristics of a triac are shown in figure. The triac has on and off state characteristics similar to SCR but now the char acteristic is applicable to both positive and negative voltages. This is expected because triac consists of two SCRs connected in parallel but opposite in direc tions.
MT2 is positive with respect to MTX in the first quadrant and it is negative in the third quad rant. As already said in previous blog posts, the gate triggering may occur in any of the following four modes.
Quadrant I operation : VMT2, positive; VG1 positive
Quadrant II operation : VMT21 positive; VGl negative
Quadrant III operation : VMT21 negative; VGl negative
Quadrant IV operation : VMT21 negative; VG1 positive
5) VARACTOR DIODE
The meaning of varactor diode is variable capacitance diode.varactor=variable reactor,referring to the voltage varying property of capacitance. A varactor diode uses a p-n junction inreverse bias and has a structure such that the capacitance of the diode varies with the reverse voltage. A voltage controlled capacitance is useful for tuning applications.
Characteristics of Varactor Diode:
A varactor diode that always operates in reverse bias and is doped to maximize the inherent capacitance of the depletion region. The depletion region, widened by the reverse bias, acts as a capacitor dielectric because of its non conductive characteristic. The p and n regions are conductive and acts as the capacitor plates, as shown in below circuit.
[pic]
The reverse biased varactor diode acts as a variable capacitor
Basic Operation of Varactor Diode:
As the reverse bias voltage increase, the depletion region widens, effectively increasing the plate separation and the dielectric thickness and thus decreasing the capacitance, when the reverse biasvoltage decreases, the depletion region narrows, thus increasing the capacitance. This action is shown below diagrams (a) and (b).
[pic]
Varactor diode capacitance varies with reverse biasing
In a varactor diode, these capacitance parameters are controlled by the method of doping near the pn junction and the size and geometry of the diode’s construction. Normal varactor capacitances are typically available from a few picofarads to several hundred picofarads.
Varactor Diode Symbol:
[pic]
Varactor Diode Symbol
Applications of Varactor Diode:
Varactor diodes are used in electronic tuning circuits and modern communications systems
6)PIN DIODE:
A PIN diode is a diode with a wide, lightly doped 'near' intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts.
PIN diode The p-i-n diode or PIN diode is a photodiode with an intrinsic layer between the P and N-regions as in Figure below. The P-Intrinsic-Nstructure increases the distance between the P and N conductive layers, decreasing capacitance, increasing speed. The volume of the photo sensitive region also increases, enhancing conversion efficiency. The bandwidth can extend to 10's of gHz. PIN photodiodes are the preferred for high sensitivity, and high speed at moderate cost.
[pic]
PIN photodiode: The intrinsic region increases the thickness of the depletion region.
7) SOLAR CELLS
A photodiode optimized for efficiently delivering power to a load is the solar cell. It operates in photovoltaic mode (PV) because it is forward biased by the voltage developed across the load resistance.
Monocrystalline solar cells are manufactured in a process similar to semiconductor processing. This involves growing a single crystal boule from molten high purity silicon (P-type), though, not as high purity as for semiconductors. The boule is diamond sawed or wire sawed into wafers. The ends of the boule must be discarded or recycled, and silicon is lost in the saw kerf. Since modern cells are nearly square, silicon is lost in squaring the boule. Cells may be etched to texture (roughen) the surface to help trap light within the cell. Considerable silicon is lost in producing the 10 or 15 cm square wafers. These days (2007) it is common for solar cell manufacturer to purchase the wafers at this stage from a supplier to the semiconductor industry.
P-type Wafers are loaded back-to-back into fused silica boats exposing only the outer surface to the N-type dopant in the diffusion furnace. The diffusion process forms a thin n-type layer on the top of the cell. The diffusion also shorts the edges of the cell front to back. The periphery must be removed by plasma etching to unshort the cell. Silver and or aluminum paste is screened on the back of the cell, and a silver grid on the front. These are sintered in a furnace for good electrical contact. (Figure below)
The cells are wired in series with metal ribbons. For charging 12 V batteries, 36 cells at approximately 0.5 V are vacuum laminated between glass, and a polymer metal back. The glass may have a textured surface to help trap light.
[pic]
Silicon Solar cell
The ultimate commercial high efficiency (21.5%) single crystal silicon solar cells have all contacts on the back of the cell. The active area of the cell is increased by moving the top (-) contact conductors to the back of the cell. The top (-) contacts are normally made to the N-type silicon on top of the cell. In Figure below the (-) contacts are made to N+ diffusions on the bottom interleaved with (+) contacts. The top surface is textured to aid in trapping light within the cell.. [VSW]
[pic]
High efficiency solar cell with all contacts on the back. Adapted from Figure 1 [VSW]
Multicyrstalline silicon cells start out as molten silicon cast into a rectangular mold. As the silicon cools, it crystallizes into a few large (mm to cm sized) randomly oriented crystals instead of a single one. The remainder of the process is the same as for single crystal cells. The finished cells show lines dividing the individual crystals, as if the cells were cracked. The high efficiency is not quite as high as single crystal cells due to losses at crystal grain boundaries. The cell surface cannot be roughened by etching due to the random orientation of the crystals. However, an antireflectrive coating improves efficiency. These cells are competitive for all but space applications.
Three layer cell: The highest efficiency solar cell is a stack of three cells tuned to absorb different portions of the solar spectrum. Though three cells can be stacked atop one another, a monolithic single crystal structure of 20 semiconductor layers is more compact. At 32 % efficiency, it is now (2007) favored over silicon for space application. The high cost prevents it from finding many earth bound applications other than concentrators based on lenses or mirrors.
8) GUNN DIODE
A gunn diode is solely composed of N-type semiconductor. As such, it is not a true diode. Figure below shows a lightly doped N- layer surrounded by heavily doped N+ layers. A voltage applied across the N-type gallium arsenide gunn diode creates a strong electric field across the lightly doped N- layer.
[pic]
Gunn diode: Oscillator circuit and cross section of only N-type semiconductor diode.
As voltage is increased, conduction increases due to electrons in a low energy conduction band. As voltage is increased beyond the threshold of approximately 1 V, electrons move from the lower conduction band to the higher energy conduction band where they no longer contribute to conduction. In other words, as voltage increases, current decreases, a negative resistance condition. The oscillation frequency is determined by the transit time of the conduction electrons, which is inversely related to the thickness of the N- layer.
The frequency may be controlled to some extent by embedding the gunn diode into a resonant circuit. The lumped circuit equivalent shown in Figure above is actually a coaxial transmission line or waveguide. Gallium arsenide gunn diodes are available for operation from 10 to 200 gHz at 5 to 65 mw power. Gunn diodes may also serve as amplifiers.
9) TUNNEL DIODES
Tunnel diodes exploit a strange quantum phenomenon called resonant tunneling to provide a negative resistance forward-bias characteristics. When a small forward-bias voltage is applied across a tunnel diode, it begins to conduct current. (Figure below(b)) As the voltage is increased, the current increases and reaches a peak value called the peak current (IP). If the voltage is increased a little more, the current actually begins to decrease until it reaches a low point called the valley current (IV). If the voltage is increased further yet, the current begins to increase again, this time without decreasing into another “valley.” The schematic symbol for the tunnel diode shown in Figure below(a).
[pic]
[pic] Construction of Tunnel diode
Tunnel diode (a) Schematic symbol. (b) Current vs voltage plot (c) Oscillator.
The forward voltages necessary to drive a tunnel diode to its peak and valley currents are known as peak voltage (VP) and valley voltage (VV), respectively. The region on the graph where current is decreasing while applied voltage is increasing (between VP and VV on the horizontal scale) is known as the region of negative resistance.
Tunnel diodes, also known as Esaki diodes in honor of their Japanese inventor Leo Esaki, are able to transition between peak and valley current levels very quickly, “switching” between high and low states of conduction much faster than even Schottky diodes. Tunnel diode characteristics are also relatively unaffected by changes in temperature.
Reverse breakdown voltage versus doping level.
Tunnel diodes are heavily doped in both the P and N regions, 1000 times the level in a rectifier. This can be seen in Figure above. Standard diodes are to the far left, zener diodes near to the left, and tunnel diodes to the right of the dashed line. The heavy doping produces an unusually thin depletion region. This produces an unusually low reverse breakdown voltage with high leakage. The thin depletion region causes high capacitance. To overcome this, the tunnel diode junction area must be tiny. The forward diode characteristic consists of two regions: a normal forward diode characteristic with current rising exponentially beyond VF, 0.3 V for Ge, 0.7 V for Si. Between 0 V and VF is an additional “negative resistance” characteristic peak. This is due to quantum mechanical tunneling involving the dual particle-wave nature of electrons. The depletion region is thin enough compared with the equivalent wavelength of the electron that they can tunnel through. They do not have to overcome the normal forward diode voltage VF. The energy level of the conduction band of the N-type material overlaps the level of the valence band in the P-type region. With increasing voltage, tunneling begins; the levels overlap; current increases, up to a point. As current increases further, the energy levels overlap less; current decreases with increasing voltage. This is the “negative resistance” portion of the curve.
Tunnel diodes are not good rectifiers, as they have relatively high “leakage” current when reverse-biased.
10) DESCRIBE IN BRIEF VARIOUS DIGITAL DISPLAY DEVICES.
Ans. The digital display devices indicate the value of the measured quantity in decimal digits. The digital display device may receive digital information in any form, but it converts that information into decimal form. The basic element in a digital display device is the display of a “single digit”, while, the “multiple digit display” is nothing but a group of single digit displays. The fig. 4 shows a multiple digit display consisting of 7 single digit displays.
[pic]
A single digit display is capable of indicating the numbers from 0 to 9 with a decimal point. The input to the digital display is a “code” indicating the particular number to be displayed or the “excitation” of one of the ten inputs. Note that I indicates “excitation” and 0 indicates “non excitation”. The digit displayed will depend on combination of excitation or non excitation. Note that the signal should be “decoded” and decoding circuits are a part of the display units.
(b) The digital display devices are of the following types:
1. Projecting displays : This projects the desired digit on a small screen by means of an optical system and an appropriate mask. Each digit appears at the same plane.
2. Segmental displays: This consists of 7 segments (LEDs). This display forms the digit to be displayed by illuminating proper segment (LED).
3. Lucit sheets: This read out system uses 10 lucit sheets with numbers 0 to 9, etched on the face of each sheet. The sheets are stacked one upon other in the order from 0 to 9 the light is placed on edge of each sheet The light passes through the sheet and comes out of the etched digit and indicates the particular digit. Note that the digits are displayed in different plane.
4. Beam switching tube The system uses a beam switching tube having 10 electrodes. The electrodes are marked from 0 to 9. The particular electrode glows by applying the proper voltage.
5. Grid illuminated dots This uses a grid of incandescent lamps or preferably LEDs. The fig. 5 shows a grid illuminated dot display.
[pic]
11)LED
L.E.D. Process of emission, materials used, construction, advantages and applications.
Ans. (a) The L.E.D. (light emitting diode) is a diode, that emits visible light, when energised (or biased) properly. In all PN junctions, near the junction, a recombination of electrons and holes takes place. In this process, the energy possessed by unbound free electrons is transferred to the another state and some of the energy is given out as heat and same in the form of light (photons). In germanium and silicon the greater percentage is given up in the form of heat and the emitted light is insignificant. In other materials such as “gallium arsenide phosphate” (Ga As P), or gallium phosphate (Ga P), the no. of photons emitted is sufficient to create a visible light. The phenomenon of giving off light (by applying an electrical source) is known as “Electro luminescence”.
(b) Process of emission
The conducting surface connected to the p type material is smaller to allow emergence of maximum number of photons. This occurs near the junction.
There is no doubt some photons are absorbed by the diode itself, but a very large percentage of photon energy is able to come out. The fig. 12 (a) shows the process of emission of light and (b) the symbol of the L.E.D.
[pic]
(c) Materials used for LED
The table shows the list of materials and other details.
Table 2.
[pic]
1.4 eV.
Gallium phosphide (Ga.P) is used for emissions in the visible spectrum. It has a gap energy of 2.26 eV and can be doped with “Zinc” and “Oxygen” to give out red light or nitrogen to give out green light.
Gallium Arsenide and gallium phosphide combine to form a solid solution of “gallium arsenide phosphate” (Ga .As P), doping with nitrogen increases the conversion efficiency and also the wavelength of emission. The Im per watt is the luminous efficiency which takes into account the sensitivity of human eye, which is most sensitive to green and hence increased luminous efficiency is needed in this region.
For infrared light, gallium arsenide (Ga. As) is used, which have an energy gap Ok
(d) Construction
The fig. 13 shows construction of a Ga. As. P LED. It is red emitting diode and has a mesh construction. It uses an N type alloy of Ga.As.P, which is grown especially on a Ga. As. substrate. The P region is diffused into this and covered with a “comb” type
[pic]
metal electrode (anode) to complete the PN junction. The electrode distributes the diode current uniformly.
The emission of photons is the result of recombination of electrons and holes, which is only possible when both “energy” and “momentum” are conserved.
A photon have a considerable energy but its momentum is very small, therefore the simplest and most probable recombination process will be that, where the electrons and holes have the same value of momentum. This condition exists in many group III and IV compound
semiconductors with minimum conduction band and maximum valence band at zero momentum position.
(e) Advantages
The advantages of LEDs in electronic displays are given below:
(1) These are small in size and can be stacked together to form alphanumeric (alphabets as well as numerals) displays in high density matrix.
(ii) The light output from LED is a function of current flowing through it, hence
intensity of emitted light can be controlled.
(iii) These have high efficiency and need little power for. operation. A voltage drop of 1.2 V and current of 20 mA is sufficient for full brightness.
(iv) These can emit radiations of many colours, such as red, green, yellow etc.
V)The switching time is less than 1 ns.
(vi) The LEDs are manufactured by the same technology used for I.Cs., hence are
economical and highly reliable.
(vu) These are rugged and can with stand shocks and vibrations
(viii) These can be operated for a temperature range of 0°C to 70°C.
(I) Applications of LEDs
(i) In solid state video displays, which are replacing cathode ray tubes
(ii) In picture phones (in image sensing circuits).
(iii) In optical fibre communications.
(iv) In data links and remote controls.
(v) In arrays for displaying alpha-numeric or for entering information into computer memories.
(vi) In calculator, watches for display.
12)PHOTODIODE & PHOTO TRANSISTOR
Introduction
Photodiodes are semiconductor light sensors that generate a current or voltage when the P-N junction in the semiconductor is illuminated by light. The term photodiode can be broadly defined to include even solar batteries, but it usually refers to sensors used to detect the intensity of light. Photodiodes can be classified by function and construction as follows:
[pic]
Figure shows a cross section of a photodiode. The P-layer material at the active surface and the N material at the substrate form a PN junction which operates as a photoelectric converter.
[pic]FIG.PHOTODIODE
Photodiode type
1. PN photodiode
2. PIN photodiode
3. Schottky type photodiode
4. APD (Avalanche photodiode)
All of these types provide the following features and are widely used for the detection of the intensity, position, color and presence of light.
Features of photodiode
1. Excellent linearity with respect to incident light
2. Low noise
3. Wide spectral response
4. Mechanically rugged
5. Compact and lightweight
6. Long life
7. Materials commonly used to produce photodiodes include
|Material |Electromagnetic spectrum |
| |wavelength range (nm) |
|Silicon |190–1100 |
|Germanium |400–1700 |
|Indium gallium arsenide |800–2600 |
|Lead(II) sulfide | ................
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