Electric Circuit and Electron Device–TRANSISTORS (Unit 4)–Lecture Notes

Anna University

Electric Circuit and Electron Device


Subject Code : EC2151

Subject Name : Electric Circuit and Electron Device

Year : 1st yr

Semester : 2nd Sem

Department : CSE & ECE


UNIT-IV

TRANSISTORS


Introduction of transistors

Transistor is a semiconductor device that can amplify electronic signals such as radio and television signals.

Advantage of the transistor

  1. Smaller in size
  2. No filament and no need of power for heating filament
  3. Low operating voltage
  4. Higher efficiency

Types of the transistor

Ø Unipolar Junction Transistor

Ø Bipolar Junction Transistor

Construction of the transistor

Ø n-p-n transistor

Ø p-n-p transistor

n-p-n transistor

It is formed by sand witching p-type semiconductor between two n-type.

p-n-p transistor

It is formed by sand witching n-type semiconductor between two p-type.

Terminals for the transistor

Ø Emitter

Ø Collector

Ø Base

Functions of Emitter, Collector & Base:

Emitter : To supply majority charge carriers.

Collector: To collect majority charge carriers.

Base: It passes most of the injected charge carriers to the collector.

Transistor Biasing

  • Applying external voltage to a transistor is called biasing.
  • In order to operate transistor properly as an amplifier, it is necessary to correctly bias the two pn junctions with external voltages.
  • Depending upon external bias voltage polarities used, the transistor works in one of the three regions.

Ø Active region.

Ø Cut-off region.

Ø Saturation region.

Sl.No.

Region

Emitter Base

Collector Base

Operation of a transistor

1

Active

Forward biased

Reverse biased

acts as an amplifier

2

Cut-off

Forward biased

Reverse biased

acts as an open switch

3

Saturation

Forward biased

Reverse biased

acts as an closed switch

Operation of NPN transistors

  • Emitter is forward biased & as a result large forward current flows across the emitter junction due to flow of majority carriers.
  • Injected electrons diffuse into the collector region due to the extremely small thickness of the base.
  • Collector is reverse bias and creates a strong electrostatic field between base &collector.
  • Field immediately collects the diffused electrons which enter the collector junction.
  • Flow of electrons into the base region when confronted with the holes, a few electrons combine & neutralize
  • Rest of the electrons of the injected electrons diffuse into the collector region and is collected by the collector electrode.

Operation of PNP transistors

  • Forward bias causes the holes in the P-type emitter to flow towards the base.
  • Reduces the potential barrier at the junction
  • Holes cross the junction & penetrate into the N-region. This constitutes emitter current IE.
  • Width of the base region is very thin & lightly doped; hence a small amount of the holes recombine with free electrons of N-regions. This constitutes base current IB & is very small.
  • Rest of the holes drift across the base and enter the collector region and are swept away by the negative collector electrode. This constitutes base current IC.
  • Current conduction I PNP transistors is by movement of holes.
  • Current conduction in the external circuit is by electrons.


Types of configuration

Ø Common Base configuration

Ø Common Emitter configuration

Ø Common Collector configuration

Common Base configuration

· Input is connected between emitter & base. Output is connected between collector &base.

· Emitter-base junction is forward biased. Collector-base junction is reverse biased.

· Emitter current IE flows in the input circuit. Collector current IC flows in output circuit.

· The ratio of collector current IC, to emitter current IE, is called the

Current amplification factor (α).

· If there is no input ac signal, then the ratio of IC to IE is called dc alpha (αdc).

· ac alpha refers to the ratio of change in IC to change in IE.

· The higher the value of α, better the transistor. α can be increased by making base thin and lightly doped.

Characteristics of CB configuration

The performance of transistors, when connected in a circuit, may be determined from their characteristic curves that relate different d.c. currents and voltages of a transistor.Such curves are known as Static characteristic curves.

Input Characteristics

· The curve drawn between Emitter current and Emitter – Base voltage for a given value of collector-Base voltage is known as input Characteristics.

· For a given value of VCB,the curve is just like a forward-biased PN junction.

· With an increase in the value of VCB,it conducts better. This is because of the effect called early effect or Base width modulation.

Output Characteristics

· The curve drawn between Collector current and Collector – Base voltage for a given value of emitter current is known as output Characteristics.

· The collector current varies with VCB for very low voltage but transistor is never operated in this region.

Common Emitter configuration

· Input is connected between base & emitter. Output is connected between collector & emitter.

· Emitter-base junction is forward biased. Collector-base junction is reverse biased.

· Base current IB flows in the input circuit. Collector current IC flows in output circuit.

· CE is commonly used because its current, voltage and power gains are quite high and output to input impedance ratio is moderate.

· The rate of change in collector current IC, to change in emitter current IE, is called amplification factor (β).

Input Characteristics

· The curve drawn between Base current and Base – Emitter voltage for a given value of collector-emitter voltage is known as input Characteristics.

· For a given value of VEC,the curve is just like a forward-biased PN junction diode.

· Input resistance is larger in CE configuration than in CB configuration. This is because the input current IB increases less rapidly with increase in VBE .

· An increment in the value of VCE, causes the input current IB to be lower for a given level of VBE . This is because of the effect called early effect.

Output Characteristics

· The curve drawn between Collector current IC and Collector – emitter voltage VCE for a given value of base current IB is known as output Characteristics.

· Output characteristics in CE configuration have some slope while CB configuration has almost horizontal characteristics. This indicates that output resistance in case of CE configuration is less than that in CB configuration.

Common Collector configuration

· Input is connected between base & collector. Output is connected between collector & emitter.

· The Collector forms the terminal common to both the input and output.

· Base current flows in the input circuit. Emitter current flows in output circuit.

· With base current IB equal to VCO, the emitter current IE is zero, so no current flows in the load resistor RL.

· With increases in input current IB, the transistor passes through the active region and finally reaches saturation.

Input Characteristics

· To determine the input Characteristics, VEC is kept at a suitable fixed value.

· The base-collector voltage VBc is increased in equal steps and the corresponding increase in IB is noted.

· This is repeated for different values of VEC.

Breakdown in Transistors

  • Avalanche Multiplication
  • Reach-Through (or) Punch through

Avalanche Multiplication

· The maximum reverse bias voltage which can be applied before breakdown between collector and base terminals of the transistor under the condition that the emitter is open-circuited.

· It is represented by the symbol BVCBO (for CB configuration).

· This breakdown voltage is a characteristic of the transistor alone.

· Breakdown occurs because of the avalanche multiplication of current ICO that crosses the collector junction.

· As a result of this multiplication, the current becomes MICO in which M is the factor by which the original current ICO is multiplied by the avalanche effect.

· At a high voltage BVCBO, the multiplication factor M becomes infinite and the region of breakdown is then attained.

· The current increases abruptly and large changes in current accompanies small changes in voltage.

Reach-Through (or) Punch through

· It results from Early effect (i.e.) as a result of increase in VCB and as the doping of the base is substantially smaller than that of the collector and the penetration of the transition region into the base is larger than into the collector

· Since the base is very thin, the transition region spreads completely across the base to reach the emitter junction.

· At this point, normal transistor action ceases and the emitter and collector are effectively shorted.

· Hence, a large current flows from the emitter to collector. This is called Reach-through.


Field Effect Transistor (FET)

  • FET is a semiconductor device which depends for its operation on the control of current by an electric field.
  • The output characteristics of FET are controlled by Input voltage and not by the Input current.
  • So, it is also known as voltage-controlled device.

Features of FET

The FET has several advantages over the conventional transistor.

  • Its operation depends upon the flow of majority carrier only. So, it is called as Unipolar device.
  • It is relatively immune to radiation.
  • It exhibits a high input resistance, typically many mega ohms.
  • It is less noisy than a tube of a Bipolar Transistor.
  • It exhibits no offset voltage at zero Drain current.
  • It has thermal stability.

Types of FET

  • Junction Field Effect Transistor (JFET)
  • Metal Oxide Field Effect Transistor (MOSFET) (or)

Insulated Gate Field Effect Transistor (IGFET)

Construction of JFET

  • JFET is a three terminal semiconductor device in which current conduction is by one type of carrier either Electrons or holes.
  • The JFET consists of a P-type or N-type silicon bar.
  • The bar is made up of N-type material which is known as N-channel JFET and if the bar is made up of P-type material, it is known as P channel JFET.
  • The current in FET is carried by the majority carriers.
  • One end of the channel is called the source and the other is called the drain.

Operation of JFET

FET works under the three conditions.

  • When VGG applied and VDD=0
  • When VDS applied and VGG=0
  • When VDD applied and VGG is applied.

Where,

Ø VGG – Gate supply voltage.

Ø VDS– Drain Source voltage.

Ø VDD– Drain supply voltage.

Characteristics of JFET

· A family of curves that relate the current and voltage are known as characteristics curve.

· There are the two important characteristics of a JFET.

Ø Transfer characteristics

Ø Drain characteristics

Characteristics Parameters of JFET

The parameters of JFET are

  • Transconductance
  • Drain resistance
  • Drain conductance
  • Amplification factor


Metal Oxide Semiconductor FET (MOSFET)

  • MOSFET is a three terminal device. Those terminals are source, gate and drain.
  • The gate of a MOSFET is insulated from the channel.
  • Because of this, the MOSFET is also known as an IGFET (Insulated gate FET).
  • The MOSFET is a second category of FET.
  • The MOSFET differs from the JFET is that it has no pn junction structure; instead the gate of the MOSFET is insulated from the channel by a silicon dioxide layer.

Types of MOSFET

  • Depletion – type MOSFET
  • Enhance – type MOSFET

Construction of MOSFET

  • Two highly doped n regions are diffused into a lightly doped p type substrate.
  • These two highly doped regions are represents source and drain. In some cases substrate is internally connected to the source terminal.
  • The source and drain terminals are connected through metallic contacts the n-doped regions linked by an n-channel.
  • The gate is also connected to a metal contact surface but remains insulted from the n-Channel by a very thin layer of dielectric material, Silicon Dioxide.
  • This layer act as one parallel plate capacitor.
  • Thus, there is no direct electrical connection between the gate terminal and the channel of a MOSFET increasing the input impedance of the device.

Characteristics of MOSFET

The different characteristics of a D-MOSFET are

Ø Drain characteristics

Ø Transfer characteristics