Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Semiconductor Devices, Analog and Digital

Electronics

Academic Consultant

Department of Physics

Uttarakhand Open University, Haldwani

Dr. Meenakshi Rana

BLOCK – I SEMICONDUCTOR DIODES, TRANSISTORS AND AMPLIFIERS

UNIT 2: Transistor

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

1. Introduction

2.Objectives

3.Transistor

4.Transistor history

5.Transistor construction

6.Comparison of transistor connection

7.Transistor biasing

8.Transistor types

• Bipolar Junction Transistor (BJT)

• Unipolar Junction Transistor (UJT)

• Field Effect Transistor (FET)

9. Construction, working, and characteristic of various types of transistors

10. Application of various types of transistor

11. Some useful links

Content

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Objectives

After studying this unit, you should be able to-

1.Evolution of transistor

2.Importance of transistor

3.Definition & transistor types

4.Transistor symbol & operation

5. Get knowledge about various types of transistors; UJT, BJT, FET,

MOS FET, CMOSFET

6. Understand construction, working, and characteristic of various

types of transistors

7. Comparison of transistor connection

8. Comparison of various types of transistors

9. Application of various types of transistor

10. Some useful links

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Transistor: Introduction

Beside diodes, the most popular semiconductor devices is transistors.

Transistors are often said to be the most significant invention of the 20th Century.

If cells are the building blocks of life, transistors are the building blocks of the digital

revolution. Without transistors, the technological wonders you use every day -- cell phones,

computers, cars -- would be vastly different, if they existed at all.

Transistors are more complex and can be used in many ways.

Most important feature: can amplify signals and as switch.

Amplification can make weak signal strong (make sounds louder and signal levels greater),

in general, provide function called Gain.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

•In the mid 1940’s a team of scientists working for Bell Telephone Labs in Murray Hill, New

Jersey, were working to discover a device to replace the then present vacuum tube technology.

Vacuum tubes were the only technology available at the time to amplify signals or serve as

switching devices in electronics. The problem was that they were expensive, consumed a lot of

power, gave off too much heat, and were unreliable, causing a great deal of maintenance.

•The scientists that were responsible for the 1947 invention of the transistor were: John

Bardeen, Walter Brattain, and William Shockley. Bardeen, with a Ph.D. in mathematics and

physics from Princeton University, was a specialist in the electron conducting properties of

semiconductors. Brattain, Ph.D., was an expert in the nature of the atomic structure of solids at

their surface level and solid-state physics. Shockley, Ph.D., was the director of transistor

research for Bell Labs.

•Their original patent name for the transistor was: “Semiconductor amplifier; Three-electrode

circuit element utilizing semi conductive materials.”

•In 1956, the group was awarded the Noble Prize in Physics for their invention of the

transistor. In 1977, John Bardeen was awarded the Presidential Medal of Freedom.

Transistor History

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

In 1947, John

Bardeen and

Walter Brattain

devised - the first

"point contact"

transistor.

The first transistor

Transistor is an electronic device made of three layers of semiconductor material that can

act as an insulator and a conductor.

The three layered transistor is also known as the bipolar junction transistor.

Transistor Definition

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

The emitter region is heavily doped and its

job is to emit carriers into the

base.

The base region is very thin and lightly

doped.

Most of the current carriers injected into the

base pass on to the collector.

The collector region is moderately doped and

is the largest of all three regions.

A transistor has three doped regions.

• For both types, the base is a narrow region sandwiched between the larger collector and

emitter regions.

Transistor Structure

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

BJT is bipolar because both holes (+) and electrons (-)

will take part in the

current flow through the device

– N-type regions contains free electrons (negative

carriers)

– P-type regions contains free holes (positive carriers)

• 2 types of BJT

– NPN transistor

– PNP transistor

• The transistor regions are:

– Emitter (E) – send the carriers into the base region and

then on to the

collector

– Base (B) – acts as control region. It can allow

none,some or many

carriers to flow

– Collector (C) – collects the carriers

Transistor Construction

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

NPN Transistor Structure

The collector is lightly doped.

The base is thin and is lightly doped.

The emitter is heavily doped.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

The base-emitter (BE) junction is forward

biased

The base-collector (BC) junction is reverse

biased.

Transistor biasing

IE=IB+IC

Transistor Types

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

• The transistor is a three-layer semiconductor device consisting of either two n- and one ptype

layers of material or two p- and one ntype layers of material.

• The former is called an npn transistor, while the latter is called a pnp transistor

• So, there are two types of BJT

i) pnp transistor ii) npn transistor

Bipolar Junction Transistors

In each transistor following points to be noted

i) There are two junction, so transistor can be considered as two diode connected back to

back.

ii) There are three terminals.

iii)The middle section is thin than other. Transistor symbol

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Transistor has three section of doped semiconductor.

• The section one side is called “emitter” and the opposite side is called “collector”.

• The middle section is called “base”.

1) Emitter:

The section of one side that supplies carriers is called emitter.

Emitter is always forward biased wr to base so it can supply carrier.

For “npn transistor” emitter supply holes to its junction.

For “pnp transistor” emitter supply electrons to its junction.

2) Collector:

The section on the other side that collects carrier is called collector.

The collector is always reversed biased wrt to base.

For “npn transistor” collector receives holes to its junction.

For “pnp transistor” collector receives electrons to its junction.

3) Base:

The middle section which forms two pn junction between emitter and collector is

called Base.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Transistor Operation

1) Working of npn transistor:

Forward bias Is applied to emitter base junction and reverse bias is

applied to collector base junction.

The forward bias in the emitter-base junction causes electrons to move toward

base. This constitute emitter current, IE

As this electrons flow toward p-type base, they try to recombine with holes. As base

is

lightly doped only few electrons recombine with holes within the base.

These recombined electrons constitute small base current.

The remainder electrons crosses base and constitute collector current.

IE=IB+IC

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

2) Working of pnp transistor: . Forward bias is applied to emitter base

junction and reverse bias is applied to collector base junction.

The forward bias in the emitter-base junction causes holes to move toward base. This

constitute emitter current, IE

As this holes flow toward n-type base, they try to recombine with electrons. As base is lightly

doped only few holes recombine with electrons within the base.

These recombined holes constitute small base current.

The remainder holes crosses base and constitute collector current.

Transistor Operation

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Transistor Symbols

Transistor Operating Modes • Active Mode

Base- Emitter junction is forward and Base-Collector junction is reverse biased.

• Saturation Mode

Base- Emitter junction is forward and Base-Collector junction is forward biased.

• Cut-off Mode

Both junctions are reverse biased.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

• Transistor can be connected in a circuit in

following three ways-

1) Common Base

2) Common Emitter

3) Common Collector

Transistor Connection

Common Base Connection

• The common-base terminology is derived from the fact that the base is common to both the

input and output sides of the configuration.

First Figure shows common base npn configuration and second

figure shows common base pnp configuration.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Current amplification factor (α) :

The ratio of change in collector current to the change in emitter current at constant

VCB is known as current amplification factor,

Practical value of is less than unity, but in the range of 0.9 to 0.99

Expression for Collector Current

Total emitter current does not reach the collector terminal, because a small portion of it

constitute base current. So,

Also, collector diode is reverse biased, so very few minority carrier passes the collector-base

junction which actually constitute leakage current.

So, collector current constitute of portion of emitter current αIE and leakage current ICBO.

IE=IB+IC

IC=αIE+ICBO

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Characteristics of common base

configuration

Input Characteristics

VBE vs IE characteristics is called input characteristics.

IE increases rapidly with VBE . It means input resistance is very

small.

IE almost independent of VCB.

Output Characteristics

VBc vs Ic characteristics is called output characteristics.

IC varies linearly with VBc ,only when VBc is very small.

As, VBc increases, Ic becomes constant.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Input Resistance: The ratio of change in emitter-base voltage to the change in emitter current is

called Input Resistance.

Input and Output Resistance of common base configuration

Output Resistance: The ratio of change in collector-base voltage to the change in collector

current is called Output Resistance.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Common Emitter Connection

The common-emitter terminology is derived from the fact that the emitter is common to both

the input and output sides of the configuration.

First Figure shows common emitter npn configuration and second figure shows common emitter

pnp configuration.

Base Current amplification factor ( β) :

• In common emitter connection input current is base current and output current is collector

current.

• The ratio of change in collector current to the β change in base current is known as base current

amplification factor,

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Normally only 5% of emitter current flows to base, so amplification factor is greater than 20.

Usually this range varies from 20 to 500.

Relation Between α and β

Expression for collector current

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Characteristics of common emitter configuration

VBE vs IB characteristics is called input characteristics.

IB increases rapidly with VBE . It means input resistance is

very small.

IE almost independent of VCE.

IB is of the range of micro amps.

Input Characteristics

Output Characteristics

VCE vs Ic characteristics is called output characteristics.

IC varies linearly with VCE ,only when VCE is very small.

As, VCE increases, IC becomes constant.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Input Resistance: The ratio of change in emitter-base voltage to the change in base current is

called Input Resistance.

Input and Output Resistance of common emitter configuration

Output Resistance: The ratio of change in collector-emitter voltage to the change in collector

current is called Output Resistance.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

• The common-collector terminology is derived from the fact that the collector is common to

both the input and output sides of the configuration.

Common Collector Configuration

First Figure shows common collector npn configuration and second figure shows common

collector pnp configuration.

Current amplification factor (γ):

• In common emitter connection input current is base current and output current is emitter

current.

• The ratio of change in emitter current to the change in base current is known as current

amplification factor in common collector configuration.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Relation between γ and α

Expression for collector current

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Comparison of Transistor Connection

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

When used as an electronic switch, the transistor is normally operated

alternately in cut-off and saturation regions.

Transistor applications

Transistor as a switch

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Due to the small changes in base current the

collector current will mimic the input with greater

amplitude.

Transistor as amplifier

Figure shows CE amplifier for npn transistor

• Battery VBB is connected with base in-order to make base forward biased, regardless of input ac

polarity.

• Output is taken across Load R

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

During positive half cycle input ac will keep the emitter base junction more forward biased. So,

more carrier will be emitted by emitter, this huge current will flow through load and we will find

output amplified signal.

• During negative half cycle input ac will keep the emitter-base junction less forward biased. So,

less carrier will be emitted by emitter. Hence collector current decreases.

• This results in decreased output voltage (In opposite direction).

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Transistor Load line analysis

Consider common emitter npn transistor circuit shown in figure.

There is no input signal.

Apply KVL in the output circuit

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Operating Point

•It is called operating point because variation of IC

takes place about this point.

•It is also called quiescent point or Q-point.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Field Effect Transistor

Field effect Transistor is a semiconductor device which depends for its operation on

the control of current by an electric field

FET has several advantages over BJT

1.Current flow is due to majority carriers only

2. Immune to radiation

3.High input resistance

4.Less noisy than BJT

5.No offset voltages at zero drain current

6.High thermal stability JFET Symbol

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Classification of FET

Based on the construction JFETS are of two types

1.N Channel FET

2.P Channel FET

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Construction of N Channel FET

Source: The source is the terminal through

which majority carriers enter the Silicon Bar

Drain: Terminal through which majority

carriers leave the bar

Gate: controls Drain current and is always

reverse biased

The operation of FET can be compared to the water flow through a flexible pipe

When one end is pressed the cross sectional area decreases hence water flow decreases

In a FET drain is similar to outlet.

Principle: To control the drain current FET makes use of channel formed in by Space charge

region between Gate and the bar. By increasing the reverse bias the width of space charge

region decreases. As a result the channel Resistance increases The Drain current decreases

Operation of FET

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

•As we increase the reverse bias on gate

(VGS)

•The channel width decreases

•Gate is reverse biased by battery 2

•The Bias voltage at which drain current

becomes zero is known as pinch off

voltage

Working of FET

Working of n channel FET and p channel FET

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

1.When Voltage is applied between source and Drain majority carriers move through the

channel between depletion region.

2. The value of Drain current is maximum when no external voltage is applied between gate and

source.

3. When gate to source reverse bias increases the depletion region widens and channel width

decreases hence Drain current decreases.

4. Hence Drain current decreases.

5. When gate to source voltage is increased further The channel completely closes.

6. This is called pinch off region.

7. This reduces Drain current to zero.

8. The Gate to source voltage at which the Drain current is zero is called “ Pinch off Voltage”

Working of FET

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Electrical behavior is described in terms of the parameters of the Device. They are

obtained from the characteristics. Important Parameters for FET are

1.DC Drain resistance

2.AC drain Resistance

3.Transconductance

JFET Parameters

Difference between N channel FET and P channel FET

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

DC Drain resistance: Defined as ratio of Drain to source Voltage VDS to Drain current ID.

Also called static or Ohmic Resistance

Mathematically

JFET Parameters

AC Drain resistance: Defined as the resistance between Drain to source when JFET is operating

in Pinch off Region or saturation Region

2. Mathematically

Transconductance (gm): It is given by the ratio of small change in drain current to the

Corresponding change in the Gate to source Voltage VGS. Also known as Forward Transmittance

Mathematically

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Difference between FET and BJT

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Drain Characteristics

Drain characteristics show the relation between the drain to source voltage

and VDS and drain current ID.

•At the Drain to source Voltage corresponding to point B Channel width reduces to a

minimum value and is known as pinch off

Drain current ID is given by

•The device gets damaged due to avalanche Breakdown mechanism.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

FET Applications

Phase shift oscillators: The high input impedance of FET is especially valuable in phase

shift oscillator to minimize the loading effect.

In voltmeters: The high input impedance of FET is useful in voltmeters to act as an

input stage.

As a buffer amplifier which isolates the preceding stage from the following stage.

FET has low noise operation. So it is used in RF amplifiers in FM tuners and

communication equipment.

FET has low input capacitance, so it is used in cascade amplifiers in measuring and test

equipment.

Since FET is a voltage controlled device, it is used as a voltage variable resistor in

operational amplifiers and tone controls.

FET has low inner modulation distortion. So it is used in mixer circuits in FM and TV

receivers, and communication equipment.

Since it is low-frequency drifts, it is used in oscillator circuits.

DISADVANTAGES OF FET OVER BJT

FETs have a drawback of smaller gain bandwidth product compared to BJT.

The high input impedance, low output impedance and low noise level make FET for

superior of the bipolar transistor.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Classification of MOSFETs

Metal oxide semiconductor field effect

transistor (MOSFET)

MOSFET is an important semiconductor device and is widely used in many circuit application.

The input impedance of a MOSFET is much more than that if a FET because of very small

leakage current.

MOSFETs has much greater commercial Importance than JFET.

The MOSFET can be used in any of the circuits covered for the FET.

Therefore all the equations apply equally well to the MOSFET and FET in amplifier

connections.

MOSFETs uses a metal gate electrode (instead of p-n junction in JFET), separated from the

semi conductor by an Insulating thin layer SiO2 to modulate the resistance of the conduction

channel.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Metal oxide semiconductor field effect transistor (MOSFET)

• It is also called as insulated gate FET (IGFET)

•MOSFETs operates both in the depletion mode as well as an the enhancement

mode

Circuit symbols of MOSFET

Differences between MOSFET and FET

There is only a single p-region. This is called substrate.

A thin layer of metal oxide is deposited over the left side of the channel. A metallic gate is

deposited over the oxide layer. As silicon dioxide is an insulator, therefore a gate is insulated

from the channel. For this reason MOSFET is some times called insulated gate FET.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

P channel Enhancement MOSFET

symbols

Enhancement MOSFET

A p-channel MOSFET consists of lightly

doped n-substrate into which two heavily

doped p+ regions act as the source and the

drain.

A thin layer of SiO2 is grown over the

surface of the entire assembly.

P channel Enhancement MOSFET

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Construction of P channel Enhancement MOSFET

Holes are cut into this SiO2 layer for making contact with p+ source and drain regions.

On the SiO2 layer, a metal (aluminum) layer is overlaid covering the

entire channel region from source to drain.

This aluminum layer constitutes the gate.

The area of MOSFET is typically 5 square mills or less.

This area is extremely small being only about 5% of the area required for a bipolar junction

transistor.

A parallel plate capacitor is formed with the metal areas of the gate and the semiconductor

channel acting as the electrodes of the capacitor.

The oxide layer acts as the dielectric between the electrodes.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

The substrate will be connected to the common terminal i.e., to the ground terminal.

A negative potential will be applied to the gate.

This results in the formation of an electric field normal the SiO2.

This electric field originates from the induced positive charges on the semiconductor side on

the lower surface of the SiO2 layer.

The induced positive charge become minority carriers in the n-type of substrate.

Working P channel Enhancement MOSFET

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

n channel Enhancement MOSFET

•It consists of a lightly doped p type substrate in to which two heavily doped n type

material are diffused.

• The surface is coated with a layer of silicon dioxide (SiO2).

• Holes are cut through the SiO2 to make contact with n-type blocks.

•Metal (Al) is deposited through the Holes to form drain and source terminals.

•The surface area between drain and source a metal plate is deposited from which gate terminal

is taken out.

Working of n channel Enhancement MOSFET

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Gate is insulated from the body of FET so it is called insulated gate FET(IGFET).

•Structurally there exits no channel between source and drain so MOSFET some times

called as N-channel enhancement type

•Because a thin layer of P-type substrate touching the metal oxide film provides channel for

electrons and hence acts like N-type material.

Drain is made positive with respect to the source and no potential is applied to the gate as

shown in figure.

• The two n-blocks and p-type substrate form back to back pn junctions connected by the

Resistance of the p-type material.

• Both the junctions cannot be forwarded at the Same time so small drain current order of few

nano amperes flows.

Working

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

•So MOSFET is cut off when gate source voltage is zero.

• That is why it is called normally- OFF MOSFET.

• The gate is made positive with respect to source substrate as shown in figure.

• A channel of electrons (n channel) is formed in between the source and drain regions.

•Behaves as a capacitor with gate metal acting as one electrode, upper surface of the

substrate as other electrode and sio2 layer as dielectric medium.

•When positive voltage is applied to gate the capacitor begin to charge.

•Consequently positive charges appears on the gate and negative charges appears

in the substrate between the drain and source.

•The n-channel thus formed is called induced n-channel or n-type inversion layer.

•As VGS increases, no. electrons in the channel increases, ID increases.

•The minimum gate source voltage which produces then induced n-channel is called

threshold voltage VGS(th).when VGS < VGS (th), ID=0.

•Drain current starts only VGS >VGS (th).

•For a given value of VDS as VGS is increased , more and more electrons accumulate

under the gate and ID increases.

•So the conductivity of the channel is enhanced by the positive bias on the gate, the device

is known as enhancement mode MOSFET.

•The n-channel MOSFET can never operate with a negative gate voltage.

Working

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Drain characteristics

It is observed that the drain current has been enhanced on

application of negative gate voltage.

•This is the reason for calling it as enhancement

MOSFET.

•By increasing the gate potential, pinch off voltage and

drain currents are increased.

•The curves are similar to drain characteristics of JFET.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Depletion type MOSFET

Circuit symbols of n channel depletion type MOSFET

•Depletion MOSFET may be fabricated from the basic

MOSFET structure.

•An n-type channel is obtained by diffusion between n+ type

source and drain in an n-channel

MOSFET.

•In depletion MOSFET a lightly doped n-type channel has

been introduced between to heavily doped source& drain

blocks,.

Construction of n channel depletion type MOSFET

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Circuit symbols of p channel depletion type MOSFET

Construction of p channel depletion type MOSFET

•Depletion MOSFET may be fabricated from the basic

MOSFET structure.

•An p-type channel is obtained by diffusion between p+

type source and drain in an p channel MOSFET.

•In p-channel depletion MOSFETs are made by using n-

type substrate and diffusing a lightly doped p-type

channel between two heavily doped P-type source &

drain blocks

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Working

•Negative gate operation of a depletion MOSFET is called

Its depletion mode Operation

•When Vgs =0 electrons can flow freely from source to

drain through the conducting channel. since a channel

exists between drain & source, Id flows even when Vgs=0.

• It is also known as normally –ON MOSFET.

•When negative voltage is applied to the gate as shown

•in Fig positive charges are induced in the channel by

capacitor action.

• The induced positive charges make the channel less

conductive and drain current decreases as VGS is made

more negative.

•With negative voltage a depletion MOSFET behave like JFET.

•When positive voltage is applied to the gate free electrons are Induced channel.

•This enhances the conductivity of the channel so increasing amount of current between terminals

• Since the action of negative voltage on gate is to deplete the channel of free n-type charge

carriers so named as depletion MOSFET.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Drain Characteristics of depletion MOSFET

•When the gate source voltage is zero considerable drain current flows.

•When the gate is applied with negative voltage, positive charge are induced in the n

channel through the SiO2 layer of the gate capacitor.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

The conduction in n channel FET is due to electrons i.e., the majority carriers.

Therefore the induced positive charges make the n-channel less conductive.

The drain current therefore gets reduced with increase in the gate bias voltage

The distribution of charges in the channel results in depletion of majority carriers.

That is why this type of FET is called depletion MOSFET.

The voltage drop due to the drain current causes the channel region nearer to the drain to

be more depleted than the region due to the source.

The depletion MOSFET can also be operated in enhancement mode simply by applying

a positive voltage to the gate

Drain Characteristics of depletion MOSFET

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Comparison of MOSFET and JFET

• The insulated gate in MOSFET s result in much greater input impedance than that of JFET

• Inter electrode capacitance are independent of bia voltage and these capacitances are smaller

incase of MOSFETs than JEFT.

• It is easier to fabricate MOSFET than JFET.

• MOSFET has no gate diode. This makes it possible to operate with +ve or –ve gate voltages

Advantages of MOSFET Over JFET

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

• Because of higher input resistance ,the enhancement type MOS devices have been used as

micro-resistor in integrated micro-circuits.

• For electrometer circuits where exceptionally low currents are to be measured MOSFETs are

most nearly ideal.

• MOSFET s are very small in size .which make them suitable for highly complex digital arrays.

•MOSFET is used for switching and amplifying electronics signals in the electronic devices.

•It is used as an inverter.

•It can be used in digital circuit.

•MOSFET can be used as a high frequency amplifier.

•It can be used as a passive element e.g. resistor, capacitor and inductor.

•It can be used in brushless DC motor drive.

•It can be used in electronic DC relay.

•It is used in switch mode power supply (SMPS).

Application of MOSFETs

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

COMPLEMENTARY MOSFET(CMOS)

CONSTUCTION OF COMPLEMENTARY MOSFET(CMOS)

•In this device two MOSFETs that are complementary to each other are used

•The drains of both the MOSFETs are combined and single terminal is taken.

•Similarly the gates of both the transistors are combined and a single gate terminal is taken out.

•Here the input is applied at the input terminal vi.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Working principle of CMOSFET

•When vi is high i.e., equal to –VDD then Q1 is turned ON and Q2 is turned OFF. The output

VO is zero.

•Similarly when the input voltage Vi is low i.e., equal to 0v, the Q2 turned ON and Q1 turned

OFF.

•So the output voltage Vo lies at –V dd level i.e., high.

•Thus the CMOSFET in this configuration works as an Inverter.

•The key advantage of using CMOS design is this extremely low power consumption

usually of the order of 50mv

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

•The UJT as the name implies, is characterized by a single pn

junction.

•It exhibits negative resistance characteristic that makes it useful in

oscillator circuits.

•With only one p-n junction, the device is really a form of diode

because two base terminal are taken from one section of the diode this

device is also called double-based diode

•The emitter is heavily doped the n-region, is lightly doped for this

reason the resistance between the base terminals is very high (5to10

kohms) when emitter lead is open.

Uni Junction Transistor (UJT)

Circuit Symbol Device

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Equivalent circuit of UJT

The PN junction behaves like a diode

The lightly doped silicon bar has high resistance can be

represented by two resistors connected in series RB1 and

RB2.

The Resistance offered by N-type bar between Base-1

and Emitter is referred as RB1.

The Resistance offered by N-type bar between Base-2

and Emitter is referred as RB2.

The Resistance of N-type bar is known as Base

spreading resistance RBB.

RBB = RB1 + RB2

Intrinsic stand of ratio

The intrinsic stand-off ratio is denoted by η.

η = RB1 /(RB1 + RB2)

The intrinsic stand-off ratio is the property of a UJT is

always less than unity.

Typical range of η is lies between 0.5 to 0.8.

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Applications of UJT

Phase control.

Relaxation oscillator.

Timing circuits.

Switching

Pulse generation.

Sine wave generator.

Voltage or current regulator supplies.

A stable triggering voltage i.e., a fixed fraction of applied inter base voltage VBB

A very low value of triggering current.

•A high pulse current capability.

• A negative resistance characteristic.

• Low cost.

Features of UJT

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani

Transistors, How do they work ?

https://www.youtube.com/watch?v=7ukDKVHnac4

Bipolar Junction Transistor (BJT)

https://www.youtube.com/watch?v=dTx9VKV0hjo

https://www.youtube.com/watch?v=d2lmY-AMs24

https://www.youtube.com/watch?v=PMOaS967Yus

https://www.youtube.com/watch?v=Q0nhtmYT6uA

Field Effect Transistor (FET)

https://www.youtube.com/watch?v=g30xTHas3aU

https://nptel.ac.in/courses/117101105/

MOSFET

Dr. Meenakshi Rana

Deptt- Physics

UOU, Haldwani