Unit 2

Field Effect Transistors

Objectives:

• BJT v/s FET• JFET• MOSFET• JFET v/s MOSFET• Handling and biasing MOSFETs• FET applications• CMOS• IGBT

5.1BJT v/s FET:

1. Current controlled – voltage controlled

2. Bipolar devices – unipolar devices

3. Zi < 1 MΩ - Zi very high (100s MΩ)

4. Less temp. stable – more temp. stable

5. Small size – smaller size

6. More gain – less gain

5.2 JFET (junction FET):

Intro.:

• JFET simpler device among family

• 3 – terminal device; 1 controls current through 2

• Types – – p - channel– n - channel

Construction

Characteristic:

Characteristic:

Transfer characteristics:

• The drain resistance (rd) in saturation region is given by:

rD

rd = --------------------

(1-VGS / VP)2

Where

rd is drain resistance at VGS = 0 & rD is drain resistance at particular VGS

• Relationship between output drain current & input gate-to-source voltage:

VGS 2

ID = IDSS 1- ------------ VP

Effect of temperature:• Better thermal stability ?• JFET temperature ↑• Depletion region width ↓ &• Increase channel width ↑• ID ↑

Thus in above steps:• T ↑ ID ↑• Positive temperature

coefficient

• Carrier mobility ↓

• ID ↓

Thus in above steps:• T ↑ ID ↓• Negative temperature

coefficient

5.3 Metal oxide FET (MOSFET):

What exactly metal oxide?

-MOSFET is insulated from the semiconductor channel by very thin oxide (SiO2) layer

• These are also known as insulated gate (IG) FET

• MOSFET types / modes:– Depletion MOSFET (De-MOSFET)– Enhancement MOSFET (E-MOSFET)

DE-MOSFET:• There is no p-

type gate• There is no

direct electrical connection between gate & channel

• Capacitive effect exists between gate & channel

Symbols:

Circuit connection:

Do not copy

Characteristics:Enhancement mode / region,

+ve charge carriers from p-substrate contribute

Depletion mode / region,

Normal operation as MOSFET

Transfer characteristics:

E-MOSFET:• ha

Notice that the channel is not

fabricated,It will be generated

Symbols:

Working:• Capacitive effect induces electrons from p-

substrate

• Creating a n-channel

Characteristics:

E-MOSFET Transfer characteristics:

Differences between JFET & MOSFET:1. Operational modes:

1. JFET -depletion mode

2. DE-MOSFET -depletion / enhancement modes

3. E-MOSFET -enhancement mode

2. MOSFET – input resistance high compared to JFET

3. JFETs have higher drain resistance rd than MOSFETs

4. Leakage current in MOSFET is less compared to JFET

5. MOSFETs are easier to construct & widely used than JFETs

Handling MOSFETs:• SiO2 layer is thin & prone to damage

• Due to static charges, potential difference between SiO2 can result in breakdown & establish conduction through it

Precautions:

1. Person handling ground himself properly

2. Connect zener diodes, back-to-back as shown

Biasing MOSFETs:Biasing DE-MOSFETs:

• Same as JFETs

• Example: Fixed bias (page 183)

Biasing E-MOSFETs:Feedback biasing configuration:

KVL at input

VDD –IG RG - ID RD - VGS = 0

VGS = VDD – ID RD -----(1)

KVL at output

VDS = VDD – ID RD -----(2)

Voltage divider bias:

Assignment #1:

Numerical examples 5.9 & 5.10 on pages 196 & 197

FET applications:

1. Amplifier

2. Analog switch

3. Multiplexer

4. Current limiter

5. Voltage variable resistors

6. Oscillators

Analog switch:

Multiplexer:

5.13 CMOS devices: CMOS Inverter

Operation:

Vin=0 Q2 ON & Q1 OFF Vout=1

Vin=1 Q1 ON & Q2 OFF Vout=0

Insulated Gate Bipolar (IGBT):

•Have positive attributes of BJT & MOSFET•Faster switching like MOSFET•Lower ON – state voltage like BJT

Application / Usage area:

•SMPS•Motor control – as high voltage handling capacity•Induction heating control

Questions:1. Explain JFET construction, biasing and

characteristics, transfer characteristics2. How better stability is achieved in JFET?3. Explain DE-MOSFET (construction

diagram, symbol, output & transfer characteristics, working)

4. Explain E-MOSFET ( --”--)5. Difference between JFET & MOSFET6. Explain biasing methods of De- & E-

MOSFETs

7. Example 5.8, page 193

8. Example 5.9, page 196

9. Explain Applications of FET

10. Explain CMOS inverter

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