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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