1. Field Effect Transistors
Unit 2
Field Effect Transistors

2. Objectives: BJT v/s FET JFET MOSFET JFET v/s MOSFET
Handling and biasing MOSFETs
FET applications
CMOS
IGBT

3. 5.1BJT v/s FET: Current controlled – voltage controlled
Bipolar devices – unipolar devices
Zi < 1 MΩ - Zi very high (100s MΩ)
Less temp. stable – more temp. stable
Small size – smaller size
More gain – less gain

4. 5.2 JFET (junction FET): Intro.: JFET simpler device among family
3 – terminal device; 1 controls current through 2
Types –
p - channel
n - channel

5. Construction

7. Characteristic:

8. Characteristic:

13. Transfer characteristics:

14. The drain resistance (rd) in saturation region is given by:
(1-VGS / VP)2
Where
rd is drain resistance at VGS = 0
& rD is drain resistance at particular VGS

15. Relationship between output drain current & input gate-to-source voltage:
VGS
ID = IDSS VP

16. Effect of temperature:
Carrier mobility ↓
ID ↓
Thus in above steps:
T ↑ ID ↓
Negative temperature coefficient
Better thermal stability ?
JFET temperature ↑
Depletion region width ↓ &
Increase channel width ↑
ID ↑
Thus in above steps:
T ↑  ID ↑
Positive temperature coefficient

17. 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)

18. DE-MOSFET: There is no p-type gate
There is no direct electrical connection between gate & channel
Capacitive effect exists between gate & channel

19. Symbols:

20. Circuit connection:
Do not copy

21. Characteristics: Enhancement mode / region,
+ve charge carriers from p-substrate contribute
Depletion mode / region,
Normal operation as MOSFET

22. Transfer characteristics:

23. Notice that the channel is not fabricated,
E-MOSFET: ha
Notice that the channel is not fabricated,
It will be generated

24. Symbols:

25. Working: Capacitive effect induces electrons from p-substrate
Creating a n-channel

26. Characteristics:

27. E-MOSFET Transfer characteristics:

28. Differences between JFET & MOSFET:
Operational modes:
JFET -depletion mode
DE-MOSFET -depletion / enhancement modes
E-MOSFET -enhancement mode
MOSFET – input resistance high compared to JFET
JFETs have higher drain resistance rd than MOSFETs
Leakage current in MOSFET is less compared to JFET
MOSFETs are easier to construct & widely used than JFETs

29. 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:
Person handling ground himself properly
Connect zener diodes, back-to-back as shown

30. Biasing MOSFETs: Biasing DE-MOSFETs: Same as JFETs
Example: Fixed bias (page 183)

31. Biasing E-MOSFETs:
Feedback biasing configuration:

32. KVL at input
VDD –IG RG - ID RD - VGS = 0
VGS = VDD – ID RD (1)
KVL at output
VDS = VDD – ID RD (2)

33. Voltage divider bias: Assignment #1:
Numerical examples 5.9 & 5.10 on pages 196 & 197

34. FET applications: Amplifier Analog switch Multiplexer Current limiter
Voltage variable resistors
Oscillators

35. Analog switch:

36. Multiplexer:

37. 5.10 Testing FETs: Damaged: Between any two: 0  short circuit
∞  open circuit

38. 5.13 CMOS devices: CMOS Inverter
Operation:
Vin=0  Q2 ON & Q1 OFF  Vout=1
Vin=1  Q1 ON & Q2 OFF  Vout=0

39. 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

40. Questions:
Explain JFET construction, biasing and characteristics, transfer characteristics
How better stability is achieved in JFET?
Explain DE-MOSFET (construction diagram, symbol, output & transfer characteristics, working)
Explain E-MOSFET ( --”--)
Difference between JFET & MOSFET
Explain biasing methods of De- & E-MOSFETs

41. Example 5.8, page 193
Example 5.9, page 196
Explain Applications of FET
Explain CMOS inverter
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