What is field effect transistor, hot it works , N type and P type field effect transistor

Field effect transistor

1. CHAPTER-4
FIELD EFFECT TRANSISTOR
10 MARKS

2. 2
What is FET?
FET is uni-polar device i.e.
operation depends on only one type
of charge carriers (h or e) . It is a
Voltage controlled Device (gate
voltage controls drain current)

3. 3
1.Very high input impedance (10
9
-10
12
)
2.Source and drain are interchangeable
3.Low Voltage Low Current Operation is
possible (Low-power consumption)
4.Less Noisy
5.No minority carrier storage (Turn off is faster)
6.Very small in size, occupies very small space
in ICs
ADVANTAGES OF FET

4. 4
Current Controlled vs Voltage Controlled
Devices

5. 5
Types of Field Effect Transistors
(The Classification)
»JFET
MOSFET (IGFET)
n-Channel JFET
p-Channel JFET
n-Channel
EMOSFET
p-Channel
EMOSFET
Enhancement
MOSFET
Depletion
MOSFET
n-Channel
DMOSFET
p-Channel
DMOSFET
FETFETFET
»JFET
MOSFET (IGFET)
FET
»JFET
MOSFET (IGFET)
FET

6. 6
JFET Construction
There are two types of JFET’s: n-channel and p-channel.
The n-channel is more widely used.
There are three terminals: Drain (D) and Source (S) are connected to n-channel
Gate (G) is connected to the p-type material

7. 7
The nonconductive depletion region becomes thicker with increased reverse bias.
(Note: The two gate regions of each FET are connected to each other.)
N-Channel JFET Operation

8. 8
Gate
Drain
Source
SYMBOLS
n-channel JFET
Gate
Drain
Source
p-channel JFET

9. 9
CHARATERISTICS
At the pinch-off point:
• any further increase in V
GS does not produce any increase in I
D.
V
GS at pinch-off is denoted as Vp.
• ID is at saturation or maximum. It is referred to as I
DSS.

10. 10
ID  IDSS
As V
GS
becomes more negative:
• the JFET will pinch-off at a lower voltage (Vp).
• I
D
decreases (I
D
< I
DSS
) even though V
DS
is increased.
• Eventually I
D
will reach 0A. V
GS
at this point is called Vp or V
GS(off)
.
• Also note that at high levels of V
DS
the JFET reaches a breakdown situation.
ID will increases uncontrollably if V
DS > V
DSmax.

11. 11
Transfer Characteristics
The input-output transfer characteristic of the JFET is not as straight
forward as it is for the BJT
In a JFET, the relationship (Shockley’s Equation) between V
GS
(input
voltage) and I
D
(output current) is used to define the transfer characteristics,
and a little more complicated (and not linear):
As a result, FET’s are often referred to a square law devices
 
 
 
2
GS
D DSS
P
V
I = I 1 -
V

12. 12
Transfer (Transconductance) Curve
From this graph it is easy to determine the value of I
D
for a given value of V
GS
It is also possible to determine IDSS and VP by looking at the knee where VGS is 0

13. 13
Case Construction and Terminal Identification

14. 14
p-Channel JFET
p-Channel JFET operates in a similar manner as the n-channel JFET except the voltage
polarities and current directions are reversed

15. 15
P-Channel JFET Characteristics
As VGS increases more positively
•the depletion zone increases
•I
D
decreases (I
D
< I
DSS
)
•eventually I
D = 0A
Also note that at high levels of VDS the JFET reaches a breakdown situation. ID increases
uncontrollably if V
DS
> V
DSmax
.

16. BJT n JFET comparision
16
Characteristi
c
BJT FET
Carriers Electrons, Holes Only one type (Electrons
or Holes)
Control Current control deviceVoltage control device
Terminal 3Terminal
(Emitter,Baise,Collector)
4Terminal
(Source,Gate,Drain, body
(substrate))
Used Amplifiers, Regulators of
currents
Digital Electronis,IC ,
Amplifiers
Other Name --------- Unipolar transistor
junctions Two 1 junctions

17. BJT n JFET comparision
17
CharacteristicBJT FET
Symbol
impedance dVi/di large (in the range of
10
10
-10
15
Ω)
Secondary
break down
Yes No
operating
frequency
Low High
switching
losses
High Low

18. 18

25. Field-effect transistors (FETs) have many applications.
Amplifiers:
 FETs are used in audio, radio frequency, and instrumentation
amplifiers because of their low noise and high input impedance.
 They are
also used as buffer amplifiers to prevent signal distortion and improve
output quality.
 

Input amplifiers:
 FETs are used as input amplifiers in devices like
oscilloscopes and voltmeters because of their high input impedance.
 

Mixer circuits:
 FETs are used in mixer circuits to reduce low
intermodulation distortions.
 

Radio frequency amplifiers:
 FETs are used to construct radio frequency
amplifiers for FM appliances.
 

TV and FM receivers:
 FETs are used for the mixer operation of TV and
FM receivers.
 

LSI (large-scale integration) and computer memory modules:
 FETs
are used in these modules because of their small size.
 

Battery-powered devices:
 FETs are power-efficient, making them a good
choice for battery-powered devices.
 
 

26. 26
MOSFET
(Metal Oxide Semiconductor FET)

27. 27
MOSFET
There are two types of MOSFET’s:
•Depletion mode MOSFET (D-MOSFET)
•Operates in Depletion mode the same way as a JFET
when VGS  0
•Operates in Enhancement mode like E-MOSFET when
VGS > 0
•Enhancement Mode MOSFET (E-MOSFET)
•Operates in Enhancement mode
•IDSS = 0 until VGS > VT (threshold voltage)

28. 28
Depletion Mode MOSFET Construction
The Drain (D) and Source (S) leads connect to the to n-doped regions
These N-doped regions are connected via an n-channel
This n-channel is connected to the Gate (G) via a thin insulating layer of SiO
2
The n-doped material lies on a p-doped substrate that may have an additional terminal
connection called SS

29. 29
D-MOSFET Symbols

30. 30
Basic Operation
A D-MOSFET may be biased to operate in two modes:
the Depletion mode or the Enhancement mode

31. 31
p-Channel Depletion Mode MOSFET
The p-channel Depletion mode MOSFET is similar to the n-channel except that the
voltage polarities and current directions are reversed

32. 32
Enhancement Mode
MOSFET’s

33. 33
Enhancement Mode MOSFET Construction
The Drain (D) and Source (S) connect to the to n-doped regions
These n-doped regions are not connected via an n-channel without an external voltage
The Gate (G) connects to the p-doped substrate via a thin insulating layer of SiO
2
The n-doped material lies on a p-doped substrate that may have an additional terminal
connection called SS

34. 34
E-MOSFET Symbols

35. 35
Basic Operation
The Enhancement mode MOSFET only operates in the enhancement mode.
VGS is always positive
IDSS = 0 when VGS < VT
As VGS increases above VT, ID increases
If VGS is kept constant and VDS is increased, then ID saturates (IDSS)
The saturation level, VDSsat is reached.

36. 36
p-Channel Enhancement Mode MOSFETs
The p-channel Enhancement mode MOSFET is similar to the n-channel except that the
voltage polarities and current directions are reversed.

37. 37
Summary Table
JFET D-MOSFET E-MOSFET