Download p-Channel JFET

Chapter-IV
Field Effect Transistor (FET)
By
Deepak Batham
Asst. Prof. (E.I.)
IET, DAVV
1
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)
2
ADVANTAGES OF FET
1.
Very high input impedance (109-1012 )
2.
Source and drain are interchangeable
3.
Low Voltage Low Current Operation is
possible (Low-power consumption)
Less Noisy
No minority carrier storage (Turn off is faster)
Very small in size, occupies very small space
in Ics.
4.
5.
6.
3
Current Controlled vs Voltage Controlled
Devices
4
Types of Field Effect Transistors
(The Classification)
»
FET
JFET
n-Channel JFET
p-Channel JFET
MOSFET (IGFET
(IGFET) (Insulated-Gate FET)
Enhancement
MOSFET
n-Channel
EMOSFET
p-Channel
EMOSFET
Depletion
MOSFET
n-Channel
DMOSFET
p-Channel
DMOSFET
5
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
6
SYMBOLS
Drain
Drain
Gate
Gate
Source
n-channel JFET
Source
p-channel JFET
7
N-Channel JFET Operation
The nonconductive depletion region becomes thicker with increased reverse bias.
(Note: The two gate regions of each FET are connected to each other.)
8
Biasing the JFET
Figure: n-Channel JFET and Biasing Circuit.
9
Operation of a JFET
Drain
-
N
Gate
P
P
+
+
+
N
Source
10
Transfer Characteristics
The input-output transfer characteristic of
forward as it is for the BJT.
the JFET is not as straight
In a JFET, the relationship (Shockley’s Equation) between VGS (input
voltage) and ID (output current) is used to define the transfer characteristics,
and a little more complicated (and not linear):
VGS 

ID = IDSS  1 
V
P


2
As a result, FET’s are often referred to a Square Law Devices
11
Transfer (Mutual) Characteristics of n-Channel JFET

V

GS
I
I
1

DS
DSS 
V

P

2




IDSS
VGS (off)=VP
Figure: Transfer (or Mutual) Characteristics of n-Channel JFET
12
Drain Characteristics of JFET
At the pinch-off point:
• any further increase in VGS does not produce any increase in ID.
VGS at pinch-off is denoted as Vp.
• ID is at saturation or maximum. It is referred to as IDSS.
13
ID  IDSS
As VGS becomes more negative:
• the JFET will pinch-off at a lower voltage (Vp).
• ID decreases (ID < IDSS) even though VDS is increased.
• Eventually ID will reach 0A. VGS at this point is called Vp or VGS(off).
• Also note that at high levels of VDS the JFET reaches a breakdown situation.
ID will increases uncontrollably if VDS > VDSmax.
14
Transfer (Transconductance) Curve
From this graph it is easy to determine the value of ID for a given value of VGS.
It is also possible to determine IDSS and VP by looking at the knee where VGS is 0
15
Output or Drain (VD-ID) Characteristics of n-JFET
Figure: Circuit for drain characteristics of the n-channel JFET and its Drain characteristics.
Non-saturation (Ohmic) Region:
I
The drain current is given by
DS
Saturation (or Pinchoff) Region:
I
DS

I
DSS
V2
P

V
V
GS
P



2

 
 

V

2I
DSS
V2
P

V 
V

DS
P 
 GS

V2


 V V
 DS
V
 GS
P  DS
2


V
 V
 V 
DS
P
 GS

V

GS
and I
I
1

DS
DSS 
V

P






2





Where, IDSS is the short circuit drain current, VP is the pinch off
voltage
16
Simple Operation and Break down of n-Channel JFET
Figure: n-Channel FET for vGS = 0.
17
N-Channel JFET Characteristics and Breakdown
Break Down Region
Figure: If vDG exceeds the breakdown voltage VB, drain current increases rapidly.
18
Case Construction and Terminal Identification
19
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.
20
P-Channel JFET Characteristics
As VGS increases more positively
• the depletion zone increases
• ID decreases (ID < IDSS)
• eventually ID = 0A
Also note that at high levels of VDS the JFET reaches a breakdown situation. ID increases
uncontrollably if VDS > VDSmax.
21
MOSFET
(Metal Oxide Semiconductor FET)
22
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 (Gate voltage is Negative)
• Enhancement Mode MOSFET (E-MOSFET)
• Operates in Enhancement mode like E-MOSFET when
VGS > 0 (Gate voltage is Positive)
• Operates in Enhancement mode IDSS = 0 until VGS > VT
(threshold voltage)
23
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 SiO2
The n-doped material lies on a p-doped substrate that may have an additional terminal
connection called SS
24
D-MOSFET Symbols
25
Basic Operation of n-channel D-MOSFET
Figure: n-Channel depletion-type MOSFET with VGS 0 V and an applied voltage VDD.
26
Basic Operation of n-channel D-MOSFET
Electrons repelled by negative
potential at gate
Figure: Reduction in free carriers in channel due to a negative potential at
the gate terminal.
27
Basic Operation
A D-MOSFET may be biased to operate in two modes:
the Depletion mode or the Enhancement mode
Figure: Drain and transfer characteristics for an n-channel depletion-type MOSFET
28
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
29
Enhancement Mode
MOSFET’s
30
MOSFET
Figure: n-Channel Enhancement MOSFET showing channel length L and channel width W.
31
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 SiO2
The n-doped material lies on a p-doped substrate that may have an additional terminal
connection called SS
32
E-MOSFET Symbols
33
Basic Operation of n-channel E-MOSFET Symbols
Figure: Channel formation in the nchannel enhancement type
MOSFET.
34
Change in channel and depletion region with increasing level
of VDS for a fixed value of VGS
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
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
Summary Table
JFET
D-MOSFET
E-MOSFET
38
39