Basic Electronics
Field Effect Transistor
C.H.Manathunga
Department of Physics ,University of Sri Jayewardenepura
J-FET (Junction Field Effect Transistor)
*The field-effect transistor (FET) controls the current
between two points but does so differently than the bipolar
transistor.
*The FET operates by the effects of an electric field on the
flow of electrons through a single type of semiconductor
material.
*This is why the FET is sometimes called a unipolar
transistor.
Types of Field Effect Transistors
(The Classification)
»
FET
JFET
MOSFET (IGFET)
Enhancement
MOSFET
n-Channel
EMOSFET
p-Channel
EMOSFET
n-Channel JFET
p-Channel JFET
Depletion
MOSFET
n-Channel
DMOSFET
p-Channel
DMOSFET
The Junction Field Effect Transistor (JFET))
n-Channel JFET.
SYMBOLS
Gate
Gate
Gate
Source
n-channel JFET
Drain
Drain
Drain
Source
n-channel JFET
Offset-gate symbol
Source
p-channel JFET
Biasing the JFET
n-Channel JFET and Biasing Circuit.
Operation of JFET at Various Gate Bias Potentials
The nonconductive depletion region becomes broader with increased reverse bias.
(Note: The two gate regions of each FET are connected to each other.)
Output or Drain (VD-ID) Characteristics of n-JFET
Circuit for drain characteristics of the n-channel JFET and its Drain characteristics.

V 
V

DS
P
 GS
Non-saturation (Ohmic) Region:
The drain current is given by
V
I
DS
I
DS

DSS
V2
P

V
 V
GS
P



DSS
V2
P

V2


 V V
 DS
V
 GS
P  DS
2


 V
 V 
DS
P
 GS
Saturation (or Pinchoff) Region:
I

2I





V
2

 
 


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
Simple Operation and Break down of n-Channel JFET
n-Channel FET for vGS = 0.
N-Channel JFET Characteristics and Breakdown
Break Down Region
If vDG exceeds the breakdown voltage VB, drain current increases rapidly.
VD-ID Characteristics of EMOS FET
Locus of pts where
VDS  VGS  VP 
Saturation or Pinch
off Reg.
Figure: Typical drain characteristics of an n-channel JFET.
Transfer (Mutual) Characteristics of n-Channel JFET

V

GS
I
I
1

DS
DSS 
V

P

2




IDSS
VGS (off)=VP
Transfer (or Mutual) Characteristics of n-Channel
JFET
JFET Transfer Curve
This graph shows the value of ID for a given
value of VGS