FET ( Field Effect Transistor)
Few important advantages of FET over conventional Transistors
1.
2.
Unipolar device i. e. operation depends on only one type of
charge carriers (h or e)
Voltage controlled Device (gate voltage controls drain
current)
3.
Very high input impedance (109-1012 )
4.
Source and drain are interchangeable in most Low-frequency
applications
5.
Low Voltage Low Current Operation is possible (Low-power
consumption)
Less Noisy as Compared to BJT
No minority carrier storage (Turn off is faster)
Self limiting device
Very small in size, occupies very small space in ICs
Low voltage low current operation is possible in MOSFETS
Zero temperature drift of output is possible
FET’s are also generally more static sensitive than BJT’s.
6.
7.
8.
9.
10.
11.
12.
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)
Figure: 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
Figure: n-Channel JFET and Biasing Circuit.
Construction and characteristics of
JFET
• JFET operation can be compared to a water faucet:
• The source of water pressure – accumulated electrons at the
negative pole of the applied voltage from Drain to Source
• The drain of water – electron deficiency (or holes) at the
positive pole of the applied voltage from Drain to Source.
• The control of flow of water – Gate voltage that controls the
width of the n-channel, which in turn controls the flow of
electrons in the n-channel from source to drain.
Construction and characteristics of
JFET
N-Channel JFET Circuit Layout
JFET Operating Characteristics
There are three basic operating conditions for a JFET:
A. VGS = 0, VDS increasing to some positive
value
B. VGS < 0, VDS at some positive value
C. Voltage-Controlled Resistor
VGS = 0, VDS increasing to some
positive value
Three things happen when VGS = 0
and VDS is increased from 0 to a
more positive voltage:
• the depletion region between p-gate
and
n-channel
increases
as
electrons from n-channel combine
with holes from p-gate.
• increasing the depletion region,
decreases the size of the n-channel
which increases the resistance of
the n-channel.
• But even though the n-channel
resistance is increasing, the current
(ID) from Source to Drain through
the n-channel is increasing. This is
because VDS is increasing.
VGS = 0, VDS increasing to some
positive value
• The flow of charge is relatively uninhibited and limited solely by
the resistance of the n-channel between drain and source.
• The depletion region is wider near the top of both p-type
materials.
• ID will establish the voltage level through the channel.
• The result: upper region of the p-type
material will be reversed biased by
about 1.5V with the lower region only
reversed biased by 0.5V (greater
applied reverse bias, the wider
depletion region).
VGS = 0, VDS increasing to some
positive value
• IG=0A  p-n junction is reverse-biased for the length of the
channel results in a gate current of zero amperes.
• As the VDS is increased from 0 to a few volts, the current will
increase as determined by Ohm’s Law.
• VDS increase and approaches a level referred to as Vp, the
depletion region will widen, causing reduction in the channel
width. (p large, n small).
• Reduced part of conduction causes the resistance to increase.
• If VDS is increased to a level where it appears that the 2
depletion regions would touch (pinch-off)
VGS = 0, VDS increasing to some
positive value
• Vp = pinch off voltage.
• ID maintain the saturation level
defined as IDSS
• Once the VDS > VP, the JFET has the
characteristics of a current source.
• As shown in figure, the current is
fixed at ID = IDSS, the voltage VDS (for
level >Vp) is determined by the
applied load.
• IDSS is derived from the fact that it is
the drain-to-source current with short
circuit connection from gate to source.
• IDSS is the max drain current for a
JFET and is defined by the conditions
VGS=0V and VDS > | Vp|.
VGS = 0, VDS increasing to some
positive value
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.
• The ohmic value of the
channel is at maximum.
Typical JFET operation
JFET modeling when ID=IDSS, VGS=0, VDS>VP
VGS < 0, VDS at some positive value
• VGS is the controlling voltage of
the JFET.
• For n-channel devices, the
controlling voltage VGS is made
more and more negative from
its VGS = 0V level.
• The effect of the applied
negative VGS is to establish
depletion regions similar to
those obtained with VGS=0V
but a lower level of VDS  to
reach the saturation level at a
lower level of VDS.
VGS < 0, VDS at some positive value
• When VGS = -Vp will be sufficiently negative to establish
saturation level that is essentially 0mA, the device has been
‘turn off’.
• The level of the VGS that results in ID = 0 mA is defined by
VGS = Vp, with Vp being a negative voltage for n-channel
devices and a positive voltage or p-channel JFETs.
• In this region, JFET can actually be employed as a variable
resistor whose resistance is controlled by the applied gate
to source voltage.
• A VGS becomes more and more negative; the slope of each
curve becomes more and more horizontal.
VGS < 0, VDS at some positive value
• The region to the right
of the pinch-off locus of
the figure is the region
typically employed in
linear amplifiers
(amplifiers with min
distortion of the applied
signal) and is commonly
referred to as the
constant-current,
saturation, or linear
amplification region.
Characteristic curves for Nchannel JFET
Voltage-Controlled Resistor
• The region to the left of the
pinch-off point is called the
ohmic region.
• The JFET can be used as a
variable resistor, where VGS
controls the drain-source
resistance (rd). As VGS
becomes more negative, the
resistance (rd) increases.
rd 
ro
(1  V GS
VP
)
2
And as summary in practical…
Operation of JFET at Various Gate Bias Potentials
Figure: The nonconductive depletion region becomes broader with increased reverse bias.
(Note: The two gate regions of each FET are connected to each other.)
Operation of a JFET
Drain
-
N
Gate
+
P
P
N
Source
+
-
+
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:
The drain current is given by
V
DS
2I

I
DS
DSS
2
V
P
Saturation (or Pinchoff) Region:
I

I
DS
DSS
2
V
P
2 

V

V  

GS
P 





V


P 
2 

V
DS 

 V
 V V


  GS
P  DS
2 




V
and
 
V
 GS
  V
 V 
DS
P 
 GS

V

I
 I
1  GS
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
Figure: n-Channel FET for vGS = 0.
N-Channel JFET Characteristics and Breakdown
Break Down Region
Figure: If vDG exceeds the breakdown voltage VB, drain current increases rapidly.
VD-ID Characteristics of EMOS FET
Locus of pts where V DS  V GS  V P 
Saturation or Pinch
off Reg.
Figure: Typical drain characteristics of an n-channel JFET.
Transfer (Mutual) Characteristics of n-Channel JFET

V

I
 I
1  GS
DS
DSS 
V

P





2
IDSS
VGS (off)=VP
Figure: Transfer (or Mutual) Characteristics of n-Channel JFET
JFET Transfer Curve
This graph shows the value of ID for a given
value of VGS
Biasing Circuits used for JFET
• Fixed bias circuit
• Self bias circuit
• Potential Divider bias circuit
JFET (n-channel) Biasing Circuits
For Fixed Bias Circuit
Applying KVL to gate circuit we get
V GG  I G R G  V GS  V GS  Fixed , I G  0

V

I
 I
1  GS
DS
DSS 
V
P





2
and
I DS  I DSS

V
 1  GS

VP





2
and V DS  V DD  I DS R D
Where, Vp=VGS-off & IDSS is Short ckt. IDS
For Self Bias Circuit
V GS  I DS R S  0
 I DS  
V GS
RS
JFET Biasing Circuits Count…
or Fixed Bias Ckt.
JFET Self (or Source) Bias Circuit
and
I
DS

V

GS
 I
1 
DSS 
V

P



V


I
1  GS

DSS 
V

P 

2
V
 
GS
R
S





2
2

V

V
V

 GS  
GS
GS
I
1

2


0



DSS 
V
R
 V


P
S
 P  


This quadratic equation can be solved for VGS & IDS
The Potential (Voltage) Divider Bias

V

 I
1  GS
DSS 
V

P

Solving this quadratic





2
V
V

G
GS
 0
R
S
equation
gives V
GS
and I
DS