Introduction to Junction Field Effect Transistor

This manuscript address the fundamental of Junction Field Effect Transistor (JFET)

1. Introduction to JFET
Dr. Varun Kumar
Dr. Varun Kumar(IIIT Surat) 1 / 16

2. Outlines
1
Introduction to FET
2
Classication of FET
3
Common Terminology in FET
4
FET Operation
5
Pinch-O voltage
6
JFET Volt-Ampere characteristics
Dr. Varun Kumar(IIIT Surat) 2 / 16

3. Introduction to FET
Basic dierence between BJT and FET
Sr no BJT FET
1 It is a bipolar device. It is a unipolar device.
2
Its operation depends
on hole and electron.
Its operation depends on
majority charge carrier.
3 It is a current controlled device.It is a voltage controlled device.
4 Input impedance is very low.Input impedance is very high.
5
It requires large physical
space for fabrication.
Small physical space
for fabrication.
Dr. Varun Kumar(IIIT Surat) 3 / 16

4. Classication of FET
1
JFET (Junction eld eect transistor)
(1)
(2)
2
MOSFET (Metal oxide semiconductor eld eect transistor)
(1)
n-channel
p-channel
(2)
n-channel
p-channel
NOTE)FET is a symmetrical device, irrespective of BJT.
Dr. Varun Kumar(IIIT Surat) 4 / 16

5. Common terminology
1Source:A terminal, where majority charge carriers enter the bar.
2Drain:A terminal, where majority charge carriers leave the bar.
3Gate:From gure, n-type semiconductor bar has been heavily doped
on two faces withp
+
material. This impurities terminals are called as
gate.
Dr. Varun Kumar(IIIT Surat) 5 / 16

6. Continued{
4Channel:There are two types (n-channel and p-channel) of JFET.
Above gure is a n-channel JFET.
5VGS!Gate to source voltage (VGS=VGG= Supply voltage)
6VDS!Drain to source voltage (VDS=VDD= Supply voltage)
7ID!Drain current,IG!Gate current
Dr. Varun Kumar(IIIT Surat) 6 / 16

7. FET Operation
)Two sides of the reverse biased p-n junction.
)There are space charged regions between p-n junction.
Q eld eect?
Ans ID) control is done by increasing the reverse bias voltage. In
other words, reverse bias creates any electric eld that aect the ow
of current or conduction of the majority charge carrier.
ID=f(VGS;VDS)
Dr. Varun Kumar(IIIT Surat) 7 / 16

8. Symbol of p and n channel JFET
)For n-channel JFET,IDandVDSare positive andVGSis negative.
)For p-channel JFET,IDandVDSare negative andVGSis positive.
Note: The sign of arrow, shows the direction of conventional current.
Dr. Varun Kumar(IIIT Surat) 8 / 16

9. FET Characteristics
)Ohmic region:The region (set ofIDandVDS), where circuit follow
the Ohm's law orVDS=RIDfor anyVGS
)Saturation region:The drain current becomes constant for
particularVGS, irrespective ofVDS.
Dr. Varun Kumar(IIIT Surat) 9 / 16

10. Continued{
)Breakdown region:Self destruction of device for particularVDSis
called as breakdown region.
)Pinch o region:The amount of negativeVGS(in case of n-channel
JFET) for which the conduction could not be happened due to
majority charge carrier.
Dr. Varun Kumar(IIIT Surat) 10 / 16

11. Pinch o voltageVp
)It is a gate to source voltage in reverse bias condition, so thatno
conduction orID0 could be possible.
)This analysis was rst made by Shockley.
)A n-type semiconductor is sandwiched between two layers of p-type
material forming two p-n junction.
)p-type region is doped withNAacceptor per cubic meter.
)n-type region is doped withNDacceptor per cubic meter.
)W=Wn+Wp!Total space charge width.
Wp!Space charge width due to hole.
Wn!Space charge width due to electron.
)IfNA>>NDthenWp<<Wn. HenceWn(x) =W(x) at a distance
xalong the channel
Dr. Varun Kumar(IIIT Surat) 11 / 16

12. Mathematical expression for pinch o voltage
W(x) =ab(x) =
n
2
qND
(V0V(x))
o
1=2
(1)
}=
}q=
}V0= x.
Dr. Varun Kumar(IIIT Surat) 12 / 16

13. WhenID= 0
}V(x)= Applied potential across space-charge region atxand is a
negative number for an applied reverse bias.
}ab(x)= PenetrationW(x)
pointxalong channel.
}Note:If drain current is zero, i.e.ID= 0,b(x) V(x)
independent ofx. Hence,b(x) =b
}If we substituteb(x) =b= 0 Von the assumption
V0<<jVj. We obtain the pinch-o voltageVp. Hence,
jVpj=
qND
2
a
2
(2)
Note:If we substituteVGS=V0V(x)
VGS=

1
b
a

2
Vp (3)
Dr. Varun Kumar(IIIT Surat) 13 / 16

14. Example
Q a= 310
4
cmand
ND= 10
15
electrons=cm
3
. Find
(a)
(b) VGS=
1
2
VpandID= 0
Hint:=r0,r!Silicon=12
Dr. Varun Kumar(IIIT Surat) 14 / 16

15. JFET VOLT-AMPERE Characteristics
)Now, smallVDSis applied between drain and source.
)SmallIDwill ow from drain to source.
)The eective channel cross section area = A = 2bw.
)2b= Channel width corresponding to zero drain current.
)w= Channel dimension perpendicular to the b direction.
)Using Ohm's law, no current ows in the depletion region. Hence the
drain current
ID=AqNDn= 2bwqNDn
VDS
L
(4)
L is the length of the channel.
Dr. Varun Kumar(IIIT Surat) 15 / 16

16. Continued{
Substitutingbfrom (3) in (4), we have for smallID,
ID=
2awqNDn
L
h
1

VGS
Vp

1=2i
VDS (5)
)Above expression shows the volt ampere characteristics for smallVDS.
)For smallVDS, JFET behaves like an ohmic resistance whose value is
determined byVGS.
)The ration
VDS
ID
at the origin is called the ON drain resistancerd;ON.
AtVGS= 0
rd;ON=
L
2awqNDn
(6)
Dr. Varun Kumar(IIIT Surat) 16 / 16