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Junction Field Effect Transistor
(JFET)
Dr. M A Islam
Assistant Professor
EEE,IIUC
1. FET comes in several forms:
• JFET( Junction FET): The control (Gate) voltage
varies the
depletion with of a reversebiased p-n junction.
• MESFET( Metal semiconductor FET): Junction
is replaced by schottky barrier
• MISFET (Metal-insulator-semiconductor FET):
Metal gate electrode separated by insulator
• MOSFET (Metal oxide semiconductor FET ):
Uses Oxide layer as insulator
2. Transistor Operation
• General Operation:
- Amplification
- Switching
The transistor is three-terminal device with
the feature that the current through two
terminal and control by third terminal. This
control feature allows to amplify small ac
signal or to switch device on and off state.
2. Transistor Operation
2.1 JFET operation can be
compared to a water spigot:
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.
3. Device Structure
Gate
G
p+
Basic structure
G
Circuit symbol
for n-channel FET
Source
D
S
Drain
S
n-channel
n
S
D
G
D
p+
p+
p+
Cross section
G
Depletion
regions
Depletion
region
n-channel
Insulation
(SiO2)
n
p
n
n
S
n-channel
D
(b)
Channel
thickness
p+
(a)
Metal electrode
Electrons in the n-type channel drift from left
to right, opposite to current flow.
The end of the channel from which electrons
flow is called source (S), and end toward
which they flow is called the drain.
The p+ regions are called the gates. If the
channel is p-type, holes will flow from source
to drain, in the same direction as the current
flow, and the gate region will be n+.
4. Type of JFET
• The channel is made of either N-type or P-type
semiconductor material; an FET is specified as
either an N-channel or P-channel device
• In N-channel devices, electrons flow so the drain
potential must be higher than that of the Source
(VDS > O)
• In P-channel devices, the flow of holes requires
that VDS < 0
5. 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
A. 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:
1. The depletion region between p-gate and nchannel increases as electrons from n-channel
combine with holes from p-gate.
2. Increasing the depletion region, decreases the
size of the n-channel which increases the
resistance of the n-channel.
3. 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.
Pinch-off
If VGS = 0 and VDS is further
increased to a more positive
voltage, then the depletion zone
gets so large that it pinches off the
n-channel.
This suggests that the current in the
n-channel (ID) would drop to 0A,
but it does just the opposite: as VDS
increases, so does ID.
Saturation
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.
6. I-V characteristics
JFET: I-V characteristics
Metal-Semiconductor
Interfaces
• Metal-Semiconductor contact
• Schottky Barrier/Diode
• Ohmic Contacts
• MESFET
Two kinds of metal-semiconductor contacts:
• Rectifying Schottky diodes: metal on lightly
doped silicon
•Low-resistance ohmic contacts: metal on
heavily doped silicon
Device Building Blocks
Schottky (MS)
HBT
p-n junction
MOS
fBn Increases with Increasing Metal Work Function
Vacuum level,E0
y M : Work Function
of metal
cSi = 4.05 eV
qyM
qfBn
Ec
c S : Electron Affinity of Si
i
Ef
Theoretically,
Ev
fBn= yM – cSi
Schottky Barriers
Energy Band Diagram of Schottky Contact
Metal
Depletion
layer
Neutral region
qfBn
Ec
Ef
• Schottky barrier height, fB ,
N-Si
E
v
E
P-Si
qfB
p
c
Ef
E
v
is a function of the metal
material.
• fB is the most important
parameter. The sum of qfBn
and qfBp is equal to Eg .
Schottky barrier heights for electrons and holes
Metal
f Bn (V)
f Bp (V)
Work
Function
y m (V)
Mg
0.4
3.7
Ti
0.5
Cr
0.61
0.61
0.5
4.3
4.5
W
0.67
Mo
0.68
Pd
0.77
0.42
4.6
4.6
Au
0.8
Pt
0.9
0.3
5.1
5.1
fBn + fBp  Eg
fBn increases with increasing metal work function
5.7
Energy band diagram of an isolated metal
adjacent to an isolated n-type semiconductor
q(fs-c) = EC – EF = kTln(NC/ND) for n-type
= EG – kTln(Nv/NA) for p-type
Energy band diagram of a metal-n semiconductor
contact in thermal equilibrium.
qfBn = qfms + kTln(NC/ND)
Measured barrier height
f
ms
for metal-Si and metal-GaAs contacts
Theory still evolving (see review article by Tung)
Energy band diagrams of metal n-type and p-type semiconductors
under thermal equilibrium
Energy band diagrams of metal n-type and p-type semiconductors
under forward bias
Energy band diagrams of metal n-type and p-type semiconductors
under reverse bias
Charge distribution
Vbi = fms (Doping does not matter!)
fBn = fms + kTln(NC/ND)
electric-field
distribution
E(x) = qND(x-W)/Kse0
Em = qNDW/Kse0
W
(Vbi-V) = - ∫E(x)dx = qNDW2/Kse0
0
Depletion
Depletion width
W  2e s (Vbi  V ) / qND
Charge per unit area
q
Q  QN
2qe s ND (Vbi  V )
DW 
Capacitance
Per unit area:
Rearranging:
Or:
qe s ND
es
Q
C


V
2Vbi  V  W
1 2Vbi  V 

2
qe s ND
C


2 
1

ND 
qe s  d 1 2 / dV 


C
 
1/C2 versus applied voltage for W-Si and W-GaAs diodes
1/C2
vs V
•If straight line – constant doping profile –
slope = doping concentration
•If not straight line, can be used to find profile
•Intercept = Vbi can be used to find fBn
fBn  Vn  Vbi
kT  ND 
Vn 
ln

q  ni 
Current transport by the thermionic
emission process
Thermal equilibrium
forward bias
reverse bias
J = Jsm(V) – Jms(V)
Jms(V) = Jms(0) = Jsm(0)
Schottky Diodes
Forward
biased
V=0
I
Reverse
biased
Reverse bias
V
Forward bias
Schottky Diodes
I 0  AKT 2 e  qf B / kT
4qmn k 2
2
2
K

100
A/(cm

K
)
3
h
I  I S M  I M S  I 0 e qV / kT  I 0  I 0 (e qV / kT  1)
4.19 Applications of Schottly Diodes
I I
I  I 0 (e qV / kT  1)
Schottky
Schottky diode
I 0  AKT 2e qfB / kT
ffBB
PN junction
PN
diode
V
V
• I0 of a Schottky diode is 103 to 108 times larger than a PN
junction diode, depending on fB . A larger I0 means a smaller
forward drop V.
• A Schottky diode is the preferred rectifier in low voltage,
high current applications.
Switching Power Supply
PN Junction
rectifier
110V/220V
AC
utility
power
Transformer
100kHz
Hi-voltage
Hi-voltage
DC
MOSFET
AC
Schottky
rectifier
Lo-voltage
50A
1V
AC
inverter
V = 1V
feedback to modulate the pulse width to keep
out
DC
Applications of Schottky diodes
Question: What sets the lower limit in a Schottky diode’s
forward drop?
• Synchronous Rectifier: For an even lower forward drop,
replace the diode with a wide-W MOSFET which is not
bound by the tradeoff between diode V and leakage current.
• There is no minority carrier injection at the Schottky
junction. Therefore, Schottky diodes can operate at higher
frequencies than PN junction diodes.
Quantum Mechanical Tunneling
Tunneling probability:
P  exp ( 2T
8 2m
(VH  E ) )
2
h
Note the difference with p-n junctions!!
In both cases, we’re modulating the population
of backflowing electrons, hence the Shockley
form, but…
V>0
V<0
V>0
V<0
• Barrier is not pinned
• Barrier from metal side is pinned
• Els with zero kinetic energy can slide
down negative barrier to initiate current
• Els from metal must jump over barrier
• Current is limited by how fast minority
carriers can be removed (diffusion rate)
• Current is limited by speed of jumping
electrons (that the ones jumping from
the right cancel at equilibrium)
• Both el and hole currents important
(charges X-over and become min. carriers)
• Unipolar majority carrier device, since
valence band is entirely inside metal band
Let’s roll up our sleeves and do the algebra !!
dkxdkydkzvxe-(Ek-EF)/kT
Jsm = 2qf(Ek-EF)vx = 2q

vx > vmin,vy,vz
(2)3/W
Vbi - V
Ek-EF = (Ek-EC) + (EC -EF)
EC - EF = q(fBn-Vbi)
Ek - EC = m(vx2 + vy2 + vz2 )/2
m*vmin2/2 = q(Vbi – V)
kx,y,z = m*vx,y,z/ħ
V>0
This means…
Jsm
∞
∞
∞
2/2kT
2
-m*v
-m*v
/2kT

dvze z
dvxvxe-m*vx2/2kT
= q(m*)3W/43ħ3dvye y


-∞
-∞
v
min
x e-q(fBn-Vbi)/kT
(2kT/m*)
(2kT/m*)
(kT/m*)e-m*vmin2/2kT
= (kT/m*)e-q(Vbi-V)kT
∞

-x2/2s2 = s2
dxe

-∞
∞

-x2/2s2 = s2e-A2/2s2
dx
xe

A
= qm*k2T2/22ħ3e-q(fBn-V)kT
= A*T2e-q(fBn-V)kT
A* = 4m*qk2/h3
= 120 A/cm2/K2
J = A*T2e-qf
BN
(eqV/kT-1)
/kT
In regular pn junctions, charge needs to move through
drift-diffusion, and get whisked away by RG processes
MS junctions are majority carrier devices, and RG is not
as critical. Charges that go over a barrier already have
high velocity, and these continue with those velocities to
give the current
Forward current density vs applied voltage of W-Si and W-GaAs diodes
Thermionic Emission over the barrier – low doping
Tunneling through the barrier – high doping
Schottky barrier becomes Ohmic !!
Fermi Level Pinning
Vacuum level,E0
cSi = 4.05 eV
qyM
qfBn
+ 
Ec
Ef
Ev
• A high density of
energy states in the
bandgap at the metalsemiconductor interface
pins Ef to a narrow
range and fBn is
typically 0.4 to 0.9 V
• Question: What is the
typical range of fBp?
Ohmic Contacts
Ohmic Contacts
Wdep
2e sf Bn

qN d
Silicide
N+ Si
fBn – V
fBn
-
-
Ec , Ef
V
Efm
Ec , Ef
Tunneling
probability:
Pe
 HfBn
Nd
T  Wdep / 2 
4
H 
h
J S M 
x
Ev
Ev
x
e sfBn / 2 qN d
e s mn / q
1
qN d vthx P  qN d
2
kT / 2mn e
 H ( fBn V ) /
Nd
Ohmic Contacts
1
Hf Bn / N d
2e
 dJ S  M 
Hf Bn /
Rc  
e
 
qvthx H N d
 dV 
Nd
Ω  cm 2
MESFET
MESFET stands for metal–semiconductor field effect transistor. It is
quite similar to a JFET in construction and terminology. The difference is
that instead of using a p-n junction for a gate, a Schottky (metalsemiconductor) junction is used.
MESFETs are usually constructed in compound semiconductor
technologies lacking high quality surface passivation such as GaAs, InP, or
SiC, and are faster but more expensive than silicon-based JFETs or
MOSFETs.
Production MESFETs are operated up to approximately 45 GHz, and are
commonly used for microwave frequency communications and radar.
The first MESFETs were developed in 1966, and a year later their
extremely high frequency RF microwave performance was demonstrated.
From a digital circuit design perspective, it is increasingly difficult to use
MESFETs as the basis for digital integrated circuits as the scale of
integration goes up, compared to CMOS silicon based fabrication.
MISFET
o A MISFET is a metal–insulation–semiconductor field-effect transistor.
o MISFET is a more general term than MOSFET and a synonym to insulated
gate field-effect transistor (IGFET). All MOSFETs are MISFETs, but not all
MISFETs are MOSFETs.
o The insulator in a MISFET is a dielectric which can be silicon oxide (in a
MOSFET), but other materials can also be employed.
o The generic term for the dielectric is gate dielectric, since the dielectric lies
directly below the gate electrode and above the channel of the MISFET.
o The term metal is used for the gate material, even though it is usually
highly doped polysilicon or some other nonmetal.