8. Schottky contacts / JFETslamp.tu-graz.ac.at/~hadley/psd/lectures18/nov21.pdf · metal -...
Transcript of 8. Schottky contacts / JFETslamp.tu-graz.ac.at/~hadley/psd/lectures18/nov21.pdf · metal -...
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Institute of Solid State PhysicsTechnische Universität Graz
8. Schottky contacts/ JFETs
Nov. 21, 2018
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Institute of Solid State Physics
metal - semiconductor contacts Technische Universität Graz
Photoelectric effect Schottky barriersSchottky diodes Ohmic contacts Thermionic emission Tunnel contacts
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Photoelectric effect
hf0 = e at threshold
workfunction f
curr
ent
threshold frequency f0
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Singh
There is a dipole field at the surface of a metal. This electric field must be overcome for an electron to escape.
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work function - electron affinity
If s < m, the semiconductor bands bend down.
If s > m, the semiconductor bands bend up.
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Singh
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p-type
Schottky contact / ohmic contact
specific contact resistance:
Ohmic contact: linear resistance
Schottky contact
12 -cmc
JRV
metal
metal
EF,s
EF,s
EF,m
EF,m
Walter Schottky
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n-type
Schottky contact / ohmic contact
specific contact resistance:
Ohmic contact: linear resistance
Schottky contact
12 -cmc
JRV
metal
metal
EF,s
EF,s
EF,m
EF,m
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Interface states
metal
b
Interface states
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http://www.springermaterials.com/navigation/#n_240905_Silicon+%2528Si%2529
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Schottky barrier
2 bin
D
V VW x
eN
0
Dn
r
eNE x x
2
2
Dn
eN xV xx
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CV measurements
-2 F m
2A
p bi
e NCx V V
2 bip
A
V Vx
eN
2
21 biA
V VC e N
GaAs has larger Eg and Vbi
V
1/C
2
( )ln vbi b BA
N TeV k TN
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Thermionic emission
1901 Richardson
Current from a heated wire is:
2 expRB
eJ A Tk T
Some electrons have a thermal energy that exceeds the work function and escape from the wire.
Owen Willans Richardson
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Vacuum diodes
diode
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Thermionic emission
EF
Fermi function
( ) exp exp exp expF FB B B B
E E E E Ef Ek T k T k T k T
The density of electrons with enough energy to go over the barriers expB
Ek T
bie V V
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Thermionic emission
expthB
eVnk T
expsm thB
eVI nk T
( 0)ms smI I V
e 1BeVk T
sm ms msI I I I
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Schottky barrier
Ism ~ 0Ims constant
Ism > Ims
Reverse bias
Ism ~ exp(eV/kBT)Ims constant
e(Vbi - V)
eV
b
Forward bias
b
e(Vbi - V)
eV
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Thermionic emission
e 1BeVk T
sm ms sI I I I
Nonideality factor = 1
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Thermionic emission
* 2 exp bs RB
eI AA Tk T
A = AreaAR* = Richardson constant
n-Si AR* = 110 A K-2cm-2
Thermionic emission dominates over diffusion current in a Schottky diode.
p-Si AR* = 32 A K-2cm-2n-GaAs AR* = 8 A K-2cm-2p-GaAs AR* = 74 A K-2cm-2
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Schottky diodes
Majority carrier current dominates.
nonideality factor = 1.
Fast response, no recombination of electron-hole pairs required.
Used as rf mixers.
Low turn on voltage - high reverse bias current
e 1BeVk T
sI I
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Tunnel contacts
For high doping, the Schottky barrier is so thin that electrons can tunnel through it.
metal p+ p
metal n+ n
Tunnel contacts have a linear resistance.
Degenerate doping at a tunnel contact
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Contacts
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Transport mechanisms
DriftDiffusionThermionic emissionTunneling
All mechanisms are always present.
One or two transport mechanisms can dominate depending on the device and the bias conditions.
In a forward biased pn-junction, diffusion dominates.
In a tunnel contact, tunneling dominates.
In a Schottky diode, thermionic emission dominates.
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Institute of Solid State Physics
JFETs - MESFETs - MODFETs
Technische Universität Graz
Junction Field Effect Transistors (JFET) Metal-Semiconductor Field Effect Transistors (MESFET)Modulation Doped Field Effect Transistors (MODFET)
n
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JFET
For NA >> ND2 ( )bi
nD
V VxeN
Depletion mode2 bi
nD
Vh xeN
Enhancement mode2 bi
nD
Vh xeN
conducting at Vg = 0
nonconducting at Vg = 0
n-channel JFET
n
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n-channel (power) JFET
depletion zone
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Power SiC JFET
p
p
n
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np p
source
gate
drain
n-channel JFET
depletion region
JFETs are often discrete devices
p+
n+
n+
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MESFET
Depletion layer created by Schottky barrier
2 ( )bin
D
V VxeN
Metal-Semiconductor Field Effect Transistors
n
Fast transistors can be realized in n-channel GaAs, however GaAs has a low hole mobility making p-channel devices slower.
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JFET
2 ( )bin
D
V VxeN
Pinch-off at h = xn2
2D
peN hV
Vp = pinch-off voltage
n-channel JFET
G
D
Sn-channel JFET
G
D
Sp-channel JFET
At Pinch-off, V = Vbi - Vp.
n