Download Zener diodes, Schottky diodes, JFETs

Avalanche breakdown
Impact ionization
causes an avalanche of
current
Occurs at low doping
Zener tunneling
Electrons tunnel from valence
band to conduction band
Occurs at high doping
Tunneling
wave decays exponentially in the
classically forbidden region
e1>e2
Tunneling is a wave phenomena. Tunneling and total
internal reflection are used in a beam splitter.
http://lampx.tugraz.at/~hadley/physikm/apps/snell/snell.en.php
Zener tunneling
Breakdown voltage is typically much lower than the
breakdown voltage of an avalanche diode and can be
tuned by adjusting the width of the depletion layer.
Used to provide a reference voltage.
Avalanche breakdown
Tunneling
Institute of Solid State Physics
Technische Universität Graz
metal - semiconductor contacts
Photoelectric effect
Workfunction
Electron affinity
Interface states
Schottky barriers
Schottky diodes
Ohmic contacts
Thermionic emission
Tunnel contacts
current
Photoelectric effect
hf0 = ef at threshold
workfunction
f
threshold frequency f0
There is a dipole field at the surface of a metal. This electric field must
be overcome for an electron to escape.
Singh
work function - electron affinity
If fs < fm, the semiconductor bands bend down.
If fs > fm, the semiconductor bands bend up.
work function - electron affinity
Electrons flow from a low work
function material to high work
function material. The high work
function material becomes
negatively charged. You have to
push the electrons uphill into the
low work function material. This
determines the band bending.
If fs < fm, the semiconductor bands bend down.
If fs > fm, the semiconductor bands bend up.
Singh
p-type
Walter Schottky
Schottky contact / ohmic contact
EF,m
metal
EF,s
Schottky contact
EF,m
metal
specific contact resistance:
EF,s
Ohmic contact:
linear resistance
 J 
Rc  

 V 
1
-cm 2
n-type
Schottky contact / ohmic contact
Schottky contact
EF,s
EF,m
metal
EF,m
metal
EF,s
Ohmic contact:
linear resistance
specific contact resistance:
 J 
Rc  

 V 
1
-cm 2
Interface states
http://www.springermaterials.com/navigation/#n_240905_Silicon+%2528S
Schottky barrier
Vbi  fs  fm
E
eN D
e re 0
 x  xn 
2e Vbi  V 
W  xn 
eN D

eN D  x 2
V
  xxn 
e  2

Like a one sided junction, the metal side is heavily doped.
0  x  xn
CV measurements
C
e
xp

2e Vbi  V 
eN A
ee N A
F m-2
2 Vbi  V 
1 2 Vbi  V 

2
C
ee N A
1/C2
xp 
V
GaAs has larger Eg and Vbi
Thermionic emission
1901 Richardson
Owen Willans Richardson
Current from a heated wire is:
 ef 
J  ART exp  

k
T
 B 
2
Some electrons have a thermal energy that
exceeds the work function and escape
from the wire.
Vacuum diodes
diode
Thermionic emission
Fermi function
e Vbi  V 
EF
 EF  E 
 EF
f ( E )  exp 
  exp 
 k BT 
 k BT

 E
 exp 

 k BT

 E 
  exp 

k
T

 B 
 E 

k
T
 B 
The density of electrons with enough energy to go over the barriers  exp 
Thermionic emission
 eV 
nth  exp 

k
T
 B 
 eV 
I sm  nth  exp 

k
T
 B 
I ms  I sm (V  0)
I  I sm  I ms
 keVT 
 I s  e B  1




Schottky barrier
Ism > Ims
Ism ~ 0
Ims constant
Thermionic emission
I  I sm  I ms
 keVT 
 I s  e B  1




Nonideality factor = 1
Thermionic emission
 efb 
I s  AAR*T 2 exp 

k
T
 B 
A = Area
AR* = Richardson constant
n-Si AR* = 110 A K-2cm-2
p-Si AR* = 32 A K-2cm-2
n-GaAs AR* = 8 A K-2cm-2
p-GaAs AR* = 74 A K-2cm-2
Thermionic emission dominates over diffusion current in a Schottky diode.
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
 keVT 
I  I s  e B  1




Tunnel contacts
For high doping, the Schottky barrier is so thin that electrons can
tunnel through it.
metal
p+
p
Degenerate doping at a tunnel contact
metal
n+
n
Tunnel contacts have a linear resistance.
Contacts
Transport mechanisms
Drift
Diffusion
Thermionic emission
Tunneling
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.
Institute of Solid State Physics
Technische Universität Graz
JFETs - MESFETs - MODFETs
Junction Field Effect Transistors (JFET)
Metal-Semiconductor Field Effect Transistors (MESFET)
Modulation Doped Field Effect Transistors (MODFET)
n
JFET
n-channel JFET
n
For NA >> ND
2e (Vbi  V )
xn 
eN D
Depletion mode
Enhancement mode
h  xn 
2eVbi
eN D
2eVbi
h  xn 
eN D
conducting at Vg = 0
nonconducting at Vg = 0
Power SiC JFET
p
n
p
n-channel (power) JFET
depletion
zone
n-channel JFET
drain
n+
depletion region
p+
p
n
p
gate
JFETs are often discrete devices
n+
source
MESFET
Metal-Semiconductor Field Effect Transistors
n
Depletion layer created by Schottky barrier
xn 
2e (Vbi  V )
eN D
Fast transistors can be realized in n-channel GaAs, however GaAs has
a low hole mobility making p-channel devices slower.
JFET
n
D
n-channel JFET
G
2e (Vbi  V )
xn 
eN D
S
n-channel JFET
Pinch-off at h = xn
eN D h 2
Vp 
2e
At Pinch-off, Vp = Vbi - V.
D
G
S
p-channel JFET
Vp = pinch-off voltage
JFET
The drain is the
side of the
transistor that
gets pinched off.