MetalSemiconductor Junction

1
Metal-Semiconductor Junction
Evacuum
Jms
Ec
Jsm
EFs
EFm
Ei
Ev
EfsgtEFm
2
Metal and Semiconductors are in contact
?mlt ?s
?mgt ?s
barriers
no barriers
Symmetric - Ohmic
Asymmetric - Schottky
Built-in voltage
Schottky Barrier
3
Thermionic Emission Current
At equilibrium JmsJsmJsT
4
Ideal Schottky Junction under bias
-Jsm
-Jms
-Jsm
-Jms
Evacuum
Evacuum
e?s
e?s
e?m
e?m
e?s
E(Vbi-Va)
e?0
Ec
e?0
e(VbiVa)
e?s
EF
EF
Ec
Ei
Ei
Ev
Ev
Forward bias
Reverse bias
The barrier for carriers going from M to S stays
the same e?0
For the carriers going from S to M it changes
with bias
Effective barrier e(VbiVa)
Effective barrier e(Vbi-Va)
5
Current under bias
At zero bias
6
Image Force
xmax
Ec
Potential maximum
7
Interface States
EF
Dif density of trapping states
8
Interface states
pinned Fermi level EFEvE0
Charge balance
Special case I
Special Case II
9
Ohmic contact
10
P-n heterojunction (abrupt)
?Ec conduction band offset
?Ev valence band offset
11
P-n heterojunction (graded)
EF2
Barriers are different
12
Heterojunction at forward bias
13
Abrupt heterojunction at forward bias
e(Vbi-Va)
?2
?2
?Ec
?1
?1
Eg2
EF2
Eg1
?Ev
14
n-n heterojunction