Institute of Solid State PhysicsTechnische Universität Graz

Metal-Semiconductor Contacts

Institute of Solid State Physics

metal - semiconductor contacts Technische Universität Graz

Photoelectric effect Schottky barriersSchottky diodes Ohmic contacts Thermionic emission Tunnel contacts

Photoelectric effect

hf0 = e at threshold

workfunction f

curr

ent

threshold frequency f0

Singh

There is a dipole field at the surface of a metal. This electric field must be overcome for an electron to escape.

work function - electron affinity

If s < m, the semiconductor bands bend down.

If s > m, the semiconductor bands bend up.

Singh

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

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

Interface states

metal

b

Interface states

http://www.springermaterials.com/navigation/#n_240905_Silicon+%2528Si%2529

Schottky barrier

2 bin

D

V VW x

eN

0

Dn

r

eNE x x

2

2

Dn

eN xV xx

CV measurements

-2 F m

2A

p bi

e NCx V V

2 bip

A

V Vx

eN

2

21 bi

A

V VC e N

GaAs has larger Eg and Vbi

V

1/C

2

( )ln vbi b B

A

N TeV k TN

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

Vacuum diodes

diode

Thermionic emission

EF

Fermi function

( ) exp exp exp expF F

B 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

Thermionic emission

expthB

eVnk T

expsm thB

eVI nk T

( 0)ms smI I V

e 1B

eVk T

sm ms msI I I I

exp exp exp expbi bith

B B B B

e V V eVE eVnk T k T k T k T

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

Thermionic emission

e 1B

eVk T

sm ms sI I I I

Nonideality factor = 1

Thermionic emission

* 2 exp bs R

B

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-2

n-GaAs AR* = 8 A K-2cm-2

p-GaAs AR* = 74 A K-2cm-2

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 1B

eVk T

sI I

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

Contacts

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.