Institute of Solid State PhysicsTechnische Universität Graz

8. Schottky contacts/ JFETs

Nov. 21, 2018

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 biA

V VC e N

GaAs has larger Eg and Vbi

V

1/C

2

( )ln vbi b BA

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 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

Thermionic emission

expthB

eVnk T

expsm thB

eVI nk T

( 0)ms smI I V

e 1BeVk T

sm ms msI I I I

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 1BeVk T

sm ms sI I I I

Nonideality factor = 1

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

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

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.

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

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

n-channel (power) JFET

depletion zone

Power SiC JFET

p

p

n

np p

source

gate

drain

n-channel JFET

depletion region

JFETs are often discrete devices

p+

n+

n+

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.

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