Lec. 61

Metal-Semiconductor Contact

Applications

① Ohmic contacts to n+ and p+ regions in semiconductor devices.

: Minimal resistance. Linear IV characteristic in both directions.

② Schottky diodes – fast switching rectifying contacts.

: The current flows easily in one direction but not in the other direction.

AlGaN layer

Source

Gate

Drain

Undoped GaN buffer layer

Ohmic Contact Schottky Contact

GS D

Lec. 62

Metal Semiconductor Contact (Schottky Contacts)

biqV

M : Metal work function (e- to vacuum)

S : Semiconductor work function

: Semiconductor electron affinity

Vbi = M - S : Built in potential

Metal Si

fmE

q

fsE

Mq sq

SM for n-type Si

Lec. 63

Schottky Barriers (Metal/n-Si)

biqV

Au Si

fmE

eV) (4.01q

fsE

Mq sq

)( m

B

qq

fE

sqMq q

biqV

① Ef = constant

② Bulk properties do not change

(M S, :constant)

③ Eo continuous.

E0,Ec,Ei, Ev // Eo

④ Built in potential qVbi= q(M - S)⑤ Schottky barrier height:

qS = q(M - )

e-

+ +++

----

Higher energy of e- in the n-side

Diffusion of e- to metal

Produces + ions at n-side and e- accumulation at metal surface

Internal electric field

Lower Efs and bend energy bands of n-side.

Lec. 6

PNMetal

----

+

2/1

D

biSn

2nD

Sbi

)qN

V2 ( x

xN 2

q

V 22

2 ndpas

bi xNxNqV ε

21112

/

dNaNqVW bis

ε

Metal-N

PN Junction vs Schottky Barriers

Lec. 6

qVoqBFor e- from n to metal,

qVo=q(m-s)

For e- from metal to n,

qB=q(m-)

For forward bias, For reverse bias

q(Vo-V)qB q(Vo+V)qB

Forward bias: more electrons in n-side are able to jump over barrier height.

Reverse bias: fewer electrons are able to jump to semiconductor

All the voltage drop occurs at n-side of the junction, since metal side is very conductive.

In this rectifying diode, current transport is mainly due to majority carriers and no recombination process is required. Faster device

Schottky Barriers under Bias

Lec. 6

1][eI /nkTqV0 I

kTI /-q

0Be

For forward bias, For reverse bias

q(Vo-V)qB q(Vo+V)qB

Schottky Barriers under Bias

For reverse saturation current, the barrier ( ) is unaffected by the bias voltages.

B

Lec. 6

(Supplementary Materials)Schottky Barriers

Lec. 68

(Supplementary Materials)

Lec. 6

Schottky Barriers (Metal/p-Si)

Efm

Ec

Ev

=

SM for p-type Si

)( mgB qEq

Lec. 6

Ohmic Contacts (Metal/n-Si)Minimal resistance. Linear IV characteristic in both directions.

In this case, the barrier to electron flow between the metal and semiconductor is small and easily overcome by a small voltage.

No depletion region occurs in semiconductor.

Lec. 6

Ohmic Contacts (Metal/p-Si)

Lec. 6

))/(2

exp( DBs N

hm

Rc

Contact Resistance vs Doping Level

Practical way to fabricate ohmic contact is to make a heavy doping at semiconductor side.

For MS contacts with lower doping concentration, the thermionic emission current is dominated.

For contacts with higher doping concentration, the barrier is narrow enough and the tunneling process is dominated. Then Rc is followed by the equation

Lec. 613

Thermionic emission current

The barrier is narrow enough and the tunneling process is dominated.

Lec. 6

Ohmic Contacts (Metal/p-Si)

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