Chapter 9 Metal-Semiconductor Contactsee130/sp06/chp9.pdf · Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-1 Chapter 9 Metal-Semiconductor Contacts Two kinds of metal-semiconductor

Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-1

Chapter 9 Metal-Semiconductor Contacts

Two kinds of metal-semiconductor contacts:

• metal on lightly doped silicon –• rectifying Schottky diodes• metal on heavily doped silicon –• low-resistance ohmic contacts


9.1 Schottky BarriersEnergy Band Diagram of Schottky Contact

• Schottky barrier height, φB , is a function of the metal material.

• φB is the single most important parameter. The sum of qφBn and qφBp is equal to Eg .

Metal Depletionlayer Neutral region

qφBn

Ec

EcEf

Ef

Ev

EvqφBp


Schottky barrier heights for electrons and holes

φBn increases with increasing metal work function

Metal Mg Ti Cr W Mo Pd Au Ptφ Bn (V) 0.4 0.5 0.61 0.67 0.68 0.77 0.8 0.9φ Bp (V) 0.61 0.5 0.42 0.3

WorkFunction 3.7 4.3 4.5 4.6 4.6 5.1 5.1 5.7ψ m (V)

φBn + φBp ≈ 1.1 V


φBn Increases with Increasing Metal Work Function

qφBn Ec

Ev

Ef

E0

qψM

χSi = 4.05 eV

Vacuum level,

Ideally, qφBn= qψM – χSi


φBn is typically 0.4 to 0.9 V

• A high density of energy states in the bandgap at the metal-semiconductor interface pins Ef to a range of 0.4 eV to 0.9 eV below Ec

• Question: What is the typical range of φBp?

qφBn Ec

Ev

Ef

E0

qψM

χSi = 4.05 eV

Vacuum level,

+ −


Schottky barrier heights of metal silicide on Si

Silicide-Si interfaces are more stable than metal-silicon interfaces. After metal is deposited on Si, an annealing step is applied to form a silicide-Si contact. The term metal-silicon contact includes silicide-Si contacts.

Silicide ErSi1.7 HfSi MoSi2 ZrSi2 TiSi2 CoSi2 WSi2 NiSi2 Pd2Si PtSiφ Bn (V) 0.28 0.45 0.55 0.55 0.61 0.65 0.67 0.67 0.75 0.87φ Bp (V) 0.55 0.49 0.45 0.45 0.43 0.43 0.35 0.23


Using CV Data to Determine φB

AW

C

qNVW

NNkTq

EEqq

dep

s

d

bisdep

d

cBn

fcBnbi

ε

φε

φ

φφ

=

+=

−=

−−=

)(2

ln

)(

Question: How should we plot the CV data to extract φbi?

Ev

Ev

Ec

Ef

Ef

Ec

qφBn

qφbiqφBn

q(φbi + V)

qV


Once φbi is known, φΒ can be determined using

22)(21

AqNV

C sd

bi

εφ +

=

d

cBnfcBnbi N

NkTqEEqq ln)( −=−−= φφφ

Using CV Data to Determine φB

V

1/C2

−φbi


9.2 Thermionic Emission Theory

2//0

//23

2

/)(2/3

2/)(

A/cm 100 where,

421

/2 /3

22

kTqo

kTqV

kTqVkTqnthxMS

nthxnth

kTVqnkTVqc

B

B

BB

eJeJ

eeTh

kqmqnvJ

mkTvmkTv

eh

kTmeNn

φ

φ

φφ

π

π

π

−

−→

−−−−

≈=

=−=

−==

==

Efn

-q(φB − V)

qφBqVMetal

N-typeSiliconV Efm

Ev

Ec

x

vthx


9.3 Schottky Diode

Ef

qφB

Efn

> qφB

qV

- - E − Ef = qφB qφB

(a) V = 0. IS M→ IM S→ = = Ι0

Efn

- - < qφB qφB

(b) Forward bias. Metal is positive wrt Si. IS M→ IM S→>> = Ι0

qV

(c) Reverse bias. Metal is negative wrt Si.

IS M→ IM S→<< = Ι0

(d) Schottky diode IV.

V

I

Reverse bias Forward bias

IS M→

IM S→

= −Ι0

≈ 0

IM S→ = −Ι0 IS M→ = Ι0 eqV/kT

-I M S→ = −Ι0

I S M→ = Ι0

Efn


)1(

)KA/(cm 1004

/00

/0

223

2

/20

−=−=+=

⋅≈=

=

→→

−

kTqVkTqVSMMS

n

kTq

eIIeIIIIh

kqmK

eAKTI B

π

φ

Ef

Efn

- - < qφB qφB

qV

IM S→ = −Ι0 IS M→ = Ι0 eqV/kT

9.3 Schottky Diode


9.4 Applications of Schottky Diodes

• I0 of a Schottky diode is 103 to 108 times larger than a PN junction diode, depending on φB . A larger I0 means a smaller forward drop V. • A Schottky diode is the preferred rectifier in low voltage, high current applications.

I

V

PN junction

Schottky

φB

I

V

PN junction

Schottky diode

φBdiode

kTq

kTqV

BeAKTI

eII/2

0

/0 )1(

φ−=

−=


Switching Power Supply

Question: What sets the lower limit in a Schottky diode’s forward drop?

Synchronous Rectifier: For an even lower forward drop, replace the diode with a wide-W MOSFET which is not bound by the tradeoff between diode V and I0 : I = I0eqV/kT

ACDC AC AC DC

utilitypower

110V/220V

PN Junctionrectifier

Hi-voltage

MOSFETinverter

100kHzHi-voltage

TransformerSchottkyrectifier

Lo-voltage 50A1.8V

feedback to modulate the pulse width to keep Vout = 1.8 V


There is no minority carrier injection at the Schottky junction. Thus, the CMOS latch-up problem can be eliminated by replacing the source/drain of the NFET with Schottky junctions.

In addition, the Schottky S/D MOSFET would have shallow junctions and low series resistance. So far, Schottky S/D MOSFETs have lower performance.

No excess carrier storage.What application may benefit from that?

9.4 Applications of Schottky Diodes

P-body

gatesilicidesource

silicide drain


GaAs MESFET

The MESFET has similar IV characteristics as the MOSFET, but does not require a gate oxide.

N-channelN+ N +

metal

gatesource drain

GaAsSemi-insulating substrate

Question: What is the advantage of GaAs over Si?


9.5 Ohmic Contacts


SALICIDE (Self-Aligned Silicide) Source/Drain

Rs RdS D

Ggate

oxide

dielectric spacercontact metal

channel

N+ source or drainCoSi2 or TiSi2

After the spacer is formed, a Ti or Mo film is deposited. Annealing causes the silicide to be formed over the source, drain, and gate. Unreacted metal (over the spacer) is removed by wet etching.

Question:• What is the purpose of siliciding the source/drain/gate?• What is self-aligned to what?


dBn NVHndthxdMS

onns

emkTqNPvqNJ

mmhmH

/)(

13/29

2/21

Vcm /104.5/4

−−→

−−

=≈

×==

φπ

επ

9.5 Ohmic Contacts

- -

x

Silicide N+ Si

- -

x

Vd

Bnsdep qN

W φε2=

dBn NHeP φ−=Ev

Ec , Ef Efm

Ev

Ec , Ef

φBn – VφBn

Tunneling probability:


2//1

cmΩ ⋅∝=

≡

−→ dBn

dBnNH

dthx

NHMS

c eNHqv

edV

dJR φφ

9.5 Ohmic Contacts

Documents

Chapter 9 Metal-Semiconductor Contactsee130/sp06/chp9.pdf · Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-1 Chapter 9 Metal-Semiconductor Contacts Two kinds of metal-semiconductor