Integrated Circuit Devices

Professor Ali Javey

Summer 2009

MS Junctions

Reading: Chapter14

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

: electron affinity

FBFCs )( EE

Metal Work Function

The work function of metals depend strongly on the “environment.”

Workfunction, M is an invariant property of metal. It is the minimum energy required to free up electrons from metal.

Question: how can you change S ?

Ideal MS Contact

Assumptions:

M and S are in intimate contact, on atomic scaleNo oxides or charges at the interfaceNo intermixing at the interface

Question: how are Vbi and Bn related?

Schottky Barrier Heights: Metal on Si

Bn tends to increase with increasing metal work function

Schottky Barrier Heights: 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 PtSi Bn (V) 0.28 0.45 0.55 0.55 0.61 0.65 0.67 0.67 0.75 0.87 Bp (V) 0.55 0.49 0.45 0.45 0.43 0.43 0.35 0.23

• Depletion Width:

Does this depletion width equation look familiar?

Question

How can you measure Bn and Bp?

Using CV Data to Determine B

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

Ev

Ev

Ec

Ef

Ef

Ec

Bn

qVbiBn

q(Vbi + V)

qV

dep

sdep W

AC

Once Vbi is known, can be determined using:

22

)(21

AqN

VV

C sd

bi

d

cBnfcBnbi N

NkTEEqV ln)(

Using CV Data to Determine B

V

1/C2

Vbi

Carrier Injection at the MS Contacts

– Thermionic emission current

– Tunneling current

– Thermally activated tunneling current

For each MS junction, 3 components contribute to the overall injection current:

What parameters affect each component?

Is the net current zero? Why?

Question:

How do p+n junctions differ from MS junctions under a forward bias?

Question

• How does the band diagram look for a MS junction with a Schottky barrier height of zero?

Applications of Schottky Diodes

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

I

V

PN junction

Schottky

B

I

V

PN junction

Schottky diode

B

diode

Ohmic MS Contacts

Two ways to achieve ohmic MS contacts:– Reduce the Schottky barrier height. How???– Reduce the Schottky barrier width (depletion

width). How????

How would each approach give you an ohmic contact?

Schottky Barriers and Fermi Level Pinning

In actual fabricated metal-Si junctions, Fermi level pinning

prevents us from ever getting zero Schottky barrier height.

Two tricks for reducing Fermi level pinning:

1. thin interfacial oxide/nitride

2. 1D semiconductors

1D nanotubeDiameter ~ 1 nm