7/27/2019 metal-semiconductor system.ppt

http://slidepdf.com/reader/full/metal-semiconductor-systemppt 1/10

1

Prof. Ming-Jer Chen

Department of Electronics Engineering

National Chiao-Tung University

03/21/2011

DEE4521 Semiconductor Device Physics

Metal-Semiconductor System

7/27/2019 metal-semiconductor system.ppt

http://slidepdf.com/reader/full/metal-semiconductor-systemppt 2/10

2

Band Bending at Surface of Semiconductors

• Depletion (suitable for a contact-

semiconductor system, as suggested by

many and many experiments done before)

• Accumulation (not suitable for a contact-

semiconductor system)

• Inversion (not suitable for a contact-

semiconductor system)

7/27/2019 metal-semiconductor system.ppt

http://slidepdf.com/reader/full/metal-semiconductor-systemppt 3/10

3

How to establish device physics for this case?

(given a Doping concentration, two Ohmiccontacts, and a supply voltage)

7/27/2019 metal-semiconductor system.ppt

http://slidepdf.com/reader/full/metal-semiconductor-systemppt 4/10

46-23

Low-resistance metal-semiconductor contacts using degenerate surface layers. Metal-n+n contact (a) and

metal-p+p contact (b). The Ohmic barrier is thin enough to permit tunneling.

Figure 6.23

How do holes and electrons communicate with each other 

at the interface?

7/27/2019 metal-semiconductor system.ppt

http://slidepdf.com/reader/full/metal-semiconductor-systemppt 5/10

5

Metal-Semiconductor System (or Junction):

• Ohmic Contact

-- Two-way conduction

-- Zero resistance or potential drop

-- Equilibrium at both sides

• Schottky Contact

-- Usually for one-way conduction

-- Considerable potential drop

-- Usually One-sided equilibrium

7/27/2019 metal-semiconductor system.ppt

http://slidepdf.com/reader/full/metal-semiconductor-systemppt 6/10

66-18

Energy band diagram as predicted by the electron affinity model for an Al:n-Si metal semiconductor

 junction: (a) Neutrality (b) equilibrium. The predicted barrier of 0.10 eV from metal to semiconductor is

much less than the experimental value of about 0.7 eV. A more refined model is required.

Figure 6.18

These two diagrams are wrong!Must be band bending up,

 NOT down, for n-type.

accumulation

7/27/2019 metal-semiconductor system.ppt

http://slidepdf.com/reader/full/metal-semiconductor-systemppt 7/10

7

6-19

(a) The neutrality diagram for the Al:n-Si Schottky barrier diode including the tunneling-induced dipole

effect. (b) The equilibrium energy band diagram for an Al:n-Si Schottky barrier diode.

Figure 6.19

This is the depletion case by bending band up for n-type.

Wrong band diagram Correct band diagram

7/27/2019 metal-semiconductor system.ppt

http://slidepdf.com/reader/full/metal-semiconductor-systemppt 8/10

86-20

Energy band diagrams for a metal:n-semiconductor Schottky barrier. (a) For forward bias, electrons flow

from semiconductor to metal. (b) For reverse bias, only a small leakage current flows. (c) For the first-order

model, the metal-semiconductor barrier ( E  B(0) = E C ( x = 0) -  E  fm) is independent of applied voltage.

Figure 6.20

Xm = (2sV j/qND)1/2

7/27/2019 metal-semiconductor system.ppt

http://slidepdf.com/reader/full/metal-semiconductor-systemppt 9/10

96-21

A Schottky barrier diode made with a p-type semiconductor. (a) Equilibrium; (b) forward bias;

(c) reverse bias.

Figure 6.21  band bending down, for p-type.

7/27/2019 metal-semiconductor system.ppt

http://slidepdf.com/reader/full/metal-semiconductor-systemppt 10/10

106-22

Comparison of the I-V a characteristics of a Schottky diode and a pn junction diode. The scale for the reverse

characteristic is compressed compared with the scale for forward bias.

Figure 6.22

Ohmic Contact