DEE4521 Semiconductor Device Physics

Metal-Semiconductor System: Contact

Prof. Ming-Jer Chen Department of Electronics Engineering National Chiao-Tung University November 6, 2014 1

Textbook pages involved

PP. 331 – 338 Section 6.4 Metal-Semiconductor Junctions

2

How to establish device physics for this circuitry? Even for the poor contact case?

(given doping concentration, two Ohmic contacts, a metal line, a supply voltage source, and a grounding system) 3

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

Can you derive an analytic model for these I-V?

and Can you derive their small-signal models?

6-22 Ohmic Contact 4

Four Situations of Band Bending at Semiconductor Surfaces – Contact Case:

• Depletion ( suitable for a metal-semiconductor interface, as suggested by many and many experiments done before) • Accumulation ( not suitable for a metal-semiconductor interface) • Inversion ( not suitable for a metal-semiconductor interface) • Flatband ( not suitable for a metal-semiconductor interface) 5

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 of textbook by Anderson’s Not so clear 6-23

How do holes and electrons communicate with each other at the interface?

6

Metal-Semiconductor Contact System (or Junction):

•

Ohmic Contact -- Two-way conducting (on) -- Nearly zero resistance or potential drop -- Equilibrium at both sides

•

Schottky Contact -- Usually for one-way conducting, with the other way off -- Considerable potential drop -- Fermi level may split off

7

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!

Band bending must go upward, NOT downward, for n-type .

accumulation 8 6-18

(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 upward for n-type semiconductor .

6-19

Wrong band diagram Correct band diagram

9

Energy band diagrams for a metal: n-type 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

Thermionic injection 6-20 X m = (2  s V j /qN D ) 1/2 10

A Schottky barrier diode made with a P-type (c) reverse bias.

semiconductor. (a) Equilibrium; (b) forward bias;

band bending down, for p-type .

Figure 6.21

Electron-hole recombination

Forward bias

Lowered barrier seen by holes

Reverse bias

6-21 Too wide to tunnel Raised barrier seen by holes 11

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

From the energy band diagram, we EE people can now derive an

analytic model

for I-V, as well as for

small-signal equivalent

circuits.

Junction Conductance Junction Capacitance Bulk (Series) Resistance

12 6-22 Ohmic Contact