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-Va 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?
Ohmic Contact
4
6-22
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
6
at the interface?
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.
Wrong band diagram
6-19
Correct band diagram
9
n-type
Energy band diagrams for a metal:
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 (EB(0) = EC(x = 0) - Efm) is independent of
applied voltage.
Figure 6.20
Thermionic injection
6-20
Xm = (2sVj/qND)1/2
10
A Schottky barrier diode made with a
(c) reverse bias.
P-type 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
Raised barrier seen by holes
6-21
Too wide to tunnel
11
Comparison of the I-Va 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
for I-V,
as well as for small-signal
model
equivalent circuits.
Junction Conductance
Junction Capacitance
Bulk (Series) Resistance
12
6-22
Ohmic Contact