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7/27/2019 metal-semiconductor system.ppt
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1
Prof. Ming-Jer Chen
Department of Electronics Engineering
National Chiao-Tung University
03/21/2011
DEE4521 Semiconductor Device Physics
Metal-Semiconductor System
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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)
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How to establish device physics for this case?
(given a Doping concentration, two Ohmiccontacts, and a supply voltage)
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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?
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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
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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
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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
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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
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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.
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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