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Prof. Ming-Jer ChenProf. Ming-Jer Chen

Department of Electronics EngineeringDepartment of Electronics Engineering

National Chiao-Tung UniversityNational Chiao-Tung University

November 6, 2014November 6, 2014

DEE4521 Semiconductor Device PhysicsDEE4521 Semiconductor Device Physics

Metal-Semiconductor System: ContactMetal-Semiconductor System: Contact

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PP. 331 – 338PP. 331 – 338

Section 6.4 Metal-Semiconductor JunctionsSection 6.4 Metal-Semiconductor Junctions

Textbook pages involved

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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)

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6-22

Comparison of the I-Va characteristics of a Schottky diode and a pn junction diode. The scale for the reversecharacteristic is compressed compared with the scale for forward bias.

Figure 6.22

Ohmic Contact

Can you derive an analytic model for these I-V?and Can you derive their small-signal models?

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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)

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6-23

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 of textbook by Anderson’s

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

Not so clear

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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

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6-18

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!Band bending must go upward, NOT downward, 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 dipoleeffect. (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 Correct band diagram

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6-20

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 (EB(0) = EC(x = 0) − Efm) is independent of applied voltage.

Figure 6.20

Xm = (2εsVj/qND)1/2

Thermionic injection

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6-21

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.

Electron-hole recombination

Lowered barrier seen by holes Forward bias

Reverse biasRaised barrier seen by holes

Too wide to tunnel

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6-22

Comparison of the I-Va characteristics of a Schottky diode and a pn junction diode. The scale for the reversecharacteristic is compressed compared with the scale for forward bias.

Figure 6.22

Ohmic Contact

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 CapacitanceBulk (Series) Resistance