ETE411 :: Lec13

Dr. Mashiur Rahman

Chapter 9 :: Metal-semiconductor junction

• Contact– Semiconductor devices must make contact with

the outside world.P-n junction

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History

• 1874- Braun : dependence of the total resistance on the polarity of the applied voltage and on the detailed surface conditions.

• 1904 : Point contact rectifier found in practical applications.

• 1938 – Schottky: potential barrier could arise from stable space charges in the semiconductor alone without the presence of a chemical layer.

Contact

1. Rectifying contacts (Schottky Barrier Diode)2. Nonrectifying contacts (Ohmic contact)

Before contact After contact

Energy Band diagram of metal and semiconductor

Ideal energy-band diagram of a metal-n-semiconductor junction for Фm > Фs

Фm = metal work functionФs = Semiconductor work functionФB0= ideal barrier height of the semiconductor contact χ= electron affinity

VB0

Schottky barrier

ФB0= (Фm- χ)

Vbi= ФB0-Фn

Mechanism : due to the flow of majority carrier electron

(Thermionic emission of majority currier)

S. M. Sze : Physics of semiconductor Devices (page 255)

Space charge region width :: W

The space charge width in a rectifying metal semiconductor contact is universally proportional to the square root of the semiconductor doping.

Vbi - VaForward bias

Measured Schottky-barrier heights (at 300 K)

S. M. Sze : Physics of semiconductor Devices

P-type semiconductorNot included in the course

χ

Reverse bias and forward bias

Reverse bias forward bias

Comparison Schottky Barrier Diode & pn junction Diode

1. Magnitudes of the reverse-saturation current density.

2. Switching characteristics

Magnitudes of the reverse-saturation current density

• Pn juction: determined by the diffusion of minority current. generation current

• Schottky barrier diode : determined by thermonic emission of majority carriers over a potential barrier.

Turn on voltage

• Frequency response. • Schottky diodes can be used in fast-switching

application.

Contact

1. Rectifying contacts2. Nonrectifying contacts (Ohmic contact)

Nonrectifying contacts (Ohmic contact)

Before contact After contact

Metal-n-semiconductor junction for Фm < Фs

Voltage applied

Positive voltage applied to the metal Positive voltage applied to the semiconductor

Example 9.7 (page 347)

Transport mechanisms

Transport mechanisms at metal–semiconductor junctions. (1) Thermionic emission (‘above’ the barrier) (2) tunneling (‘through’ the barrier), (3) recombination in the depletion layer, (4) hole injection from metal

Forward bias

S. M. Sze : Physics of semiconductor Devices (page 254)

Transport of electrons from the semiconductor over the potential barrier into the metal. Dominent process for Schottky diodes with moderately doped semicondutor (Si with ND ≤1017cm-3) operated at moderate temperature (room temp.).

Thermionic emission

tunneling (‘through’ the barrier)

Quantum-mechanical tunneling of electrons through the barrier (important for heavily doped semiconductors and responsible for most ohmic contacts).

recombination in the depletion layer

Recombination in the space-charge region identical to the recombination process in a p-n junction.

hole injection from metal

Hole injection from the metal to the semiconductor Recombination in the neutral region.

Not included in the course