ETE411 :: Lec14

Dr. Mashiur Rahman

Contact

1. Rectifying contacts2. Nonrectifying contacts (Ohmic contact)

Nonrectifying contacts (Ohmic contact)

Before contact After contact

Metal-n-semiconductor junction for Фm < Фs

Schotkey BarrierOhmic contact

Tunneling barrier

The space charge width in a rectifying metal semiconductor contact is universally proportional to the square root of the semiconductor doping. The width of the depletion region decrease as the doping concentration in the semiconductor increases.

Energy-band diagram of a heavily doped n-semiconductor-to-metal junction

Voltage applied

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

Ohmic contact :: metal –p -semiconductor

(not included in the course)

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.

Various metal-semiconductor device structure

S. M. Sze : Physics of semiconductor Devices (page 297)

Chapter 10

The Bipolar Transistor

History

• Bardeen, Brattain & Shockley– 1948 : Invented the transistor– 1956 : Received Nobel

• Post war effort to replace vacuum tube.• They used Germanium: it was possible to

obtain high purity material.

Block diagrams and circuit symbols

++ = Heavily doped+= moderately doped

Bipolar: its operation involves both type of mobile carriers – electrons & holes.

n++ n n

npn pnp

Doping profile

nn++

Conventional ICs

Conventional npn transistor An oxide-isolated npn bipolar transistor

From Muller and Kamins

Forward active operating mode

1. Electron injected: E → B2. Create excess concentration of minority carrier3. Diffuse across the base region: B → C4. Electric field will swap the electrons into the collector.

n++ n

Minority carrier distribution

Energy band diagram