Semiconductor Devices

1. Introduction

A semiconductor is a material whose electrical conductivity lies between that of a good conductor and a good insulator, for example silicon (Si) and germanium (Ge), both tetravalent. In a pure semiconductor the valence band and conduction band are separated by a small energy gap ($E_g \approx 1.1\ \text{eV}$ for Si, $0.7\ \text{eV}$ for Ge), narrow enough that thermal energy can lift some electrons into the conduction band. This chapter builds from the two kinds of semiconductor up to the p-n junction diode, its forward and reverse characteristics, the Zener diode, rectifiers, and finally logic gates.

2. Intrinsic and Extrinsic Semiconductors

2.1 Intrinsic semiconductor

An intrinsic semiconductor is a semiconductor in its purest form (no added impurity).

• The number of free electrons equals the number of holes, $n_e = n_h$, because each electron that jumps to the conduction band leaves behind one hole.
• Its conductivity is low at room temperature and rises with temperature as more electron-hole pairs are created.

2.2 Extrinsic semiconductor

An extrinsic semiconductor is formed by adding a small, controlled amount of impurity (doping) to a pure semiconductor. Doping greatly increases the conductivity. There are two types.

n-type semiconductor:

• Formed by doping with a pentavalent impurity (P, As, Sb), which donates a free electron. The impurity is a donor, and its donor energy level lies just below the conduction band.
• Electrons are the majority carriers and holes are the minority carriers.

p-type semiconductor:

• Formed by doping with a trivalent impurity (B, Al, In), which creates a hole. The impurity is an acceptor, and its acceptor energy level lies just above the valence band.
• Holes are the majority carriers and electrons are the minority carriers.

In both cases the crystal as a whole remains electrically neutral.

3. The p-n Junction and Semiconductor Diode