Electrons in Solids

Electric Conduction in Semiconductors

Electrons in Solids

It is proved in physics that electrons in a solid cannot possess just any arbitrary energy. Each electron can only have a particular discrete energy called an energy level. The distribution of electrons among the ener­gy levels is usually shown on a diagram such as appears in Fig. 1.1. The horizontal lines drawn across the energy diagram represent each the energy E that an electron residing at that level has.

Fig.1.1. Energy-level (or energy-band) diagram of (a) a metal and (b) a semiconductors and dielectric

The electrons occupying the external shell of an atom fill a number of energy levels that form what is known as the valence band. Metals and semiconductors have a great number of electrons occupying the higher ener­gy levels. These levels constitute the conduction band.

The conduction band is separated from the valence band by what is called a 'band gap', that is, one where no electrons can reside (Fig. 1.lb). The width, of the band gap Eg, or the difference in energy between the top level of the valence band E_V and the bottom level of the conduction band E_C, is a few electron-volts.

At this writing, semiconductor devices and IC are most often fabricated from silicon (Si) and germanium (Ge) both of which are four-valent sub­stances. The lattice of silicon or germanium consists of atoms bound together by valence electrons. This type of linkage is known as the covalent bond and is shown in Fig. 1.2. As is seen, each atom of a pair contributes one electron to the shared pair that constitutes an ordinary chemical bond.

Fig1.2. Covalent bonding between silicon atoms

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