Semiconductors

• Elemental silicon has the same structure as diamond.
• Compared to diamond, in the silicon there is a smaller energy gap between the filled and empty molecular orbitals, hence a few electrons can cross the gap at 25^oC, making silicon a semiconducting element, or semiconductor.
• At higher temperatures the conductivity of silicon increases.
• There are two types of semiconductors:

1 – n-type semiconductor – substance whose conductivity is increased by doping it with atoms having more valence electrons than the atoms in the host crystal.

2 – p-type semiconductor – substance whose conductivity is increased by doping it with atoms having fewer valence electrons than the atoms of the host crystal.

• Most important applications of semiconductors involve connection of a p-type and an n-type to form a p–n junction.
• At the junction, a small number of electrons migrate from the n-type region into the p-type region, where there are vacancies in the low-energy molecular orbitals. The effect of these migrations is to place a negative charge on the p-type region (since it now has a surplus of electrons) and a positive charge on the n-type region (since it has lost electrons, leaving holes in its low-energy molecular orbitals). This charge build-up, called the contact potential, or junction potential, prevents further migration of electrons.
• Now suppose an external electric potential is applied by connecting the negative terminal of a battery to the p-type region and the positive terminal to the n-type region, as shown in figure (b). Electrons are drawn toward the positive terminal, and the resulting holes move toward the negative terminal—exactly opposite to the natural flow of electrons at the p–n junction. The junction resists the imposed current flow in this direction and is said to be under reverse bias. No current flows through the system.
• If the battery is connected so that the negative terminal is connected to the n type region and the positive terminal is connected to the p-type region (Figure c) the movement of electrons (and holes) is in the favored direction. The junction has low resistance, and a current flows easily. The junction is said to be under forward bias.