Short Notes:

A semiconductor is a crystalline material that conducts electricity but not as well as metals; it also resists electricity but not as well as many insulators. In general, semiconductors' electrons are tightly bound to their nuclei. But if a few atoms of antimony - with a surplus of electrons - are incorporated in a semiconductor such as silicon, the free electrons give it a negative charge. By replacing a few atoms with indium, which has room for extra electrons, the semiconductor has holes where electrons should be; the holes relay a positive charge.

These properties make semiconductors useful in transistors to amplify current, to block current, or to let current flow in only one direction. In a typical NPN transistor, a layer of positive (P) semiconductor, the base, is sandwiched between two negative (N) layers, the emitter and the collector. When, for example, a small signal from an intercom is channelled through the base, the movement of electrons amplifies the signal.