Transistor

The first transistors were made from germanium, but now most transistors are made from silicon. Silicon (Si) and germanium (Ge) form similar crystal structures with similar physical properties, but silicon is preferred over germanium because of silicon's superior thermal characteristics. Crystals of Si and Ge are neither good electrical insulators nor good electrical conductors, but conduct electricity at a level midway between metallic conductors (which have very low resistance to electric current) and nonmetallic insulators such as glass (which have a very high resistance to electric current). Transistor action is made possible by semiconduction.

Each atom in a silicon or germanium crystal lattice has four atoms as close neighbors. That is, each atom is held in its place in the crystal's orderly structure because each atom shares its four outermost electrons with the outermost electrons of four nearby atoms. This sharing holds the atoms together by the process termed covalent bonding. Covalent bonding also prevents these outermost electrons from moving through the crystal (i.e., flowing as an electric current) as easily as do the conduction electrons in metals. They are not bound too tightly to break loose if given a small amount of extra energy, but cannot wander easily through the crystal. Heat, light, or ionizing radiation may all increase the semiconductor's electrical conductivity by liberating these electrons to support current. Usually these effects are unwanted, because one does not want the properties of a circuit to vary with temperature. Ionizing radiation, furthermore, may cause transistors to fail by permanently altering the crystal structure. The first active communications satellite placed in orbit by the United States, Telstar (1962), failed when its transistors were exposed to unexpected levels of ionizing radiation. (Early satellites such as Echo had been passive radio reflectors, containing no amplifying transistor circuits to be affected by radiation.)