doping

Doping is the art of controlling a semiconductor by contaminating it on purpose. Add a few impurity atoms, sometimes just one per million host atoms, and you can transform a nearly insulating crystal into a well-behaved conductor and decide, at will, whether its current is carried by negative electrons or positive holes. This deliberate, exquisitely controlled poisoning is the knob that makes all of semiconductor electronics possible.

There are two kinds. A donor impurity, such as phosphorus in silicon, has one more valence electron than the atom it replaces; that spare electron sits in a shallow level just below the conduction band and is easily freed, adding mobile electrons and making the material n-type, with electrons as the majority carriers. An acceptor impurity, such as boron, has one fewer valence electron; it creates a level just above the valence band that readily captures an electron, leaving a mobile hole and making the material p-type, with holes as majority carriers. In band terms, doping shifts the Fermi level, upward toward the conduction band for n-type and downward toward the valence band for p-type. In the extrinsic regime the carrier density is set by the dopant concentration and is nearly independent of temperature.

Doping is what turns semiconductors into devices: joining a p-type and an n-type region makes a p-n junction, the diode, and stacking doped regions makes the transistor. One point of honesty about naming: the labels n and p refer only to the sign of the majority carriers, not to any net charge; a doped crystal remains electrically neutral overall, because each donated electron or hole is balanced by a fixed ionized dopant of the opposite sign.