Doping of semiconductors

For a pure semi-conductor the concentration of holes and electrons can be determined using a simple equation from thermal physics, but this ratio can also be manipulated by adding a certain amount of impurity atoms to the semi- conductor crystals in a process called doping. By introducing impurities with a different number of valence electrons, the number of available charge carriers in the semi-conductor can be increased. An important consequence of doping is the creation of intermediate energy levels in the forbidden region because of the excess charge carriers.

To take the most simple example, consider Silicon. Since Silicon belongs to group IV of the periodic table, it has for valence electrons. In the crystal form, each atom shares an electron with a neighbouring atom. In this state it is an intrinsic semiconductor. B, Al, In, Ga all have three electrons in the valence band. When a small proportion of these atoms, (less than 1 in 10 6 ), is incorporated into the crystal the dopant atom has an insufficient number of bonds to share bonds with the surrounding Silicon atoms. One of the Silicon atoms has a vacancy for an electron. It creates an a hole that contributes to the conduction process at all temperatures. Dopents that create holes in this manner are known as acceptors. This type of extrinsic semiconductor is known as p-type as it create positive charge carriers.

The most common n-type dopants for silicon are phosphorus and arsenic. Notice that the latter two elements are in Group V of the periodic table, and silicon is in Group IV. When silicon is doped with arsenic or phosphorus atoms, these dopant atoms replace silicon atoms in the semiconductor crystal, but since they have one more outer-shell electron than silicon they tend to contribute this electron to the conduction band. By far the most common p-type dopants for silicon is the Group III element boron, which lacks an outer-shell electron compared with silicon and thus tends to contribute a hole to the valence band.

Heavily doping a semiconductor can increase its conductivity by a factor greater than a billion. In modern integrated circuits, for instance, heavily-doped polycrystalline silicon is often used as a replacement for metals.