Transistor

An pure or "intrinsic" silicon crystal contains about one non-silicon impurity atom for every 100 million or so silicon atoms. These impurity atoms are implanted in the crystal by a process termed doping. They are located in the crystal lattice as if they were themselves silicon atoms, but change the properties of the lattice radically because of their distinct properties.

When doping adds impurity atoms with five electrons in their outermost (valence) orbit, the result is termed an n-type semiconductor. Arsenic, for example, has five valence electrons and is often used to produce n-type semiconductor. Pentavalent impurity atoms share only four of their five valance electrons with their four closest silicon neighbors; the fifth is free to move through the crystal in response to any electric field that may be present, almost like a conduction electron in an ordinary metal. An n-type semiconductor thus conducts electricity more easily than an intrinsic semiconductor.

If an impurity with only three valence electrons (e.g., boron, aluminum, gallium, and indium) is used, p-type semiconductor results. These atoms are short one of the electrons needed to establish a covalent bond with all four of its silicon neighbors and so introduce a defect into the crystal lattice, a positively-charged location where a negatively-charged electron would be found if a silicon atom had not been displaced by the impurity atom. This defect, termed a hole, can move when a neighboring electron slips into the hole, leaving a new hole behind. The hole will have moved from one location to another within the crystal, behaving much like a positive counterpart of an electron.

Holes travel somewhat more slowly than electrons within a an electrical field of given strength, but this difference in speed is usually not important in practice. Both the excess electrons donated in n-type semiconductor by pentavalent impurity atoms and the holes created in p-type semiconductor by trivalent impurity atoms increase the conductivity of the semiconductor; for example, at 86°F (30°C) the conductivity of n-type silicon with one pentavalent impurity atom per 100 million silicon atoms is 24,100 times greater than that of intrinsic silicon.