A semiconductor material has a resistivity lying between that of a conductor and that of an insulator. In contrast to the granular materials used for resistors, however, a semiconductor establishes its conduction properties through a complex quantum mechanical behavior within a periodic array of semiconductor atoms, that is, within a crystalline structure. For appropriate atomic elements, the crystalline structure leads to a disallowed energy band between the energy level of electrons bound to the crystal’s atoms and the energy level of electrons free to move within the crystalline structure (i.e., not bound to an atom). This energy gap fundamentally impacts the mechanisms through which electrons associated with the crystal’s atoms can become free and serve as conduction electrons. The resistivity of a semiconductor is proportional to the free carrier density, and that density can be changed over a wide range by replacing a very small portion (about 1 in 106) of the base crystal’s atoms with different atomic species (doping atoms). The majority carrier density is largely pinned to the net dopant impurity density. By selectively changing the crystalline atoms within small regions of the crystal, a vast number of small regions of the crystal can be given different conductivities. In addition, some dopants establish the electron carrier density (free electron density), whereas others establish the hole carrier density (holes are the dual of electrons within semiconductors). In this manner, different types of semiconductor (n type with much higher electron carrier density than the hole density and p type with much higher hole carrier density than the electron carrier density) can be located in small but contacting regions within the crystal.