Doping

N-type doping
The purpose of n-type doping is to produce an abundance of carrier electrons in the material. To help understand how n-type doping is accomplished, consider the case of silicon (Si). Si atoms have four valence electrons, each of which is covalently bonded with one of four adjacent Si atoms. If an atom with five valence electrons, such as those from group VA of the periodic table (eg. phosphorus (P), arsenic (As), or antimony (Sb)), is incorporated into the crystal lattice in place of a Si atom, then that atom will have four covalent bonds and one unbonded electron. This extra electron is only weakly bound to the atom and can easily be excited into the conduction band. At normal temperatures, virtually all such electrons are excited into the conduction band. Since excitation of these electrons does not result in the formation of a hole, the number of electrons in such a material far exceeds the number of holes. In this case the electrons are the majority carriers and the holes are the minority carriers. Because the five-electron atoms have an extra electron to "donate", they are called donor atoms.

P-type doping
The purpose of p-type doping is to create an abundance of holes. In the case of silicon a trivalent atom, such as boron, is substituted into the crystal lattice. The result is that an electron is missing from one of the four possible covalent bonds. Thus the atom can accept an electron from the valence band to complete the fourth bond, resulting in the formation of a hole. Such dopants are called acceptors. When a sufficiently large number of acceptors are added, the holes greatly outnumber the excited electrons. Thus, the holes are the majority carriers, while electrons are the minority carriers in p-type materials. Blue diamonds (Type IIb), which contain boron impurities, are an example of a naturally occurring p-type semiconductor.

P-n junctions
A p-n junction may be created by doping adjacent regions of a semiconductor with p-type and n-type dopants. If a positive bias voltage is placed on the p-type side, the dominant positive carriers (holes) are pushed toward the junction. At the same time, the dominant negative carriers (electrons) in the n-type material are attracted toward the junction. Since there is an abundance of carriers at the junction, current can flow through the junction from a power supply, such as a battery. However, if the bias is reversed, the holes and electrons are pulled away from the junction, leaving a region of relatively non-conducting silicon which inhibits current flow. The p-n junction is the basis of an electronic device called a diode, which allows electric current to flow in only one direction. Similarly, a third region can be doped n-type or p-type to form a three-terminal device, such as the bipolar junction transistor (which can be either p-n-p or n-p-n).