Semiconductors are materials with electrical conductivities that are intermediate between those of conductors and insulators. What makes semiconductors useful for electronic purposes is that they have the property of being able to carry an electric current by electron propagation or "hole" propagation. Electron propagation is the same sort of current flow seen in a standard copper wire - heavily ionized atoms pass excess electrons down the wire from one atom to another in order to move from a more negatively ionized area to a less negatively ionized area. "Hole" propagation is a rather different proposition - in the case of a semiconductor experiencing hole propagation, the charge moves from a more positively ionized area to a less positively ionized area by the movement of the electron hole created by the absence of an electron in a nearly-full electron shell.
The properties of semiconductors, e.g. the number of carriers (and therefore the prevalence of electron propagation or hole propagation), can be controlled by "doping" the semiconductor blocks with impurities. A semiconductor with more electrons than holes is called an n-type semiconductor, while a semiconductor with more holes than electrons is called a p-type semiconductor.
Semiconductors are the fundamental materials in many modern electronic devices.
Electronic Structure of Semiconductors
Semiconductors exhibit a number of useful and unique properties related to their electronic structure. In solids the electrons tend to occupy various energy bands. The highest energy band occupied by electrons in their ground state is called the valence band. The lowest energy band occupied by excited electrons is called the conduction band. As the name implies, electrons in the conduction band are able to conduct electricity. The energy spacing between the valence band and the conduction band is called the Band gap and corresponds to the energy necessary to excite an electron from the valence band into the conduction band. For some metals, such as magnesium, the valence and conduction bands overlap, corresponding to a negative Band gap. In this situation, there are always some electrons in the conduction band and the material is highly conductive. Other metals, such as copper, have empty states in the valence band. In this case electrons in the valence band can conduct electricity by moving between the various states and again the material is highly conductive. For insulators the valence band is completely filled and the Band gap is relatively large, preventing conduction. Semiconductors have an electronic structure similar to that of insulators, but with a relatively small Band gap, generally less than 2 eV. Because the Band gap is relatively small, electrons can be thermally excited into the conduction band, making semiconductors somewhat conductive.
Electrons in the conduction band are free to move through the material conducting electricity. In addition, when an electron is excited into the conduction band it leaves behind an empty state in the valence band, corresponding to a missing electron in one of the covalent bonds. Under the influence of an electric field,an adjacent valence electron may move into the missing electron position, effectively moving the location of the missing electron. Thus, like the electron, this missing electron or hole is also able to move through the material, conducting electricity. Holes are considered to have a charge of the same magnitude as an electron (1.6E-19 C), but of opposite charge. Thus, in the presence of an electric field excited electrons and holes move in opposite directions. Electrons are somewhat more mobile than holes and are thus more efficient at conducting electricty. Because both electrons and holes are capable of carrying electricity, they are collectively called carriers.
The concentration of carriers is strongly dependent on the temperature. Increasing the temperature leads to an increase in the number of carriers and a corresponding increase in conductivity. This contrasts sharply with most conductors, which tend to become less conductive at higher temperatures. This principle is used in thermistors.
Doping and Extrinsic semiconduction
Intrinsic semiconductors are those in which the electrical behavior depends on the electronic structure of the pure material. For the case of intrinsic semiconductors, all carriers are created by exciting electrons into the conduction band. Thus equal numbers of electrons and holes are created. An extrinsic semiconductor is a semicondutor that has been doped with various impurities to modify the number of holes and excited electrons.
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 the those from group VA of the periodic table (eg. phosphorous (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 non-bonding 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.
The purpose of p-type doping is to create and abundance of holes. In this case a trivalent atom, usually 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 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.
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 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 flows through the junction. 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 prevents current flow. The p-n junction is the basis of an electronic device called a diode, which allows electricty to flow in only one direction. Similar npn and pnp junctions form the basis for most semiconductor devices including the transistor.