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A semiconductor is a material that is an insulator at very
low temperature, but which has a sizable electrical
conductivity at room temperature. The distinction between a
semiconductor and an insulator is not very well-defined, but roughly, a semiconductor is an insulator with a band gap small enough that its conduction band is appreciably thermally populated at room temperature. Silicon dioxide is an example of a nearly-perfect insulator, while silicon is the archetypical semiconductor. Many materials that in the past would have been considered insulators
are now called wide bandgap
semiconductors.
Semiconductors are useful in electronics because their electronic
properties can be greatly altered in a controllable way by adding small amounts of impurities. These impurities, called dopants,
add extra electrons or holes. A semiconductor with extra electrons is called an n-type semiconductor, while a semiconductor with extra holes is called a p-type semiconductor. Semiconductors are fundamental materials in
modern electronic devices (e.g. diodes, transistors, and integrated circuits) and
electro-optic devices (e.g. laser diodes and LEDs).
Intrinsic semiconductors
An intrinsic semiconductor is one which is pure enough that impurities do not appreciably affect its electrical
behavior. In this case, all carriers are created by thermally or optically exciting electrons from the full valence band into the empty conduction band. The band gap, or energy spacing between the valence band and the conduction band, corresponds to the energy necessary
to free charge carriers in this way. Note that both the electron and the hole it leaves behind are charge carriers, the hole having a charge of the same magnitude (1.6×10−19 C) but opposite sign of the electron. Their effective masses, i.e. their resistance to acceleration, may differ considerably. Electrons and holes flow
in opposite directions in an electric field. Equal numbers of electrons and holes are present in an intrinsic semiconductor.
The concentration of carriers is strongly dependent on the temperature. At low temperatures, the valence band is completely full, making the material an insulator (see electrical conduction for
more information). Increasing the temperature leads to an increase in the number of carriers and a corresponding increase in
conductivity. This principle is used in thermistors. This behavior contrasts
sharply with most conductors, which tend to become less conductive at higher temperatures due to increased carrier
scattering.
Doping and extrinsic semiconductors
An extrinsic semiconductor is one that has been doped with impurities to modify the number and type of free charge
carriers.
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. These n-p-n and p-n-p junction devices form the basis for most
semiconductor devices including the transistor.
See also
Encompassing fields
Sub-fields
Concepts
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