Semiconductor

Semiconductors are materials with electrical conductivities that are
intermediate between those of conductors and insulators. Semiconductors are
useful for electronic purposes because they can carry an electric current by
electron propagation or hole propagation, and because this current is
generally uni-directional and the amount of current may be influenced by an
external agent (see diode, transistor, amplifier etc.). 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.

While silicon dioxide or sand is an insulator, pure silicon is a semiconductor.

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 energy band associated with electrons in their ground
state is called the valence band. These electrons are static. The energy
band of excited electrons is called the conduction band. These electrons
move freely and are usually higher energy. 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 at room
temperature.

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
(1.6×10−19 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 electricity. 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 semiconductor that has been doped with various
impurities to modify the number of holes and excited electrons. Natural blue
diamonds (Type IIb) which contain boron which has a valency of 3 thus
replacing carbon atoms which have a valency of 4 have extra holes and thus
are naturally occurring p-type semiconductors.

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 the 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 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.

p-type doping

The purpose of p-type doping is to create an 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.

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. Similarily, 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.