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.
