INTRODUCTION:
SEMICONDUCTORS
Semiconductors are crystalline solids with distinct electrical characteristics. Their electrical resistance is high than conductor but, lower than that of insulators.
They are divide in to two:
v Intrinsic
Semiconductors
v Extrinsic Semiconductors
Intrinsic
Semiconductors:
The
semiconductor material which does not have any impurities is called intrinsic
Semiconductor (pure semiconductors). Silicon and Germanium, which belong to the
fourth group element, behave like a semiconductor. Each atom of silicon and
germanium share an electron with their neighbors. A Silicon atom and its neighbors
share a pair of electrons in covalent bonding. Whenever a covalent bond break,
an electron-hole pair is formed. To remove the valence electrons from the outer shells a semiconductor atom needs the
energy of the order 1.1 eV. The vacancy in the covalent bond is called a hole(positively
charge). Any other electron can fill this hole. In other words, a hole shifts
from one covalent bond to another. We can assume that the hole is a positive
charge carrier since the direction of the hole is opposite to that of the
electron. In an intrinsic semiconductor, electrons and holes move in random
directions and the number of free electrons (ne) and holes (nh)
remain same.
Extrinsic Semiconductors: The introduction of the extrinsic
semiconductor is due to the excess holes or excess electrons present in
silicon. Pure semiconductors are of no use as there are very few charge
carriers which can cause conduction process. By adding some impurities to the
pure semiconductor the conductivity can be improved. This process is called
doping.
Depending on the type of doping material used,
extrinsic semiconductors can be divided into:
v
N-type semiconductors
v
P-type semiconductors
N-type Semiconductors: The N-type semiconductor has a large
number of electrons in the conduction band and less number of holes in the
valence band, so electrons are called majority carriers and holes are called
minority carriers.
A pentavalent
impurity such as phosphorous or arsenic is added to the silicon crystal.
Out of five valence electrons, four silicon atoms take part in covalent bonding
with one arsenic/phosphorus atom. The fifth electron is loosely bound to the
silicon atom. Such a silicon crystal is still electrically neutral as the extra
electron does not show up as an additional charge in the atom.
P-type Semiconductors: The P-type semiconductor has a
large number of hole in the conduction band and less number of electrons in the
valence band, so holes are called majority carriers and electrons are called
minority carriers. A trivalent impurity such as Boron is mixed with the silicon
atoms. Boron can share three valence electrons with the silicon atom; the boron
atom takes one electron from nearby covalent bonds with the silicon atom in
order to complete eight electrons in its valence shell. As the trivalent
impurity atoms accept electrons from the silicon atom, it is known as an
acceptor impurity. The p-type silicon crystal so obtained is called p-type
extrinsic semiconductor and the holes created are extrinsic carriers.