SEMICONDUCTORS
The substances (cg. silicon, germanium,
carbon etc. ) whose electrical conductivity lies in between the conductors and
insulators are called as semiconductors. The semiconductors behave like
insulators at O°K, because no electrons are available in the conduction band.
As the temperature is increased or small amount of energy is applied more
valence electrons cross over to the conduction band, and the conductivity of
the material increases.
TYPES OF SEMICONDUCTORS
In every element, the atoms are tied
together by lie bonding action of valence electrons. Si and Ge atoms contain
only 4 valence electrons. These electrons have a tendency to fill the last
outermost orbit,. In this way the electrons placed in the last orbit of an atom
share the electrons with their neighbouring atoms. Similarly all electrons are
tied together with their neighbouring atoms. For this, they form a bond, Called
co-valent bond.
The semiconductors are classified as
follows.
A pure semiconductor is called intrinsic
Semiconductor. The silicon and germanium atoms
Contains only four electrons in the outermost
orbit. So they are called tetravalent atoms. The co-valent bond structure of
germanium atom is shown in the given figure.
At low temperature (Q0 K), the
semiconductor behaves as a perfect insulator. Now no electrons get away from
the co-valnt bond. So the current flow (electron flow) is zero. At room
temperature, some of the valence electrons may acquire sufficient energy. The
bonds may be broken, the electrons become free and are shifted to the
conduction band, as shown in the given figure.
The motion of electrons constitutes
.electron current. The vacancy created by this electron in the valence band is
known as holes, and acquires a positive charge. The combination of electron and
hole is known as electron-hole pair. In the intrinsic semiconductors, the
number of electrons is equal to the number of holes. The amount of current flow
depends upon the number ‘of electron-hole pairs broken, depends upon the
applied electric field (voltage).
When an external electric field is applied
across the intrinsic semiconductor, more number of
Electron-hole pair combinations will be
broken. According to the amount of electric field, many free electrons are
generated in the valence band. The free electrons are moved to the positive
potential through holes, called electron current. Now the holes are moved
towards the negative potential, called hole current. The Slim of electron
current and hole current is known as electric current.
EXTRINSIC SEMICONDUCTORS
The electrical conductivity of pure
semiconductor is increased by adding some impurities, in it. The
Resultant semiconductor is called extrinsic
semiconductor the process of adding impurities to a pure semiconductor is known
as doping. The purpose of adding impurities in the pure semiconductor is to
Increase the number of free electrons or
holes, for
Increasing their conductivity.
The extrinsic semiconductors are divided
into Two types. They are N-type semiconductors and P-type semiconductors.
N-TYPE SEMICONDUCTOR
N-type semiconductor is formed by adding a
Small amount of pentavalnt impurities (such as Arsenic, antimony or
phosphorous) to a semiconductor (such as Silicon or Germanium) material. The
added Impurities are called donar impurities because they will donate
electrons.
Germanium atom has four valence electrons,
and antimony has five valence electrons. The antimony atom forms co-valent
bonds with their surrounding four germanium atoms. The co-valent bond structure
energy band diagram of N-type semiconductor is shown in the given figure. the
four valence electrons of antimony ‘atom form co-valent bonds with four valence
electron of individual germanium atom. The fifth valence electron of antimony
is left free, loosely bound to antimony atom.
This loosely bound electron can be easily
excited from the valence band to the conduction band by the application of
small electric field. The extra electron creates impurity because it can donate
one
Electron for conduction. Thus the addition
of pentavalent impurities increases the number of electrons in the conduction
band, thereby increasing the conductivity of the semiconductor. Now the
semiconductor contaj1js more electrons and less holes. Hence it is Called
N-type semiconductor. So the electrons are called majority carriers and holes
are called minority carriers.
P-TYPE SEMICONDUCTOR
P-type semiconductor is formed by adding a
small amount of trivalent impurities(such as Aluminium or Boron) to a pure semiconductor
(such as Silicon or Germanium) material. Three valence electrons in aluminium
form co-valnt bond with four surrounding atoms of Ge. Now one co-valent bond is
incomplete, which gives rise to a hoJe. The co-valnt bond structure and energy
band diagram are shown in the Given figure.
For this, more number of holes (positive
charge) are generated. The holes increase the conductivity of the P-type
semiconductor. This impurities are known as acceptor impurities, because the
holes created can accept electrons. The number of holes is more than the number
of electrons. In P-type semiconductors holes are majority carriers and
electrons are minority Carriers.
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