What is Semiconductor and types of Semiconductor ?
Semiconductors
The materials whose electrical conductivity lies between those of conductors and insulators, are known as semiconductors.
Silicon 1.1 eV
Germanium 0.7 eV
Cadmium Sulphide 2.4 eV
Silicon is the most widely used semiconductor.
NOTE:- Semiconductors have negative temperature coefficients of resistance, i.e. as temperature increases resistivity decreases
Energy Band Diagram
 |
Insulator
|
 |
| Metal |
 |
Pure Semiconductor
(room temperature) |
Forbidden energy band is small for semiconductors.
Less energy is required for electron to move from valence to conduction band.
A vacancy (hole) remains when an electron leaves the valence band.
Hole acts as a positive charge carrier.
Types of Semiconductors
- Intrinsic Semiconductor
- Extrinsic Semiconductor
Intrinsic Semiconductor
A semiconductor, which is in its extremely pure form, is known as an intrinsic semiconductor. Silicon and Germanium are the most widely used intrinsic semiconductors.
Both silicon and germanium are tetravalent, i.e. each has four electrons (valance electrons) in their outermost shell.
Each atom shares its four valence electrons with its four immediate neighbours, so that each atom is involved in four covalent bonds.
When the temperature of an intrinsic semiconductor is increased, beyond room temperature a large number of electron-hole pairs are generated.
Since the electrons and holes are generated in pairs so,
Free electron concentration (n) = concentration of holes (p) = Intrinsic carrier concentration (ni)
Extrinsic Semiconductor
Pure semiconductors have negligible conductivity at room temperature. To increase the conductivity of intrinsic semiconductor, some impurity is added. The resulting semiconductor is called impure or extrinsic semiconductor.
Impurities are added at the rate of ~ one atom per 10^6 to 10^10 semiconductor atoms. The purpose of adding impurity is to increase either the number of free electrons or holes in a semiconductor.
Two types of impurity atoms are added to the semiconductor
Atom containing 5 valance electrons
(Pentavalent impurity atoms)
e.g. P, As, Sb, Bi
N-type semiconductor
Atoms containing 3 valance electrons(Trivalent impurity atoms)
e.g. Al, Ga, B, In
N-type Semiconductor
The semiconductors which are obtained by introducing pentavalent impurity atoms are known as N-type semiconductors.
Examples are P, Sb, As, Bi . These elements have 5 electrons in their valance shell. Out of which 4 electrons will form covalent bonds with the neighbouring atoms and the 5th electron will be available as a current carrier. The impurity atom is thus known as donor atom.
In N-type semiconductor current flows due to the movement of electrons and holes but majority of through electrons. Thus electrons in a N-type semiconductor are known as majority charge carriers while holes as minority charge carriers.
P-type Semiconductor
The semiconductors which are obtained by introducing trivalent impurity atoms are known as P-type semiconductors.
Examples are Ga, In, Al and B . These elements have 3 electrons in their valance shell which will form covalent bonds with the neighbouring atoms.
The fourth covalent bond will remain incomplete. A vacancy, which exists in the incomplete covalent bond constitute a hole. The impurity atom is thus known as acceptor atom.
In P-type semiconductor current flows due to the movement of electrons and holes but majority of through holes. Thus holes in a p-type semiconductor are known as majority charge carriers while electrons as minority charge carriers.
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