Classification of solids into Metals, Semiconductors and Insulators:
Based on the energy band structure, arrangement of electrons and forbidden band, solid materials classified
into the following three categories:
a) Conductors(Metals)
b) Insulators
c) Semiconductors
Conductors:
A material which conducts electric current when a potential
difference is applied across them is known as conductors. In case of
conductors, the valence band is completely filled while conduction band is
half filled as shown in figure. When a small potential difference is applied to a
solid material, it provides sufficient energy to the electron in the valence
band to shift to the conduction band. The shifting of electrons from the
valence band to the unfilled conduction band results in the flow of current in
the material.
Eg: Copper, Lithium, Aluminum, etc..
Insulators:
Solid materials which do not conduct electric current under normal
conditions are known as insulators. In insulators, the valence band is
completely filled and it has no electrons in the conduction band. The
forbidden energy gap will be very high when compared with conductor. The
energy required to shift an electron from the valence band to the
conduction band to make electrical conduction is very high. Hence it is not
possible to provide enough energy by an ordinary electric field. We can
achieve electric conduction in an insulator with very high voltage known as
breakdown voltage.
Semiconductors:
Semiconductors are materials whose electrical conductivity
lies between that of conductors and insulators. The conductivity of
semiconductors is in the order of 104 to 10-4 mho.m-1 . The magnitude
of forbidden energy gap of a semiconductor lies in between the
forbidden energy gap of insulators and conductors.
Semiconductors such as silicon, germanium, Gallium
Arsenide(GaAs) , Cadmium Sulphide(CdS), are compound
semiconductors. Some of oxide semiconductors are Bi2O3, Te2O3, ZnO3,
Cu2O, etc,.
Padmasri Dr. B.V.Raju Institute of Technology, Narsapur, Medak Dist
, Engineering Physics Date:
SEMI CONDUCTOR PHYSICS
Introduction:
Semiconductors are associated with relatively narrow forbidden bandgap, for example, Germanium has
0.7eV, Silicon has 1.01eV and GaAs has 1.5eV. Semiconductors are classified as elemental semiconductors and
compound semiconductors. Silicon and Germanium are example for elemental semiconductors. Compound
semiconductors are made of two or three elements. Some of the simple compound semiconductors are: III – IV
group compound semiconductors like GaAs, GaP, GaN, InSb, ….. and II – VI group compound semiconductors like
CdSe, PbTe. Ternary semiconductors are formed by the addition of small quantity of the third element of the
compound semiconductors can be extended to include quaternary materials such as GaxIn1-xAsyP1-y / GaInNAs.
Semiconductors are divided into two categories. They are 1. Intrinsic 2. Extrinsic semiconductors. Intrinsic
semiconductors are pure semiconductors. Extrinsic semiconductors are impurity doped semiconductors.
Intrinsic semiconductors:
The intrinsic semiconductors are insulators at 0K. The valence band is completely filled and the conduction
band is completely empty. The Fermi level in these semiconductors lies midway between the bottom of conduction
band and the top of the valence band. At any temperature above 0K, some electrons transfer to empty levels of
conduction band. The formation electron-hole pair is a characteristic of a semiconductor.
At any temperature, some electrons would be present in the conduction band and equal number of holes would be
present in the valence band.
Extrinsic Semiconductor:
Extrinsic semiconductors are of two types. They are 1. N-type semiconductor 2. P-type semiconductor.The N-
type semiconductors are obtained by doping the intrinsic semiconductors with pentavalent atoms like Arsenic,
Phosphorous and Antimony. Bond satisfaction need only four electrons to be shared and extra electron stays
attached to the lattice. It requires very little energy to take an electron into the conduction band.
The energy level is
represented by Ed. And if
electrons go into the
conduction band, the
donar atoms are ionized
indicated by Nd+ . The
donor levels are placed
just below the conduction
band Ec.
P-type semiconductors are
obtained when trivalent atoms like Boron, Aluminum, Gallium or Indium are doped. Only three of the four bonds
needed are satisfied and the missing electron is a hole. The impurity accepts electrons and therefore called
acceptors.
Padmasri Dr. B.V.Raju Institute of Technology, Narsapur, Medak Dist
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