1. Semiconductors are the basic materials used in the present solid state
electronic devices like diode, transistor, ICs, etc.
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2. Lattice structure and the atomic structure of constituent elements
decide whether a particular material will be insulator, metal or
semiconductor.
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3. Metals have low resistivity (10–2 to 10–8 Wm), insulators have very high
resistivity (>108 W m–1), while semiconductors have intermediate values
of resistivity.
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4. Semiconductors are elemental (Si, Ge) as well as compound (GaAs,
CdS, etc.).
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5. Pure semiconductors are called ‘intrinsic semiconductors’. The presence
of charge carriers (electrons and holes) is an ‘intrinsic’ property of the
material and these are obtained as a result of thermal excitation. The
number of electrons (ne
) is equal to the number of holes (nh
) in intrinsic
conductors. Holes are essentially electron vacancies with an effective
positive charge.
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6. The number of charge carriers can be changed by ‘doping’ of a suitable
impurity in pure semiconductors. Such semiconductors are known as
extrinsic semiconductors. These are of two types (n-type and p-type).
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7. In n-type semiconductors, ne
>> nh
while in p-type semiconductors nh
>> ne
.Ans
8. n-type semiconducting Si or Ge is obtained by doping with pentavalent
atoms (donors) like As, Sb, P, etc., while p-type Si or Ge can be obtained
by doping with trivalent atom (acceptors) like B, Al, In etc.
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9. ne
nh
= ni
2
in all cases. Further, the material possesses an overall charge
neutrality.
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10. There are two distinct band of energies (called valence band and
conduction band) in which the electrons in a material lie. Valence
band energies are low as compared to conduction band energies. All
energy levels in the valence band are filled while energy levels in the
conduction band may be fully empty or partially filled. The electrons in
the conduction band are free to move in a solid and are responsible for
the conductivity. The extent of conductivity depends upon the energy
gap (Eg
) between the top of valence band (EV
) and the bottom of the
conduction band EC
. The electrons from valence band can be excited by