- Semiconductors are the basic materials used in the present solid state
electronic devices like diode, transistor, ICs, etc.
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- Lattice structure and the atomic structure of constituent elements
decide whether a particular material will be insulator, metal or
semiconductor.
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- 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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- Semiconductors are elemental (Si, Ge) as well as compound (GaAs,
CdS, etc.).
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- 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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- 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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- In n-type semiconductors, ne
nh
while in p-type semiconductors nh
ne
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- 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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- ne
nh
= ni
2
in all cases. Further, the material possesses an overall charge
neutrality.
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- 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
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heat, light or electrical energy to the conduction band and thus, produce
a change in the current flowing in a semiconductor.
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- For insulators Eg > 3 eV, for semiconductors Eg
is 0.2 eV to 3 eV, while
for metals Eg
ª 0.
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- p-n junction is the ‘key’ to all semiconductor devices. When such a
junction is made, a ‘depletion layer’ is formed consisting of immobile
ion-cores devoid of their electrons or holes. This is responsible for a
junction potential barrier.
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- By changing the external applied voltage, junction barriers can be
changed. In forward bias (n-side is connected to negative terminal of the
battery and p-side is connected to the positive), the barrier is decreased
while the barrier increases in reverse bias. Hence, forward bias current
is more (mA) while it is very small (mA) in a p-n junction diode.
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- Diodes can be used for rectifying an ac voltage (restricting the ac voltage
to one direction). With the help of a capacitor or a suitable filter, a dc
voltage can be obtained.
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- The energy bands (EC
or EV
) in the semiconductors are space delocalised
which means that these are not located in any specific place inside the
solid. The energies are the overall averages. When you see a picture in
which EC
or EV
are drawn as straight lines, then they should be
respectively taken simply as the bottom of conduction band energy levels
and top of valence band energy levels.
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- In elemental semiconductors (Si or Ge), the n-type or p-type
semiconductors are obtained by introducing ‘dopants’ as defects. In
compound semiconductors, the change in relative stoichiometric ratio
can also change the type of semiconductor. For example, in ideal GaAs
the ratio of Ga:As is 1:1 but in Ga-rich or As-rich GaAs it could
respectively be Ga1.1 As0.9 or Ga0.9 As1.1. In general, the presence of
defects control the properties of semiconductors in many ways.
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