Download engineering physics ii semiconducting materials

ENGINEERING PHYSICS II
SEMICONDUCTING MATERIALS
2.14 VARIATION OF FERMI ENERGY LEVEL WITH TEMPERATURE AND
IMPURITY CONCENTRATION IN P-TYPE SEMICONDUCTOR
 When the temperature is increased, some of the electrons in the valence band will go to
acceptor energy levels by breaking up the covalent bonds and hence the Fermi level is
shifted in upward direction for doping level of Nd = 1021 atoms/m3 as shown in figure.
 From the figure it can be seen that for the same temperature, if the impurity atoms doping
level is increased say Na = 1024 atoms/m3, the hole concentration increases and hence the
Fermi level decreases.
 Therefore at low temperature the Fermi energy level may be increased upto the level of
intrinsic energy level (Ei)
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ENGINEERING PHYSICS II
SEMICONDUCTING MATERIALS
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ENGINEERING PHYSICS II
SEMICONDUCTING MATERIALS
2.15 VARIATION OF CARRIER CONCENTRATION WITH TEMPERATURE
 The carrier concentration in n-type semiconductor varies with the increase in the
temperature.
Electron concentration
 At very low temperature at 0 K, 1/T is high, the Fermi level will be exactly in the middle
between Ec and Ed and the carrier concentration is at the most zero in the conduction band
and hence point A is obtained in the graph.
 Now when the temperature is slowly increased, the donar atom gets ionized and the
electrons move towards conduction band.
 Hence the carrier concentration increases slowly in the conduction band for electrons.
 Since this range is obtained due to impurity atoms, it is called impurity range shown by
curve AB in figure.
 The range is obtained due to exhaustion of donor atoms it is called exhaustion range, shown
by curve BC in figure.
 Since the number of available electrons in donor energy level is almost exhausted, many
numbers of electrons are shifted from valence band to conduction band and thus the carrier
concentration increases rapidly, tracing the curve CD.
 Since the material practically becomes intrinsic in this range, this range is known as
intrinsic range.
Hole Concentration
 At 0K electrons concentration is zero in conduction band.
 Now when temperature is increased slowly the electrons will move from donor energy level
to conduction band.
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ENGINEERING PHYSICS II
SEMICONDUCTING MATERIALS
 Now when temperature is very high the electrons are transferred from valence band to
conduction band and holes are created in valence band.
 The electron concentration in conduction band equalizes the hole concentration in valence
band.
 So electron concentration curve and hole concentration curve overlaps only in the intrinsic
region.
Conductivity of Extrinsic semiconductor
 The variation in the conductivity of an extrinsic semiconductor, with the increase in
temperature is as shown in figure.
 When the temperature is slowly increased from 0K impurity atoms are slowly ionized and
goes to conduction band and hence conductivity increases.
 When all the impurities are ionized the mobility of charge carriers decreases slightly and
therefore logσ becomes constant (or) may slightly decrease.
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