Physics for Information Science: Unit II: Semiconductor Physics
Variation of Fermi Energy Level with Temperature and Impurity Concentration in 'n'-Type Semi Conductor
When the temperature is increased some electrons in Ed level may be shifted to conduction band and hence some vacant sites will be created in Ed levels.
VARIATION OF FERMI
ENERGY LEVEL WITH TEMPERATURE AND IMPURITY CONCENTRATION IN 'n'-TYPE SEMI
CONDUCTOR
When
the temperature is increased some electrons in Ed level may be
shifted to conduction band and hence some vacant sites will be created in Ed
levels. Therefore the Fermi level shifts down to separate that empty levels and
the filled valence band level as shown in Fig. 2.17, for the doping level of Nd
= 1021 atoms/m3.
From
the Fig. 2.17 it can be seen that for the same temperature, if the impurity
atoms i.e., doping level is increased (Nd = 1024 atoms/m3),
the electron concentration increases and hence the Fermi level increases.
Note:
Ei is the fermi energy level of the intrinsic semiconductors given
just as reference energy level in order to indicate that the variation of EF
can fall only upto Ei.
Ionisation Energy of Donor
To
find the ionisation energy of donor equation (8) can be written as
ne
= Ce ̄ΔE/2KBT, where C is a constant
If
a graph is plotted between log ne and 1/T we get lines (Fig. 2.18)
for various values of donor impurity levels. From the slope - ΔE/2KB,
the value of ionisation energy of donor can be calculated.
Note:
If T is very high then the slope become -
Eg/2KB as that of intrinsic semiconductor.
Results
•Density
of electrons (ne) in conduction band is proportional to the square
root of donor concentration (Nd).
•When
temperature is increased the Fermi level falls below the donor level and may
approach only upto Ei (Fermi level of the intrinsic semi conductor).
Electrical Conductivity of n-type
We
know the electrical conductivity (σ) is given by
σ
= ne e (μe +μh)
For
n-type semiconductor the acceptors are at the most zero. Therefore the mobility
of charge carriers (holes) is zero. i.e., μh = 0.
σ
=ne e μe
Physics for Information Science: Unit II: Semiconductor Physics : Tag: : - Variation of Fermi Energy Level with Temperature and Impurity Concentration in 'n'-Type Semi Conductor
Physics for Information Science: Unit II: Semiconductor Physics
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