Fermi-level
Fermi level indicates the level of energy in the forbidden gap.
1. Fermi-level for an Intrinsic semiconductor
• We know that the Intrinsic semiconductor acts as an insulator at absolute zero temperature because there are free electrons and holes available but as the temperature increases electron hole pairs are generated and hence number of electrons will be equal to number of holes.
• Therefore, the possibility of obtaining an electron in the conduction band will be equal to the probability of obtaining a hole in the valence band.
• If Ec is the lowest energy level of Conduction band and Ev is the highest energy level of the valence band then the fermi level Ef is exactly at the center of these two levels as shown above.
2. Fermi-level in a semiconductors having impurities (Extrinsic)
a) Fermi-level for n-type Semiconductor
• Let a donar impurity be added to an Intrinsic semiconductor then the donar energy level (ED) shown by the dotted lines is very close to conduction band energy level (Ec).
• Therefore the unbonded valence electrons of the impurity atoms can very easily jump into the conduction band and become free electros thus, at room temperature almost all the extra electrons of pentavalent impurity will jump to the conduction band.
• The donar energy level (ED) is just below conduction band level (Ec) as shown in figure. Due to a large number of free electrons, the probability of electrons occupying the energy level towards the conduction band will be more hence, fermi level shifts towards the conduction band.
b) Fermi-level for P-type semiconductor
• Let an acceptor impurity be added to an Intrinsic semiconductor then the acceptor energy level (Ea) shown by dotted lines is very close to the valence band shown by dotted lines is very close to the valence band energy level (Ev).
• Therefore the valence band electrons of the impurity atom can very easily jump into the valence band thereby creating holes in the valence band.
• The acceptor energy level (EA) is just above the valence band level as shown in figure.
• Due to large number of holes the probability of holes occupying the energy level towards the valence band will be more and hence, the fermi level gets shifted towards the valence band.
Fermi level indicates the level of energy in the forbidden gap.
1. Fermi-level for an Intrinsic semiconductor
• We know that the Intrinsic semiconductor acts as an insulator at absolute zero temperature because there are free electrons and holes available but as the temperature increases electron hole pairs are generated and hence number of electrons will be equal to number of holes.
• Therefore, the possibility of obtaining an electron in the conduction band will be equal to the probability of obtaining a hole in the valence band.
• If Ec is the lowest energy level of Conduction band and Ev is the highest energy level of the valence band then the fermi level Ef is exactly at the center of these two levels as shown above.
2. Fermi-level in a semiconductors having impurities (Extrinsic)
a) Fermi-level for n-type Semiconductor
• Let a donar impurity be added to an Intrinsic semiconductor then the donar energy level (ED) shown by the dotted lines is very close to conduction band energy level (Ec).
• Therefore the unbonded valence electrons of the impurity atoms can very easily jump into the conduction band and become free electros thus, at room temperature almost all the extra electrons of pentavalent impurity will jump to the conduction band.
• The donar energy level (ED) is just below conduction band level (Ec) as shown in figure. Due to a large number of free electrons, the probability of electrons occupying the energy level towards the conduction band will be more hence, fermi level shifts towards the conduction band.
b) Fermi-level for P-type semiconductor• Let an acceptor impurity be added to an Intrinsic semiconductor then the acceptor energy level (Ea) shown by dotted lines is very close to the valence band shown by dotted lines is very close to the valence band energy level (Ev).
• Therefore the valence band electrons of the impurity atom can very easily jump into the valence band thereby creating holes in the valence band.
• The acceptor energy level (EA) is just above the valence band level as shown in figure.
• Due to large number of holes the probability of holes occupying the energy level towards the valence band will be more and hence, the fermi level gets shifted towards the valence band.
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