Write Fermi Dirac distribution function. With the help of diagram. Explain the variation of Fermi level with temperature in n-type semiconductor.
Each energy band in a crystal accommodates a large number of electron energy levels. According to Pauli’s exclusion principle any energy level can be occupied by two electrons only, one spin up and down . however, all the available energy states are not filled in an energy band.
The separation between the consecutive energy level is very small around 10-27 eV due to which the energy states are not filled in an energy band.
FERMI DIRAC DISTRIBUTION FUNCTION.
The carrier occupancy of the energy states is represented by a continuous distribution function known as the Fermi-Dirac distribution function, given by
`f(E) = 1/(1+e^((E-E_F)/(kT))`
This indicates the probability that a particular quantum state at the energy level E is occupied by an electron. Here k is Boltzmann’s constant and T is absolute temperature of the semiconductor. The energy EF called Fermi energy that corresponds to a reference level called Fermi level.
IN n-TYPE SEMICONDUCTOR.
⦁ At 0K the fermi level EFn lies between the conduction band and the donor level.
⦁ As temperature increases more and more electrons shift to the conduction band leaving behind equal number of holes in the valence band. These electron hole pairs are intrinsic carriers.
⦁ With the increase in temperature the intrinsic carriers dominate the donors.
⦁ To maintain the balance of the carrier density on both sides the fermi level EFn gradually shifts downwards.
⦁ Finally at high temperature when the donor density is almost negligible EFn is very close to EFn .