Fermi level in intrinsic and extrinsic semiconductors.

fermi level in intrinic and extrinsic semiconductors ..

please explain me that what is the physical meaning of the statements that with increase in temperature the fermi level moves downwards .. and with increase in donor level the fermi level move upwards.

Re: fermi level in intrinic and extrinsic semiconductors ..

The difference between an intrinsic semi. and extrinsic semi. is the amount of impurities or dopants. Pure semi's like type IV; Silicon (Si) or Germanium (Ge) and type III-V ; like GaAs or GaN use different dopants for each type.
The dopants uses as donors raise the fermi level and make it less temperature dependant.

A common dopant for n-type silicon is phosphorus.
In an n-type semiconductor, the Fermi level is greater than that of the pure intrinsic semiconductor and lies closer to the conduction band than the valence band.

A common p-type dopant for silicon is boron. For p-type semiconductors the Fermi level is below the pure intrinsic Fermi level and lies closer to the valence band than the conduction band.

Since heat is an energy source, the use of dopants makes the semi-conductor less temperature sensitive to conduction or insulation with electrical bias and more dependent on the ideal donors or acceptors used to "contaminate" or dope the pure semiconductor. The dopants add more mobile electrons and holes to make the flow of charges in the lattice by electrical bias rather than controlled by temperature.

Although temperature still has an effect of reducing the forward voltage on PN junctions with rising temperature which reduces the incremental resistance somewhat with an applied voltage. But it tends to increase the conduction resistance in MOSFETs with rising temperature.