Fermi level in n-type or p-type semiconductors

[Cathyzhang]

Hi, everyone. I'm learning basic theories about semiconductors but can't quite understand the concept of Fermi level. is this just a imaginary energy level or true existence? and why Fermi level is close to the conduction band of n-type semiconductor and valence band of p-type semiconductor? the last question is, if applying a potential on the semiconductor materials, the Fermi level will shift up and down with the increase or decrease of the voltage-why?
thanks _Cathy

 

[mfb]

is this just a imaginary energy level or true existence?

It does not have to be a possible energy level for electrons.
"imaginary" versus "true" is not physics.

and why Fermi level is close to the conduction band of n-type semiconductor and valence band of p-type semiconductor?

I think this is easier to see if you imagine an additional energy level in the bandgap. Where would it have to be to be 50% filled?
For n-type semiconductors, close to the conduction band, as you have additional electrons there.
For p-type semiconductors, close to the valence band, as you have additional holes there.

the last question is, if applying a potential on the semiconductor materials, the Fermi level will shift up and down with the increase or decrease of the voltage-why?

A voltage changes electron energies.

 

[Cathyzhang]

It does not have to be a possible energy level for electrons.
"imaginary" versus "true" is not physics.

I think this is easier to see if you imagine an additional energy level in the bandgap. Where would it have to be to be 50% filled?
For n-type semiconductors, close to the conduction band, as you have additional electrons there.
For p-type semiconductors, close to the valence band, as you have additional holes there.

A voltage changes electron energies.

Thanks! that's help :)