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Q & A: semiconductor Fermi levels

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Most recent answer: 09/01/2013
Q:
1) why the energy band bends near the insulator semi conductor interface in MOSFET? 2)why the fermi level at the metal portion goes upward or backward due to the application of gate potential in a MOSFET? 3)instead of using the insulator between the semi conductor and metal gate in a MOSFET if we directly insert the gate to the channel region then what will happen? is there any advantage using the insulator? 4)for doping by the donar type impurity to a semy conductor the fermi level moves upward and come closer to conduction band. why?"
- soumitra (age 25)
kolkata,westbengal,india
A:

4. Say you have an undoped semiconductor. Since the bulk must be neutral, the Fermi level is just at the right position to leave equal numbers of holes in the valence band and electrons in the conduction band. Now put in a few donors.  These each have an extra positive charge an an extra electron compared to the atoms they replace. The highest bound state on each donor is weakly bound, with energy just a little below the conduction band minimum. If the Fermi level didn't change, then these donor states would only be slightly more occupied than the sparsely occupied conduction band states. That would mean that the number of electrons added wouldn't match the number of added positive charges. Whoops. The Fermi level must have moved up. increasing the occupancy of (mainly) the conduction band and (much less important) the donor states, and decreasing the number of holes in the valence band.

3. If the metal were directly in contact with the doped semiconductor, the gate would draw much more current. That would take away a key feature of a MOSFET- it's ability to amplify a signal with very low current and low power.

2. The electrical potential from the gate doesn't stop abruptly at the edge of the gate but extends out until it's screened by the electrons and holes in the semiconductor. In that screening layer, there must be some charge imbalance. That means that the gate voltage changed the concentration of holes and electrons, so it changed the Fermi level.

1. Maybe here it's best just to point to another source: .

Mike W.


(published on 09/01/2013)

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