Question about Fermi Levels at equilibrium

Hi,

I know that in a pn junction, at equalibrium, the Fermilevels in the p and n sides reach the same level, because the elcetron will flow from both sides to the other side, untill the fremi - levels become equal.

Now, I want to know, as to why, at equilibrium, the fermi levels in the metal and semiconductor are equal in a MOS capacitor. The oxide cannot support the flow of electrons from either side. Then how the fermi levels become equal ????

Re: Fermi Levels

Fermi level is the minimum energy level that an lectron could achieve at absolute zero, as in MOS FET , at thermal equillibrium, no electron flows through semiconductor to metal through oxide or viceversa, that mean the fermi level are merged.

Becz if there were difference in fermi level that mean difference in energy level and elctron must folw, so the fermi level is same in metal oxide and semiconductor.

but there is difference in free energy level and conduction band amongs this three layer.

Re: Fermi Levels

According to me, the equalization of fermi levels in MOSFET, in equilibrium condition, happens because of the formation of image carriers in the metal region
for better understanding i would suggest you to refer BG Streetman.

Re: Fermi Levels

If the workfunctions are different for the metal (say Al) and silicon, then How come the fermi levels become same at equilibrium. I know that band bending happens and make the fermi-leveals same ?

What causes the bands to bend and make the fermi levels equal, when no current can flow through oxide.

In non-oxide junctions, the electron transfer takes place to make the fermi level equal......how the fermi -levels become equal in MOS cap. ?

Re: Fermi Levels

check the attachment i think that will help. tahnk you.

Re: Fermi Levels

I don't think the repliers understood alok's question here. I had been asking myself the same question when studying semi-conductor physics. Unfortunately none of my books provided me with an answer (I have Neamen, Pierret and Muller & Kamins). This is what I remember:

For the fermi level of two materials in contact to become equal, some charge has to flow (diffuse) from one material to the other. The direction of the flow for electrons is from the material with high fermi level to the material with low fermi level. For the holes it is the other way around. This flow of charge causes a depletion region with an accompaning voltage difference across it. This voltage difference is just enough to lift the (initially) low fermi energy to the level of the high fermi level.

And like alok, I could not figure out how this flow of charge happened in a MOS (between metal and semiconductor). The only thing I could think of was tunneling through the insulator. But my intuition said that this would be too slow.

Fermi Levels

Analog Guru's thinking is the same as mine.
Can somebody help me understand this fermi level doubt ???

Fermi Levels

well , i am not sure but lets talk about that,
*the MOS is called MOS cap >>> where a cap. as far as i know allow displacment current to pass therefore there maybe no problem .

*i think that the idea is not that charges flow but that the charges will exist at the surface of the metal and the semiconductor and these are opposite charges <just like a cap.>
and when u apply voltage across the MOS cap the only thing that will happen is the amount of surface charges both at the metal and semiconductor.