# MOS capacitor flatband voltage

1. May 21, 2006

### antonantal

Why is the flatband voltage of a MOS capacitor equal to the voltage corresponding to the energy difference between the work functions of the semiconductor and the metal of the gate? In other words, why and how are the energy bands bent when the gate-source voltage is zero?

2. May 23, 2006

### URI

Hello,
i don't know if I've understood you right, because my english is not as good as it should be

So, i try to explain like i've understood you.

Well, there are two kinds of MOSFET
First the enhancement transistor and then the depletion transistor.

The second one transistor will conduct if U_GS is either positiv or negativ << and not zero >>!
But at this point i'm not sure, so i'll repost when i've read my study documents at home and you still need it.

I'll explain the functionality of a MOSFET to you later, because i don't know if this is the answer that you need and this theme is too big and complicated for answering for nothing.

So, when this is what you are searching for, post again.

regards URI

______________________________
hope my english is not too bad.
My knowledge of electricity is not as bad, but my english :uhh: !
So if you notice gramatical mistakes please improve me.

3. May 23, 2006

### antonantal

Thanks, but my question is not "how does a MOSFET work?". It is much more specific.

4. May 23, 2006

### es

I am not exactly sure what you are asking but I think the answer is, because that's how the physicists chose to define flatband voltage. I mean, a voltage is the potential difference between two points, so those are the points they picked.

For why bands bend see section 2.3.5 of

Is gate and source, the anode and cathode of the MOS cap? Is so then,
for how see Figure 3.2.3 or Figure 3.2.4 with Va=0.

Hope that helps...

5. May 23, 2006

### antonantal

Thanks es. That answered my question.

6. Nov 15, 2010

### sandipta

If you draw the band diagram of MOS structure you can see a band bending of semiconductor region. The bending of bond depends on the work function difference of metal and semiconductor to make flat of the bend bond there is a requirement of voltage which makes generation of charge which makes a bend of energy level in opposite direction that makes a flat band.

7. May 3, 2011

### andy2c

I realise this is an old thread, but I came across it when I was looking on Google, so thought I would add a bit incase it helps any students etc. looking here for an answer.

The band structures in the different materials are different, which I guess is obvious. But in semiconductors it is a very close match across the various interfaces. A simplistic way to think about this is to imagine the mismatch in the band structure between the metal gate and the bulk silicon. the flatband voltage is the voltage applied to the gate electrode in an effort to try and line up the band structure across the different materials, so that the potential distribution going from the metal, to the oxide, to the bulk does not change, it is flat (hence flatband voltage)

Flat band voltage is effected by a few perameters in the device, but generally it is the difference in work functions between the metal and the bulk silicon. As the work function is related to energies in the band structures they can be described in terms of potential and so result in an offset voltage to compensate, i.e. so the zero (or low voltage condition) is actually a few tenths of a volt bigger than or less than 0V.

Please note: other device perameters can effect the flatband voltage such as oxide charge.