Re: For MOS Cap curve, does the depletion region in MOS when VFB < VGB < 0?
Hi erikl :
I know the depletion region is light green.
Could you help me to know the depletion cap structure? like Cox.
I wish to know the two plates of Cdep.
I think the Ctol = Cox serial Cdep in the depletion region that means the structure is:
one plate is gate
other plate is bulk
The distance between the gate and the bulk is tox (the width of the oxide) + Wdep (the width of the depletion region).
==>The distance is increased, so the cap value is decrease.
If I am wrong, please correct me.
Thanks.
mpig
You are very close, but not right there.
The total thickness - Tox plus Tdep - contains two distinctively different materials (dielectric/oxide, and depleted Si) - that have different dielectric constants (eps~4.2 in SiO2, and eps ~11.9 in Si).
That's why it is meaningful to treat these two regions as two capacitors connected in series, rather than one capacitor.
- - - Updated - - -
Erikl - you are confusing hole with an energy state in atom.
The holes are mobile (as mobile as electrons - with somewhat different mobility), atoms are fixed and not moving (after device is formed).
Hole is kind of a missing electron in the valence band.
Mobile electrons are in the conduction band.
Electrons normally do not hop from one acceptor atom to another (unless acceptor concentration is so high that they form a, energy band where electrons can move - this normally does not happen in semiconductor devices).
Your explanation of depletion region is wrong.
In neutral p-type semiconductor, concentration of (free) holes is equal to concentration of ionized (negatively charged - by capturing electrons from the valence band) acceptors - that's why it is electrically neutral, charge density is zero.
When positive voltage is applied to the gate, the holes are pushed away from the dielectric/semiconductor interface, and depletion regions is formed - which contains no holes, but contains negatively charged acceptors.
There is no mobile electrons in p-type neutral region (it is extremely low).
Electrons can appear in the depletion region due to thermal generation (electron excitation from the valence band, across band gap, into conduction band).
Holes are not fixed, they are mobile!!!