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[SOLVED] For MOS Cap curve, what does the inversion region in MOS when VGB > Vth?

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mpig09

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For MOS Cap curve, does the inversion region in MOS when VGB > Vth?

Hi all:

How to explanation the green region of MOS C-V curve when the VGB > Vth?

Thanks.

mpigView attachment 139180
 

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erikl

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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).

Thermal generation (s. above) is a rather slow process. So you'll get the curve segment in the green rectangle (i.e. the increasing capacitance in the inversion region) only from a slow DC sweep. From a fast DC sweep you'd get the shown green segment, and the segment called HF you'd get if you hold the corresponding inversion DC voltage constant, and measure the capacitance using a small superimposed RF (HF). (This segment - shown as constant vs. Vg - in reality should also go downwards with increasing Vg - I think -, because the depletion zone still extends).

For the latter 2 segments, thermal generation isn't fast enough to supply enough electrons for strong inversion, so the depleted serial capacitor is still there and reduces the oxide capacitance.
 
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mpig09

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Hi Erikl:

1. Is there an inversion layer between the Drain and Source terminal when VGB a little bigger than Vth?

Thermal generation (s. above) is a rather slow process.
2. what region operates the thermal generation? (drain or source or ...)

3. the inversion layer is composed of the electrons that from the thermal generation?

mpig
 

erikl

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1. Is there an inversion layer between the Drain and Source terminal when VGB a little bigger than Vth?
VGB > Vth is the definition for a conductive channel due to an inversion layer between source an drain.

2. what region operates the thermal generation? (drain or source or ...)
Thermal generation/recombination occurs everywhere in a semiconductor. It constitutes the main part of leakage current. It increases exponentially with temperature.

3. the inversion layer is composed of the electrons that from the thermal generation?
Yes, and they get attracted to the SiO2/Si interface by the positive VGB (for NMOSFETs), there forming the inversion channel connecting drain & source.
 

mpig09

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Hi erikl:

I try to describe what I understand:

The thermal generation/recombination occurs everywhere in the semiconductor all the time.
For P-type sub, the electrons are attracted to the SiO2/Si interface by the positive VGB (NMOS), and the inversion layer between Drain and Source is formed. The capacitance is formed.(top plate is metal / bottom plate is inversion layer / dielectric is SiO2)

The difference increased speed of VGB will get the difference capacitance (difference C-V curve when VGB > Vth).

If I am wrong, please correct me.

mpig
 

erikl

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I think your understanding now is right.
 

mpig09

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Hi erikl:

Thanks for your help.

mpig
 

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