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n-channel enhanecement MOSFET: why is it called an n-type inversion layer?

In this MOSFET we have a p-type substrate, and n+ source/drain.

When we apply a +ve voltage to the Gate, we repel holes (the majority charge carrier) in the p-type substrate downwards, thus uncovering fixed acceptor ions/atoms which are now negativly charged (electrons in the substrate are drawn to this region under the Gate).

We thus create a layer of fixed negative ions (a acceptor atom with a electron) and Sedra calls this newly formed structure a n-type inversion layer.

But in n-type materials we normally have fixed donor atoms that have donated an electron and are now +ve ions.

So.. n-type == '+ve dopant ion, plenty of e' inversion == '-ve dopant ion'

Therefore, what is our basis for calling the channel a n-type layer and stating that it is conducive to the flow of electrons?

Electrons ought to be repelled by the -ve Ions except that there's a larger attractive force (+ve field) but then again that field is directed downwards and not sideways and without charge-carriers in the channel.. additionally somehow you would be attracting minority charge carriers (electrons)..

I suppose there is a mathematical and complicated explanation in Sze for all of this but could someone explain this more simply so I do not need to memorize blindly.

Reply to
Veek M
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It's like applying suction to the skin and drawing blood to the surface. And, do that for long enough and hard enough, and you'll get sweat and plasma and blood through the skin too... which is conductive! So there you go. ;-)

The dopant terminology refers to the charge carriers, so N type has electrons and such. Thus, a positive charge near the surface, will bring electrons to the surface, inducing a surface N effect, even if the bulk (and the ions contained within) is P type.

The key is, it's a very localized phenomenon: whereas BJTs work fine over distances of microns (e.g., base thickness, limited by charge diffusion length), the MOS induced surface layer is limited by stuff like Debye shielding distance, on the order of 10s to 100s of nm.

Which is one strong limitation on the current density of MOSFETs, why they always used to be larger, more expensive, and poorer performing, than BJTs. Until fabs finally got finer feature sizes, and new vertical MOS structures were developed (VDMOS, TrenchMOS, and now SuperJunction as well), to push current densities up near the same range.

Tim

-- Seven Transistor Labs Electrical Engineering Consultation Website:

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Reply to
Tim Williams

thanks - i got it

Reply to
Veek M

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