Donor concentrations between the Gate and Drain

Question

When attempting to determine whether a MOSFET is NMOS, PMOS, or at intrinsic concentration, assuming the Drain and Gate are dopped to Nd = X and the bulk is doped to Na = Y, I get a little confused.

My initial thought is that Nd = Na, regardless of two regions of Nd existing (drain is at Nd, Source is at Nd), thus since Nd = Na the whole MOSFET is at Na = Nd = Ni, is this wrong?

Answer 1

First let's link Nd (donor concentration) and Na (acceptor concentration) to N-type and P-type:

A donor adds an electron so we get N-silicon

An acceptor adopts an electron so we get P-silicon.

So:

Nd => N-type (your X)

Na => P-type (your Y)

If Na = Nd then all you end up with intrinsic (Ni) silicon as the two compensate for eachother.

Then just remember this image:

For an NMOS an N-channel is formed in a P-substrate

For a PMOS a P-channel is formed in an N-substrate (called N-well here but it is still N-type).