Question

a) Plot the structure of p-channel MOS-FET (Show semiconductor structure, gate, drain, source
and voltage polarities) and summarize the working principle.
b) Explain what happens if you change drain source voltage polarity.
c) Plot the band diagram, approximate distributions of charge, electric field, and electrostatic potential in
the ideal MOS capacitor in inversion for the p-channel case.

Answer 1

a)We know that,a P-Channel MOSFET is composed of a majority of holes as current carriers. When the MOSFET is activated and is on, the majority of the current flowing are due to the presence of holes moving through the channels.

Note that in order to turn on a P-Channel Enhancement-type MOSFET,we need to  apply a positive voltage VS to the source of the MOSFET and apply a negative voltage to the gate terminal of the MOSFET (the gate must be sufficiently more negative than the threshold voltage across the drain-source region (VGDS). This makes sure that a current will flow through the source-drain channel.

Therefore we see that with a sufficient positive voltage,also and sufficient negative voltage applied to the gate, the P-Channel Enhancement-type MOSFET is fully functional and is the the ON mode.

In the sketch below I have labbeled drain as D,Source as S and gate as G.

b)We know from the characteristics in general that greater the voltage difference from the Source the more the MOSFET can conduct. Usually we see that the Source pin must be more positive than the Drain. However since we are discussing P -channel MOSFET this is not the case since in this case there is reverse polarity protection already built-in.

c)MOS capacitance vs voltage for P-type has the following plot

Now Ei-Ef is related to the potential as seen below

ΦF is positive for p channcel

suppose that ΦF=ΦS for instance with NA=10^17 cm^-3 approximately