PMOS/NMOS current direction and digital logic

Question

What happens when the PMOS source is connected to negative Vcc (-Vcc).
What I understand is that when the gate voltage is <=0 then the drain-source is connected. Normally I would expect current to flow from source to drain but since the source is connected to -Vcc. Is this correct? Also, how can I think the behaviour for of multiple MOSFET connected together.
ie I have the PMOS as described above and at the drain another PMOS is connected. Assumming that the above statement is correct and current flows from drain to source does that mean that the second PMOS will be OFF?

I hope this is not very confusing...I am new to this.

Answer 1

Here is a typical PMOS circuit. I drew in the body diode so you can see which way it points. Note how the body diode in NMOS points in the opposite direction from the body diode in PMOS.

This circuit works as follows: When VCC is present, M2 turns ON and pulls down the gate of M1, which then turns M1 on.

When VCC is not present, the gate of M2 is low, so M2 is off, which means that M1's gate will be high (if the external signal is high) and M1 will be off. R1 ensures that M1 will remain off unless M2 is on.

MOSFET's are controlled by the gate-to-source voltage. For NMOS, driving Vgs high turns it on. For PMOS, driving Vgs low (less than zero) turns it on.

I should probably mention that I am talking about enhancement mode MOSFET's, which are the most common ones. I just don't want to talk about depletion mode MOSFET's at all because it might be confusing.

In this circuit, if the drain of M1 is higher than the source, then the body diode will become forward biased and will conduct. M1 cannot block current from flowing into the external signal wire. It just acts like a diode in that direction.

^{simulate this circuit – Schematic created using CircuitLab}