How Could you determine if a P-Channel MOSFET is on or off?

Question

How Could you determine if a P-Channel MOSFET is on or off?

Answer 1

If you drain to source voltage exceeds |(Vto)| - |(Gate to source voltage)|, and greater than zero, the PMOS is in saturation mode, meaning the transistor channel runs directly to the drain, and the switch is on. Note that this situation cause the PMOS to behave like a Dependant voltage source. The PMOS is in the triode region when |(Vto)| - |(Gate to source voltage)| is greater the the drain to source voltage and greater than zero. The PMOS behaves like a resistor in this situation because the channel region is between the drain and the source. Cuttoff region, or off, occurs when Vto is less than the gate to source voltage.

When solving, assume the transistor is in the saturation mode. Use the current eqn for saturation mode, and see if your gate to source voltage is higher than your Vto. If it is, the transistor is switched off.  If not, see if it meets the voltage guideline for saturation mode. If not, use the triode region current eqn instead.

I'm sorry if it's confusing, it's really hard to explain this in written word. If you need the PMOS current equations to find your voltages, add a detail to your question and I will include them as quick as I can.

Basically,  I think this will answer your question...

Cuttoff - switched off,  Vgs >= Vto  

Saturation - switched on, |Vds| >= |Vgs-Vto| >= 0