How is a PMOS transistor used as a switch?
If you want to understand why PMOS passes a bad 0 value take a look at the circuit below:







simulate this circuit &ndash; Schematic created using CircuitLab

If we assume \$ V_{in} =0\;V\$ and \$ V_{DD}=0\;V \$and also the capacitor is initially charged and has a voltage of \$5\;V\$, Source is the pin connected to the Capacitor because it has a higher voltage therefore Drain is the pin connected to \$V_{DD}\$. Also let's assume that \$ V_{tp}=-1\;V \$.Because \$V_{S}-V_{G}&gt;|V_{tp}|\$ therefore PMOS is turned ON and there is a path for the capacitor to discharge. As soon as the capacitor's voltage reaches \$1\;V\$ , the transistor will be OFF because \$V_{S}\$ is equal to \$|V_{tp}|\$.