A pMOSFET is "ON", meaning that it presents a resistive path of, generally, less than 0.5 ohms at full rated current, when the gate is pulled low (in this case, away from the source, which would be connected to Vcc. The load is at the drain, and, since relays rated at 5 volts drop 5 volts at rated input circuit (coil) current, a PFET with low on resistance would probably be advisable. Since many of them are below 0.1 ohms, and some even below 0.05 ohms, this should not be a problem.

see http://www.industrialcontroldesignline.com/howto/199.../199204060 and its second part as well. For a bit of a write-up.

If this is to be driven from Vcc=5.0 volts, well, it will be somewhat out of specified limits, but will probably work fine. If the relay supply is, say, 12 volts, then the 5 volts of the coil plus the forward voltage of the pMOSFET, plus an appropriate resistor's forward voltage should sum up to 12 volts.

The gate impedance is so high that an arbitrarily large resistor will do the job, though it could affect turn-on time, depending on gate capacitance.

Read the spec's and choose the components carefully.

RE