A touch sensitive on/off switch can be made with a single MOSFET. The following schematics illustrate such a switch using only a power MOSFET (IRFZ22).
In this example, the switching capability is achieved by the dielectric nature of MOSFET’s gate. Since the gate is insulated from other device regions, the leakage current is relatively small and thus the capacitor can hold its charge for a relatively long period of time.
When the gate and the positive lead are touched, the relatively small gate capacitor is rapidly charged (dry skin has a resistance of several mega-ohm) and the switch turns on. And when the gate and the ground are touched the gate capacitor quickly discharges and the switch turns off.
