This just occurred to me, and it's probably a stupid question, but I've got to ask: I've seen lots of circuits for DC motor speed control using the PWM principle and a MOS FET to control the DC current through a motor. The gate of the FET is modulated by a square wave of variable duty cycle. All of the diagrams I have seen about the construction of a MOS FET seem to indicate a device that is symmetric on the source and drain ends about the gate electrode. If that is really the case, could one switch source and drain connection ends in a such a circuit and still have the same operational characteristics? If so, could the same arrangement be used to control an AC current through a load?