What is important is that from now on the positive voltage will be applied to the other connector (which has not been chosen so far) with respect to this chosen connector in order to DRAIN this third connector. It is up to us to decide which of the two remaining terminals will be selected for this purpose. Either of the other two can be chosen as the SOURCE, and the gate voltage with respect to that terminal is made greater than the threshold voltage for the device. The gate connector is fixed for obvious reasons. Suppose we want an n-channel MOSFET to conduct electrons from the source to the drain. Once the channel is formed by applying an appropriate positive potential between the source and gate terminals, electrons flow from the source to the drain when a positive potential is applied from the source to the drain. That is, an n-channel MOSFET (taken here as an example) conducts when the gate-source voltage is above a threshold voltage and the drain voltage is greater than the source voltage to support electron flow through the induced channel. The MOSFET is a bidirectional device because the terminals referred to as “source” and “drain” can be replaced. The MOSFET transistor is a bidirectional device, but the current can only flow through the source-drain if the voltage in the source is greater than the drain. The MOSFET is a symmetrical device in which the source and drain can be interchanged (if the body is not short-circuited to the drain or source).

Power mosfet bidirectional switch