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). 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 bidirectional device because the terminals referred to as “source” and “drain” can be replaced. 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 .
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.
Suppose we want an n-channel MOSFET to conduct electrons from the source to the drain. The gate connector is fixed for obvious reasons. 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.
It is up to us to decide which of the two remaining terminals will be selected for this purpose. 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.
Thus, the source and drain can be selected in two ways, and therefore we can have two different directions of current flow through the same device under two different conditions.
When the Vgs is biased so that the MOSFET is fully on, it conducts current evenly in both directions with the same ON resistance between source and drain.
However, due to the inherent substrate diode between the source and drain, it can only block in one direction. However, if you use two MOSFETs in series with the sources in series, the combination blocks the current in both directions when it is off and conducts the current in both directions when it is on.
