A metal oxide semiconductor field effect transistor (MOSFET) is essentially a four terminal device: gate, drain, source and body. In almost all conventional MOSFETs the body is permanently internally attached to the source, and only three terminals are brought out for connection to other circuit elements. An N-Channel MOSFET, often written NMOSFET, will not block positive current flow from the source to the drain, irrespective of the gate voltage, if the drain to source voltage, denoted VDS, is <−0.7V due to the body diode inherent in the NMOSFET. Similarly, a P-Channel MOSFET, often written PMOSFET, will not block positive current flow from the drain to the source, irrespective of gate voltage, if VDS is >+0.7V due to the body diode inherent in the PMOSFET. It is possible to implement back-to-back MOSFETs, in which the drain of a first MOSFET is connected to the source of a second MOSFET of a similar channel so as to block this body diode sourced current flow, but the penalty is that the back-to-back MOSFETs have twice the channel resistance of a single MOSFET.
As described above, for an NMOSFET, if the drain is at a lower potential than the body by a diode drop, current will flow through the body to the drain, which from the outside world appears as current flow from source-to-drain. Similarly, for a PMOSFET, if the drain is at a higher potential than the body by a diode drop, current will flow through from the drain to the body, which from the outside world appears as current flow from drain to source. There is no inherent requirement that the body of the MOSFET be attached to the source, but to prevent body diode conduction, it is necessary that the body be held at an appropriate potential in relation to either side of the MOSFET channel.
Many circuits, particularly synchronous secondary side regulation schemes, would benefit from a MOSFET that when turned off would block current flow from both drain to source and from source to drain, over a wide range of potentials.