The parasitic diode is in between drain and source. The MOSFET structure itself contains parasitic diode or body drain diode. This parasitic diode allows the reverse flow of current when the MOSFET is in off state. The figure shows the internal parasitic diode. This parasitic diode have slow switching characteristics as compare to MOSFET channel. The integral diode have slow reverse recovery characteristics. In parasitic diode, drain acts as cathode and source acts as the anode. The integral diode have pn-n+.
Use of parasitic diode:
This parasitic diode is useful in the flow of reverse current in the inverter with an inductive load.
MOSFET structure also have parasitic BJT. The figure shows parasitic BJT. The n-channel MOSFET will have npn parasitic transistor. The body region of MOSFET acts as the base of BJT, the source of MOSFET acts as an emitter, the drain of MOSFET as a BJT collector. Parasitic BJT operates in the cut-off region.