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If your studies lean toward VLSI design rather than discrete circuit design then you may have also learned that a MOSFET is really a **4-terminal** device: gate, source, drain, and body. In VLSI design it is common to connect the bodies of all NMOS transistors to the lowest voltage in the circuit (ground, for digital circuits) and to connect the bodies of the PMOS transistors to the highest voltage in the circuit.

When MOSFETS are used as discrete devices, such as in power supplies or amplifiers, it is generally more convenient to connect the body to the source and treat the transistor as a **3-terminal** device. This allows the MOSFETs to use the same packages as high-power BJT transistors and usually doesn't cause any problems. However, since the body is connected to the source a PN junction now exists between the combined body/source connection and the drain connection. In an NMOS transistor the diode's anode is the p-type body and the diode's cathode is the n-type drain. So, effectively there is a diode between the source and drain. In normal use this diode would be reverse biased.