MOSFETSourcenDrainBereichen

MOSFET source and drain are the two heavily doped diffusion regions of a metal-oxide-semiconductor field-effect transistor (MOSFET). They form the current path between which charge carriers move when the device is turned on, with the gate electrode controlling the conductivity of the channel that forms between them.

In operation, the gate voltage modulates an inversion layer in the silicon near the semiconductor-oxide interface, creating a conductive channel that allows current to flow between source and drain. For an n-channel MOSFET (NMOS), current primarily flows when the drain voltage is higher than the source voltage, while for a p-channel MOSFET (PMOS) the source is at the higher potential and current flows toward the drain when the gate is appropriately biased.

The source and drain are not identical in all respects. The transistor body (substrate) forms p-n junctions with both source and drain regions, and in most devices the body is tied to the source to prevent unwanted forward biasing of these junctions. This creates a built-in diode between the drain and source, oriented from source to drain in NMOS devices. In many circuits this diode remains reverse-biased during normal NMOS operation.

Because MOSFETs are largely symmetric, source and drain can be interchanged in terms of conduction paths, but the body connection and the intrinsic diode fix a preferred orientation in practical use. The designation of source and drain is therefore tied to the device’s body connection and the typical operating polarity for a given type (NMOS or PMOS).