When the drain voltage equals the pinch-off voltage in a field-effect transistor (FET), the drain current becomes nearly constant with further increase in drain voltage. At this pinch-off voltage, the channel near the drain gets "pinched off," and the transistor enters saturation. Beyond this point, increasing the drain voltage does not significantly increase the drain current, which remains roughly at a saturation current level.
Explanation
- The pinch-off voltage VPV_PVP is the drain-source voltage at which the conducting channel narrows so much near the drain that it limits current flow.
- At VDS=VPV_{DS}=V_PVDS=VP, the drain current IDI_DID reaches a saturation level IDSSI_{DSS}IDSS.
- For VDS>VPV_{DS}>V_PVDS>VP, the depletion region near the drain extends slightly, but the channel resistance from source to the pinch-off point remains largely unchanged.
- As a result, the drain current IDI_DID remains almost constant even if the drain voltage continues to increase.
This behavior makes the FET act as a constant current source in this saturation or pinch-off region.
