Vds

Vds, written as VDS, stands for the drain–source voltage of a transistor, most commonly a MOSFET. It is the potential difference between the drain terminal and the source terminal and is a fundamental voltage that influences how the device conducts current.

In a MOSFET, the drain current (ID) is controlled primarily by the gate–source voltage (VGS), while VDS helps determine the region of operation. When VDS is small (below VGS minus the threshold voltage, Vth), the device operates in the triode or ohmic region, where ID increases roughly with VDS. When VDS exceeds that boundary, the device enters the saturation region, where ID becomes relatively independent of VDS (for an ideal long-channel MOSFET). In real devices, effects such as channel-length modulation and velocity saturation cause some Dependence of ID on VDS even in saturation.

VDS is defined as VD − VS, and conventional sign conventions depend on device type and current direction; for an n-channel MOSFET, VDS is typically considered positive when the drain is at a higher potential than the source.

In practice, VDS is a critical specification in device datasheets. The maximum drain–source voltage, VDS(max) or BVDS, sets the voltage limit before breakdown or avalanche occurs. Designers must respect VDS, along with ID ratings and thermal limits, to ensure reliability. In power electronics and switching applications, VDS stress can be a major factor in transient responses and protection strategies, such as snubbers and proper layout.