What is a depletion-mode field-effect transistor (FET)?

Field-effect transistors (FETs) come in two primary types based on their zero-bias behavior: depletion-mode and enhancement-mode. A depletion-mode FET is 'normally on,' meaning the channel is already formed and conductive when no voltage is applied to the gate. Because the channel is naturally conductive, current can flow freely between the source and the drain at zero gate voltage. To reduce or stop this current, a reverse-bias voltage must be applied to the gate to 'deplete' the channel of charge carriers. In contrast, an enhancement-mode FET is 'normally off' and requires a gate voltage to create a conductive channel. Recognizing the difference between normally on (depletion) and normally off (enhancement) is essential for FET circuit design.