12.18 : MOSFET（金属氧化物半导体场效应晶体管）：耗尽型

Depletion-mode MOSFETs are already ON without a gate bias but need a gate-source voltage to switch OFF.

They function like JFETs, where the drain-source channel is inherently conductive due to channel doping. This creates a low resistance path between the drain and source, enabling current to flow without any gate bias.

For n-channel depletion MOSFETs, the drain characteristics show that an increasing positive gate voltage widens the channel, thereby increasing the drain current.

As the gate voltage becomes more negative, the drain current decreases. This behavior is reversed for p-channel depletion MOSFETs.

The gate threshold voltage is the gate-to-source voltage at which the channel closes completely, stopping the current flow.

The saturation current is the maximum current that can flow through the device at zero gate-to-source voltage.

Depletion MOSFETs function as a normally on switch, making them useful in digital logic circuits as load resistors.

They are extensively used in start-up auxiliary power supply circuits, voltage sweeping circuits, current monitor circuits, and solid-state relay, among other applications.