MOSFET: Depletion Mode

Depletion-mode MOSFETs are normally on devices that conduct current without gate bias, unlike enhancement-mode MOSFETs which require positive gate-source voltage to turn on. The key difference lies in channel doping: depletion devices have a pre-doped channel creating inherent conductivity, while enhancement-mode MOSFETs require gate voltage to form a conductive channel. This makes depletion MOSFETs useful as load resistors in digital logic circuits.

Applying a negative gate-source voltage to an n-channel depletion MOSFET narrows the channel, reducing drain current. As the gate voltage becomes more negative, the channel progressively closes until it reaches the gate threshold voltage, where current flow stops completely. This behavior allows the device to function as a controllable switch despite being normally on.

The gate threshold voltage is the specific gate-to-source voltage at which the channel closes completely, stopping all current flow through the device. This critical parameter determines the voltage required to turn off a normally on depletion MOSFET. Understanding this voltage is essential for designing circuits that rely on depletion devices for switching and amplification applications.

Both depletion-mode MOSFETs and JFETs operate with inherently conductive channels due to channel doping, creating a low-resistance path for current flow without gate bias. This shared characteristic means both devices are normally on and require a control voltage to reduce or stop current. The similarity in operation makes depletion MOSFETs useful alternatives to JFETs in applications requiring field-effect transistor behavior.

Saturation current is the maximum drain current that flows through a depletion-mode MOSFET when the gate-source voltage is zero. This parameter represents the device's peak current-carrying capability under zero gate bias conditions. Saturation current is a fundamental characteristic used to specify device performance and predict behavior in circuit applications.

In n-channel depletion MOSFETs, positive gate voltage widens the channel and increases drain current, while negative voltage narrows it. P-channel devices exhibit opposite behavior: positive gate voltage decreases drain current and negative voltage increases it. This polarity reversal is fundamental to device design and must be considered when selecting components for specific circuit configurations.

Depletion-mode MOSFETs are used as load resistors in digital logic circuits, start-up auxiliary power supply circuits, voltage sweeping circuits, current monitor circuits, and solid-state relays. Their normally on characteristic makes them ideal for power amplifiers in radio transmitters, where they enable continuous signal transmission until a control voltage explicitly turns the device off.