Depletion-mode MOSFETs represent a unique subset of MOSFET technology, functioning fundamentally differently from their enhancement-mode counterparts. Unlike enhancement MOSFETs, which require a positive gate-source voltage (V_gs) to turn on, depletion-mode MOSFETs are inherently conductive and "normally on" devices.

The primary characteristic of depletion-mode MOSFETs is their ability to conduct current between the drain and source terminals without gate bias. This inherent conductivity arises due to the doping of the channel, which creates a low-resistance path for current flow. The device operates similarly to Junction Field-Effect Transistors (JFETs), where the channel's conductivity is predetermined by its material properties.

In n-channel depletion-mode MOSFETs, applying a positive V_gs enhances the channel width, thereby increasing the drain current (i_d). Conversely, applying a negative V_gs narrows the channel, reducing i_d. This behavior is the opposite for p-channel depletion MOSFETs, where a positive V_gs decreases i_d and a negative V_gs increases it.

A critical parameter for these MOSFETs is the gate threshold voltage (V_TH), the specific V_gs at which the channel closes entirely, stopping current flow. Another vital characteristic is the saturation current, the maximum current that flows through the MOSFET at zero V_gs.

Due to their default "on" state, depletion-mode MOSFETs are especially useful in applications like power amplifiers for radio transmitters. Here, they enable continuous signal transmission, maintaining the signal's presence until a control voltage explicitly turns the device off. This ability to remain continuously active makes depletion-mode MOSFETs integral to applications requiring reliable, uninterrupted operation.