12.18:
MOSFET: Depletion Mode
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.
12.18:
MOSFET: Depletion Mode
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 (Vgs) 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 Vgs enhances the channel width, thereby increasing the drain current (id). Conversely, applying a negative Vgs narrows the channel, reducing id. This behavior is the opposite for p-channel depletion MOSFETs, where a positive Vgs decreases id and a negative Vgs increases it.
A critical parameter for these MOSFETs is the gate threshold voltage (VTH), the specific Vgs 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 Vgs.
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.