Question

What is a depletion-mode MOSFET and how does it differ from an enhancement-mode MOSFET?

Answer 1

A Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is a type of transistor used in electronic devices for switching and amplification. Depletion-mode and enhancement-mode MOSFETs are two different operational modes of MOSFETs, distinguished by how they respond to the control voltage applied to the gate terminal.

Depletion-Mode MOSFET (D-MOSFET):
In a depletion-mode MOSFET, the channel between the source and drain terminals is inherently present even when no voltage is applied to the gate terminal. This is because the channel is created by a fixed charge within the semiconductor material itself. When a negative voltage is applied to the gate terminal, it counteracts this fixed charge and reduces the size of the channel, effectively restricting the flow of current between the source and drain terminals. Thus, a depletion-mode MOSFET is normally "on" (conducting) when no voltage is applied to the gate and can be turned "off" by applying a negative voltage.

Enhancement-Mode MOSFET (E-MOSFET):
In an enhancement-mode MOSFET, the channel between the source and drain terminals is initially absent or very small when no voltage is applied to the gate terminal. To enable current flow between the source and drain terminals, a positive voltage (greater than a certain threshold voltage) is applied to the gate. This positive voltage attracts charge carriers (usually electrons) to form a conductive channel between the source and drain terminals, allowing current to flow. An enhancement-mode MOSFET is normally "off" (non-conducting) when no voltage is applied to the gate and can be turned "on" by applying a positive voltage.

In summary, the main difference between depletion-mode and enhancement-mode MOSFETs lies in their default operational states. Depletion-mode MOSFETs are naturally conducting without an applied voltage, and a negative voltage turns them off. Enhancement-mode MOSFETs are naturally non-conducting without an applied voltage, and a positive voltage turns them on.

It's worth noting that enhancement-mode MOSFETs are more commonly used in modern digital circuits due to their ability to be controlled more precisely and their compatibility with CMOS (Complementary Metal-Oxide-Semiconductor) technology. Depletion-mode MOSFETs are used less frequently and often find applications in specialized circuit designs.