What is a MOSFET enhancement-mode device, and how does Vgs determine conduction?

• A. Enhancement-mode MOSFET conducts when Vgs exceeds threshold Vth; below threshold, the channel is off and conduction is minimal.
• B. It conducts when Vgs is below threshold.
• C. It conducts when Vds exceeds Vgs.
• D. It conducts when the gate is at 0 V.

Answer: (A)

In an enhancement-mode MOSFET, there is no conducting channel between drain and source when the gate bias is zero. The gate is insulated from the channel, so applying a gate-to-source voltage creates an electric field that forms an inversion layer and establishes a conductive path. Conduction begins only when the gate voltage exceeds a threshold value, Vth; this is the point at which the channel is created. Once Vgs is above Vth, increasing Vgs makes the channel more conductive, so the drain–source current increases for a given drain–source voltage. If Vgs is below Vth, the channel isn’t formed and conduction is essentially off. The role of Vgs is to turn the device on and control how strongly it conducts; Vds then governs how much current flows once the channel exists, through the device’s operating region.