Depletion-mode MOSFET gate biasing capability Which statement correctly describes allowable gate bias polarities for a depletion-mode MOSFET (of appropriate type)?

Introduction / Context:
Depletion-mode MOSFETs (both n-channel and p-channel) conduct at zero gate-source voltage, unlike enhancement-mode devices that require a threshold to turn on. Understanding how positive and negative gate biases affect channel conductivity is essential for analog switches, constant-current sources, and amplifier loads.

Given Data / Assumptions:

• Depletion-mode device with an existing channel at VGS = 0.
• Proper polarity is referenced to the device type (n-channel or p-channel).
• Safe operating limits are observed.

Concept / Approach:
Because a depletion-mode MOSFET already conducts at zero bias, applying a reverse-polarity gate voltage relative to the source can “deplete” the channel (reducing current), while applying the opposite polarity can “enhance” it (increasing current). Thus, it can be operated with both positive and negative gate voltages (relative to the source) within ratings to modulate conductance over a wide range.

Step-by-Step Solution:
At VGS = 0: device conducts nominally (IDSS-like behavior).Apply gate voltage that increases depletion: channel narrows; current decreases toward cutoff.Apply gate voltage that enhances: channel widens; current increases above the zero-bias level.Therefore, both polarities (with correct reference for n or p type) are usable.

Verification / Alternative check:
Transfer characteristics ID vs VGS for depletion-mode devices show conduction at VGS = 0 and modulation for both signs of VGS within limits.

Why Other Options Are Wrong:
Only positive or only negative: contradicts depletion-mode behavior.Cannot operate in ohmic region: false; MOSFETs operate in ohmic (linear) region when VDS is small relative to VGS − VTH.

Common Pitfalls:
Confusing depletion- and enhancement-mode devices; most power MOSFETs are enhancement-mode, but depletion-mode parts exist for special uses.

Final Answer:
can operate with positive as well as negative gate voltages