Explain the concept of MOSFET subthreshold conduction and why it matters for leakage power.

• A. In subthreshold region, drain current flows linearly with Vgs.
• B. Subthreshold conduction is negligible at room temperature.
• C. Subthreshold conduction occurs when Vgs exceeds Vth.
• D. In subthreshold region, drain current flows exponentially with Vgs below Vth; affects leakage and static power.

Answer: (D)

Subthreshold conduction is the channel current that flows when the gate voltage is below the threshold, in the weak inversion region. Even though the device isn’t strongly turning on, there’s a diffusion-driven current that makes the MOSFET never truly off. This leakage current is what sets static power dissipation in many circuits.

The key point is that this current changes exponentially with the gate voltage in this region. Specifically, the drain current follows a form like I_d ≈ I0 · exp[(Vgs − Vth) / (n·Vt)], where Vt is the thermal voltage (about 26 mV at room temperature) and n is the subthreshold slope factor. Because of the exponential dependence, a small change in Vgs near the threshold causes a large change in current, which is why leakage can be a dominant factor in modern, low-voltage designs. Temperature also affects this leakage: as temperature rises, Vt changes and the intrinsic carrier behavior shifts, typically increasing leakage.

This leakage current means that power is continuously drawn even when the transistor is supposed to be off, so leakage power becomes a critical consideration in design. Designers mitigate it with techniques like using higher-threshold devices, longer channel lengths, leakage-aware sizing, body biasing, and power-gating strategies.

In short, subthreshold conduction involves an exponential dependency of drain current on Vgs below Vth, and this leakage current directly influences static power in circuits.