Why is subthreshold conduction important for leakage power in modern ICs?

• A. Because drain current flows only above Vth.
• B. Because leakage is eliminated by subthreshold effects.
• C. Because drain current flows exponentially with gate voltage below Vth, contributing to leakage.
• D. Because it is negligible in all technologies.

Answer: (C)

Subthreshold conduction is the current that flows when the gate-to-source voltage is below the threshold, because a weak inversion channel forms and carriers can drift and diffuse from drain to source. This current is not zero in the off state; it changes exponentially with gate voltage, roughly following I_sub ∝ exp((Vgs − Vth)/(nVt)). In modern ICs, devices are scaled and threshold voltages are pushed toward lower values, while we still operate many transistors with Vgs well below threshold in the off state. The result is a leakage current that can be sizeable, and when multiplied by Vdd, becomes a notable portion of static (leakage) power. This exponential sensitivity also means leakage can grow strongly with temperature and process variations, making subthreshold leakage a dominant concern in deep submicron and nanoscale technologies. So the reason subthreshold conduction matters for leakage power is precisely that the drain current flows exponentially with gate voltage below Vth and contributes to off-state power dissipation.