Subthreshold conduction: definition

Beginner

The small trickle that still flows when a transistor is “off,” like a tap that never quite stops dripping.

Novice

Below the threshold voltage the channel isn’t fully formed, but some electrons still have enough energy to cross from source to drain. The current falls by a fixed factor for every step down in gate voltage instead of dropping to zero.

Expert

Diffusion current over a gate-controlled barrier: $I_{\mathrm{D}} \propto \exp[(V_{\mathrm{GS}} - V_{\mathrm{t}})/(n\,kT/q)]\,(1 - e^{-V_{\mathrm{DS}}/(kT/q)})$ . It is the dominant leakage in modern logic and the reason $V_{\mathrm{t}}$ cannot fall freely.