The subthreshold slope is a feature of a MOSFET's current–voltage characteristic.
In the subthreshold region, the drain current behaviour—though being controlled by the gate terminal—is similar to the exponentially decreasing current of a forward biased diode.
Therefore, a plot of drain current versus gate voltage with drain, source, and bulk voltages fixed will exhibit approximately log-linear behaviour in this MOSFET operating regime.
= thermal voltage The minimum subthreshold swing of a conventional device can be found by letting
(known as thermionic limit) and 60 mV/dec at room temperature (300 K).
A typical experimental subthreshold swing for a scaled MOSFET at room temperature is ~70 mV/dec, slightly degraded due to short-channel MOSFET parasitics.
[2] A dec (decade) corresponds to a 10 times increase of the drain current ID.
A device characterized by steep subthreshold slope exhibits a faster transition between off (low current) and on (high current) states.