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. Its slope is the subthreshold slope.

The subthreshold slope is also the reciprocal value of the subthreshold swing Ss-th which is usually given as:

S s − t h = ln ⁡ ( 10 ) k T q ( 1 + C d C o x ) {\displaystyle S_{s-th}=\ln(10){kT \over q}\left(1+{C_{d} \over C_{ox}}\right)}

C d {\displaystyle C_{d}} = depletion layer capacitance

C o x {\displaystyle C_{ox}} = gate-oxide capacitance

k T q {\displaystyle {kT \over q}} = thermal voltage

The minimum subthreshold swing of a conventional device can be found by letting C d → 0 {\displaystyle \textstyle {C_{d}}\rightarrow 0} and/or C o x → ∞ {\displaystyle \textstyle {C_{ox}}\rightarrow \infty } , which yield S s − t h , min = ln ⁡ ( 10 ) k T q {\displaystyle S_{s-th,\min }=\ln(10){kT \over q}} (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.

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.