Subthreshold slope

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:
formula_1
formula_2 = depletion layer capacitance
formula_3 = gate-oxide capacitance
formula_4 = thermal Voltage
The minimum subthreshold swing of a conventional device can be found by letting formula_5 and/or formula_6, which yield formula_7(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.