Subthreshold slope

The subthreshold slope is a feature of a MOSFET's current–voltage characteristic. 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 = l n ( 10 ) k T q ( 1 + C d C o x ) {displaystyle S_{s-th}=ln(10){kT over q}(1+{C_{d} over C_{ox}})} 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 extstyle {C_{d}} ightarrow 0} and/or C o x → ∞ {displaystyle extstyle {C_{ox}} ightarrow infty } , which yield S s − t h , min = l n ( 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.