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 ''S_s-th'' which is usually given as:''Physics of Semiconductor Devices'', S. M. Sze. New York: Wiley, 3rd ed., with Kwok K. Ng, 2007, chapter 6.2.4, p. 315, .

$S_ = \ln(10) \left(1+\right)$

$C_d$ = depletion layer capacitance

$C_$ = gate-oxide capacitance

$$ = thermal voltage

The minimum subthreshold swing of a conventional device can be found by letting $\textstyle \rightarrow 0$ and/or $\textstyle \rightarrow \infty$, which yield $S_ = \ln(10)$(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 ''I_D''.

A device characterized by steep subthreshold slope exhibits a faster transition between off (low current) and on (high current) states.

References

External links

Optimization of Ultra-Low-Power CMOS Transistors
Michael Stockinger, 2000

{{DEFAULTSORT:Subthreshold Slope
Transistor modeling