The subthreshold slope of is a feature of a MOSFET

MOSFET

The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...

's current–voltage characteristic.

In the subthreshold region the drain current behaviour – though being controlled by the gate terminal – is similar to the exponentially increasing current of a forward biased diode. Therefore a plot of logarithm

Logarithm

The logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...

ic drain current versus gate voltage with drain, source, and bulk voltages fixed will exhibit approximately linear behaviour in this MOSFET operating regime. Its slope is the subthreshold slope.

The subthreshold slope is closely related to its reciprocal value called subthreshold swing S_s-th which is usually given as:



= depletion layer

Depletion region

In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region or the space charge region, is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have diffused away, or have been forced away by an...

capacitance

= gate-oxide capacitance.

Subthreshold swing typical value at room temperature: .

A (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.

External links

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