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:

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.