Subthreshold slope

The subthreshold slope is a feature of a <a href="/facts/MOSFET/eTDJT2Km">MOSFET</a>'s <a href="/facts/Current%25E2%2580%2593voltage_characteristic/wft425cq">current–voltage characteristic</a>.
In the <a href="/facts/Subthreshold_conduction/bB9yIDqF">subthreshold</a> region, the <a href="/facts/Field-effect_transistor/TerYC6Nz">drain</a> current behaviour—though being controlled by the <a href="/facts/Field-effect_transistor/TerYC6Nz">gate</a> terminal—is similar to the exponentially decreasing current of a <a href="/facts/Diode/JyaoyGnk">forward biased diode</a>. Therefore, a plot of drain current versus gate voltage with drain, <a href="/facts/Field-effect_transistor/TerYC6Nz">source</a>, and <a href="/facts/Field-effect_transistor/TerYC6Nz">bulk</a> 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 <a href="/facts/Reciprocal_(mathematics)/yczdYyCw">reciprocal value</a> 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}}
 
 = <a href="/facts/Depletion_region/54fB1pfE">depletion layer</a> capacitance

 
 
 
 
 C
 
 o
 x
 
 
 
 
 {\displaystyle C_{ox}}
 
 = gate-oxide capacitance

 
 
 
 
 
 
 k
 T
 
 q
 
 
 
 
 {\displaystyle {kT \over q}}
 
 = <a href="/facts/Boltzmann_constant/5DIejyR3">thermal voltage</a>
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.

Subthreshold slope open-in-new

Subthreshold slope