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The '''subthreshold slope''' is a feature of a [[MOSFET]]'s [[current–voltage characteristic]]. | |||
In the [[Subthreshold conduction|subthreshold]] region the [[Drain_(transistor)#Terminals|drain]] current behaviour – though being controlled by the [[Gate_(transistor)#Terminals|gate]] terminal – is similar to the exponentially increasing current of a [[Diode#Current.E2.80.93voltage_characteristic|forward biased diode]]. Therefore a plot of [[logarithm]]ic drain current versus gate voltage with drain, [[Source_(transistor)#Terminals|source]], and [[Bulk_(transistor)#Terminals|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 (mathematics)|reciprocal value]] called '''subthreshold swing''' ''S<sub>s-th</sub>'' which is usually given as:<ref>''Physics of Semiconductor Devices'', S. M. Sze. New York: Wiley, 3rd ed., with Kwok K. Ng, 2007, chapter 6.2.4, p. 315, ISBN 978-0-471-14323-9.</ref> | |||
<math> S_{s-th} = ln(10) {kT \over q}(1+{C_d \over C_{ox}}) </math> | |||
<math>C_d</math> = [[Depletion region|depletion layer]] capacitance | |||
<math>C_{ox}</math> = gate-oxide capacitance | |||
<math>{kT \over q}</math> = [[thermal energy]] divided by the [[elementary charge]] | |||
The minimum subthreshold swing of a conventional device can be found by letting <math>\textstyle {C_{ox}} \rightarrow \infty </math> which yields 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.<ref name="Intel22">{{Cite doi|10.1109/VLSIT.2012.6242496}}</ref> | |||
A ''dec'' (decade) corresponds to a 10 times increase of the drain current ''I<sub>D</sub>''. | |||
A device characterized by steep subthreshold slope exhibits a faster transition between off (low current) and on (high current) states. | |||
==References== | |||
{{reflist}} | |||
==External links== | |||
* [http://www.iue.tuwien.ac.at/phd/stockinger/node13.html Optimization of Ultra-Low-Power CMOS Transistors]; Michael Stockinger, 2000 | |||
{{DEFAULTSORT:Subthreshold Slope}} | |||
[[Category:Transistor modeling]] | |||
Latest revision as of 23:04, 12 July 2013
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 increasing current of a forward biased diode. Therefore a plot of logarithmic 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 Ss-th which is usually given as:[1]
= depletion layer capacitance
= thermal energy divided by the elementary charge
The minimum subthreshold swing of a conventional device can be found by letting which yields 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.[2]
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.
References
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External links
- Optimization of Ultra-Low-Power CMOS Transistors; Michael Stockinger, 2000
- ↑ Physics of Semiconductor Devices, S. M. Sze. New York: Wiley, 3rd ed., with Kwok K. Ng, 2007, chapter 6.2.4, p. 315, ISBN 978-0-471-14323-9.
- ↑ Template:Cite doi