Editing Subthreshold conduction

{{Use dmy dates|date=August 2019|cs1-dates=y}}
[[File:FET subthreshold leakage.png|thumbnail|Subthreshold leakage in an nFET]]
'''Subthreshold conduction''' or '''subthreshold leakage''' or '''subthreshold drain current''' is the [[electric current|current]] between the source and drain of a [[MOSFET]] when the [[transistor]] is in subthreshold region, or [[MOSFET#Modes of operation|weak-inversion]] region, that is, for gate-to-source [[voltage]]s below the [[threshold voltage]].<ref name="Tsividis_1999"/>

The amount of subthreshold conduction in a transistor is set by its [[Threshold Voltage|threshold voltage]], which is the minimum gate voltage required to switch the device between ''on'' and ''off'' states. However, as the drain current in a MOS device varies exponentially with gate voltage, the conduction does not immediately become zero when the threshold voltage is reached. Rather it continues showing an exponential behavior with respect to the subthreshold gate voltage. When plotted against the applied gate voltage, this subthreshold drain current exhibits a [[Semi-log plot|log-linear slope]], which is defined as the [[subthreshold slope]]. Subthreshold slope is used as a figure of merit for the switching efficiency of a transistor.<ref>''Physics of Semiconductor Devices'', S. M. Sze. New York: Wiley, 3rd ed., with Kwok K. Ng, 2007, chapter 6.2.4, p.&nbsp;315, {{ISBN|978-0-471-14323-9}}.</ref>

In digital circuits, subthreshold conduction is generally viewed as a parasitic [[Leakage (electronics)|leakage]] in a state that would ideally have no conduction. In micropower [[analog circuit]]s, on the other hand, weak inversion is an efficient operating region, and subthreshold is a useful transistor mode around which circuit functions are designed.<ref name="Toumazou_1996" />

Historically, in CMOS circuits, the threshold voltage has been insignificant compared to the full range of gate voltage between the ground and supply voltages, which allowed for gate voltages significantly below the threshold in the ''off'' state. As gate voltages [[Dennard scaling|scaled down with transistor size]], the room for gate voltage swing below the threshold voltage drastically reduced, and the subthreshold conduction became a significant part of the off-state leakage of a transistor.<ref name="Soudris_2002" /><ref name="ULVD_2015" /> For a technology generation with [[threshold voltage]] of 0.2 V, subthreshold conduction, along with other leakage modes, can account for 50% of total power consumption.<ref name="Roy_2004" /><ref name="Al-Hashimi_2006" /> 

==Sub-threshold electronics ==
Some devices exploit sub-threshold conduction to process data without fully turning on or off. Even in standard transistors a small amount of current leaks even when they are technically switched off. Some sub-threshold devices have been able to operate with between 1 and 0.1 percent of the power of standard chips.<ref name="tr_2015"/>

Such lower power operations allow some devices to function with the small amounts of power that can be scavenged without an attached power supply, such as a wearable [[EKG]] monitor that can run entirely on body heat.<ref name="tr_2015"/>

==See also==
* [[Integrated circuit]]
* [[Moore's law]]
* [[Multi-channel length]]
* [[Subthreshold slope]]

==References==
{{reflist|refs=
<ref name="Tsividis_1999">{{cite book |author-first=Yannis |author-last=Tsividis |title=Operation and Modeling of the MOS Transistor |date=1999 |page=[https://archive.org/details/isbn_9780073032313/page/99 99] |edition=2 |publisher=[[McGraw-Hill]] |location=New York |isbn=0-07-065523-5 |url=https://archive.org/details/isbn_9780073032313/page/99 }}</ref>
<ref name="Toumazou_1996">{{cite book |title=Circuits and systems tutorials |chapter=The Fundamentals of Analog Micropower Design |editor-first1=Chris |editor-last1=Toumazou |editor-first2=Nicholas C. |editor-last2=Battersby |editor-first3=Sonia |editor-last3=Porta |author-first=Eric A. |author-last=Vittoz |publisher=[[John Wiley and Sons]] |date=1996 |isbn=978-0-7803-1170-1 |pages=365–372 |chapter-url=https://books.google.com/books?id=WTInL9njOKAC&pg=PA367}}</ref>
<ref name="Roy_2004">{{cite book |author-first1=Kaushik |author-last1=Roy |author-first2=Kiat Seng |author-last2=Yeo |title=Low Voltage, Low Power VLSI Subsystems |date=2004 |publisher=[[McGraw-Hill Professional]] |isbn=0-07-143786-X |url=https://books.google.com/books?id=jXm4pNxCSCYC |no-pp=true |pages=Fig. 2.1, p. 44}}</ref>
<ref name="Soudris_2002">{{cite book |title=Designing CMOS Circuits for Low Power |editor-first1=Dimitrios |editor-last1=Soudris |editor-first2=Christian |editor-last2=Piguet |editor-first3=Costas |editor-last3=Goutis |publisher=Springer |date=2002 |isbn=1-4020-7234-1 |url=https://books.google.com/books?id=86oXI7MWw8AC&pg=PA20}}</ref>
<ref name="Al-Hashimi_2006">{{cite book |editor-first=Bashir M. A |editor-last=l-Hashimi |title=System on a Chip: Next Generation Electronics |date=2006 |page=429 |publisher=Institution of Engineering and Technology |isbn=0-86341-552-0 |url=https://books.google.com/books?id=NqNvUtZcKA4C&pg=PA419}}</ref>
<ref name="ULVD_2015">{{cite book |title=Ultra-Low-Voltage Design of Energy-Efficient Digital Circuits |author-first1=Nele |author-last1=Reynders |author-first2=Wim |author-last2=Dehaene |series=Analog Circuits And Signal Processing (ACSP) |date=2015 |edition=1 |location=Heverlee, Belgium |publisher=[[Springer International Publishing AG Switzerland]] |publication-place=Cham, Switzerland |isbn=978-3-319-16135-8 |issn=1872-082X |doi=10.1007/978-3-319-16136-5 |lccn=2015935431}}</ref>
<!-- <ref name="Narendra_2006">{{cite book |editor-first1=Siva G. |editor-last1=Narendra |editor-first2=Anantha |editor-last2=Chandrakasan |title=Leakage in Nanometer CMOS Technologies |date=2006 |page=307 |publisher=Springer Publications |isbn=0-387-25737-3 |url=https://www.springer.com/engineering/circuits+%26+systems/book/978-0-387-25737-2,M1}}</ref> -->
<ref name="tr_2015">{{cite news |title=A Batteryless Sensor Chip for the Internet of Things |author-first=Suzanne |author-last=Jacobs |date=2014-07-30 |access-date=2018-05-01 |url=https://www.technologyreview.com/2014/07/30/12892/a-batteryless-sensor-chip-for-the-internet-of-things/}}</ref>
}}

==Further reading==
* {{cite book |author-first=Vincent C. |author-last=Gaudet |chapter=Chapter 4.1. Low-Power Design Techniques for State-of-the-Art CMOS Technologies |editor-first=Bernd |editor-last=Steinbach |editor-link=:de:Bernd Steinbach |title=Recent Progress in the Boolean Domain |publisher=[[Cambridge Scholars Publishing]] |publication-place=Newcastle upon Tyne, UK |location=Freiberg, Germany |edition=1 |date=2014-04-01 |orig-date=2013-09-25 |isbn=978-1-4438-5638-6 |pages=187–212 |url=https://books.google.com/books?id=_pwxBwAAQBAJ |access-date=2019-08-04}} [https://web.archive.org/web/20210228020207/https://www.tau.ac.il/~ilia1/publications/rpbd_book.pdf<!-- https://m.tau.ac.il/~ilia1/publications/rpbd_book.pdf draft version 2013-09-25 -->] (xxx+428 pages)

[[Category:Transistor modeling]]
[[Category:Electric current]]
[[Category:MOSFETs]]