Subthreshold conduction or subthreshold leakage or subthreshold drain current is the current between the source and drain of a

MOSFET upright=1.3, Two power MOSFETs in amperes">A in the ''on'' state, dissipating up to about 100 watt">W and controlling a load of over 2000 W. A matchstick is pictured for scale. In electronics, the metal–oxide–semiconductor field- ...

when the

transistor A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch electrical signals and electric power, power. It is one of the basic building blocks of modern electronics. It is composed of semicondu ...

is in subthreshold region, or weak-inversion region, that is, for gate-to-source

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

s below the threshold voltage. The amount of subthreshold conduction in a transistor is set by its 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 log-linear slope, which is defined as the

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 A gate or gateway is a point of entry to or from a space en ...

. Subthreshold slope is used as a figure of merit for the switching efficiency of a transistor.''Physics of Semiconductor Devices'', S. M. Sze. New York: Wiley, 3rd ed., with Kwok K. Ng, 2007, chapter 6.2.4, p. 315, . In digital circuits, subthreshold conduction is generally viewed as a parasitic leakage in a state that would ideally have no conduction. In micropower

analog circuit Analogue electronics () are electronic systems with a continuously variable signal, in contrast to digital electronics where signals usually take only two levels. The term ''analogue'' describes the proportional relationship between a signal ...

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. 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 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. 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.

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. 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.

Sub-threshold electronics

See also

References

Further reading