Subthreshold conduction

Subthreshold conduction or subthreshold leakage or subthreshold drain current is the <a href="/facts/Electric_current/Dc1xdnUX">current</a> between the source and drain of a <a href="/facts/MOSFET/eTDJT2Km">MOSFET</a> when the <a href="/facts/Transistor/Bmr8zwL0">transistor</a> is in subthreshold region, or <a href="/facts/MOSFET/eTDJT2Km">weak-inversion</a> region, that is, for gate-to-source <a href="/facts/Voltage/EN2po6N2">voltages</a> below the <a href="/facts/Threshold_voltage/IparzT9A">threshold voltage</a>.
The amount of subthreshold conduction in a transistor is set by its <a href="/facts/Threshold_Voltage/IparzT9A">threshold voltage</a>, 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 <a href="/facts/Semi-log_plot/H2a0ANuF">log-linear slope</a>, which is defined as the <a href="/facts/Subthreshold_slope/xm0dqlQs">subthreshold slope</a>. Subthreshold slope is used as a figure of merit for the switching efficiency of a transistor.
In digital circuits, subthreshold conduction is generally viewed as a parasitic <a href="/facts/Leakage_(electronics)/Nd6PNxhV">leakage</a> in a state that would ideally have no conduction. In micropower <a href="/facts/Analog_circuit/Bnlnfwwv">analog circuits</a>, 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 <a href="/facts/Dennard_scaling/f84TN2w7">scaled down with transistor size</a>, 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 <a href="/facts/Threshold_voltage/IparzT9A">threshold voltage</a> of 0.2 V, subthreshold conduction, along with other leakage modes, can account for 50% of total power consumption.

Subthreshold conduction open-in-new

Subthreshold conduction