Subthreshold conduction

Subthreshold conduction or subthreshold leakage or subthreshold drain current is the current between the source and drain of a MOSFET when the transistor is in subthreshold region, or weak-inversion region, that is, for gate-to-source voltages below the threshold voltage. The terminology for various degrees of inversion is described in Tsividis.

In digital circuits, subthreshold conduction is generally viewed as a parasitic leakage in a state that would ideally have no current. In micropower analog circuits, on the other hand, weak inversion is an efficient operating region, and subthreshold is a useful transistor mode around which circuit functions are designed.

In the past, the subthreshold conduction of transistors has usually been very small in the off state, as gate voltage could be significantly below threshold; but as voltages have been scaled down with transistor size, subthreshold conduction has become a bigger factor. Indeed, leakage from all sources has increased: for a technology generation with threshold voltage of 0.2 V, leakage can exceed 50% of total power consumption.

The reason for a growing importance of subthreshold conduction is that the supply voltage has continually scaled down, both to reduce the dynamic power consumption of integrated circuits (the power that is consumed when the transistor is switching from an on-state to an off-state, which depends on the square of the supply voltage), and to keep electric fields inside small devices low, to maintain device reliability. The amount of subthreshold conduction is set by the threshold voltage, which sits between ground and the supply voltage, and so has to be reduced along with the supply voltage. That reduction means less gate voltage swing below threshold to turn the device off, and as subthreshold conduction varies exponentially with gate voltage (see MOSFET: Cut-off Mode), it becomes more and more significant as MOSFETs shrink in size.

Subthreshold conduction is only one component of leakage: other leakage components that can be roughly equal in size depending on the device design are gate-oxide leakage and junction leakage. Understanding sources of leakage and solutions to tackle the impact of leakage will be a requirement for most circuit and system designers.

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