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Subthreshold leakage in an nFET. 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 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 decreasing current of a forward biased diode.
A nanowire MOSFET's current–voltage characteristic (left, using logarithmic y-axis) and a simulation of the electron density (right) forming a conductive inversion channel which connects at the ~0.45 V threshold voltage. Extremely little current flows below this voltage. The threshold voltage, commonly abbreviated as V th or V GS(th), of a ...
As threshold voltage is reduced, the transistor cannot be switched from complete turn-off to complete turn-on with the limited voltage swing available; the circuit design is a compromise between strong current in the on case and low current in the off case, and the application determines whether to favor one over the other. Subthreshold leakage ...
In classical MOSFET devices, the 60 mV/decade is a fundamental limit to power scaling. The ratio between on-current and the off-current (especially the subthreshold leakage — one major contributor of power consumption) is given by the ratio between the threshold voltage and the subthreshold slope, e.g.:
In textbooks, channel length modulation in active mode usually is described using the Shichman–Hodges model, accurate only for old technology: [2] where = drain current, ′ = technology parameter sometimes called the transconductance coefficient, W, L = MOSFET width and length, = gate-to-source voltage, =threshold voltage, = drain-to-source voltage, =, and λ = channel-length modulation ...
DIBL also affects the current vs. drain bias curve in the active mode, causing the current to increase with drain bias, lowering the MOSFET output resistance. This increase is additional to the normal channel length modulation effect on output resistance, and cannot always be modeled as a threshold adjustment.
The mode can be determined by the sign of the threshold voltage (gate voltage relative to source voltage at the point where an inversion layer just forms in the channel): for an N-type FET, enhancement-mode devices have positive thresholds, and depletion-mode devices have negative thresholds; for a P-type FET, enhancement-mode have negative ...