thresholdvoltage

Threshold voltage, commonly denoted Vth, is a characteristic of a field-effect transistor (FET), such as a metal-oxide-semiconductor (MOS) transistor. It is defined as the gate-to-source voltage at which the device begins to form an inversion or conduction channel, allowing appreciable drain current to flow. For enhancement-mode MOSFETs, Vth marks the boundary between the off state (VGS below Vth) and the on state (VGS above Vth). Depletion-mode devices have a nonzero current at zero gate bias and require a gate voltage of opposite polarity to suppress conduction.

Vth depends on many factors: the transistor type (n-channel or p-channel), substrate bias (body effect), manufacturing process, temperature, and short-channel effects. In conventional MOS technology, Vth is influenced by oxide thickness, channel dopant concentration, and the body potential; and the threshold can shift with source-bulk voltage, described by the body effect.

Around threshold, subthreshold conduction can occur, with current flowing even when VGS < Vth. In the subthreshold region, drain current follows an exponential relation with VGS, approximately ID ≈ I0 exp((VGS − Vth)/(n VT)), where n is the subthreshold slope factor and VT is the thermal voltage. This region defines leakage behavior and analog operation; a steeper threshold corresponds to a sharper switch.

Temperature generally lowers Vth in many silicon processes, and short-channel effects such as drain-induced barrier lowering can also shift the threshold. Designers account for process variations and operating conditions by specifying ranges and margins for Vth, and by using techniques such as body biasing to adjust it.

In applications, Vth determines switching thresholds in digital logic and influences transconductance, gain, and noise in analog circuits. Accurate control of threshold voltage is essential for power, speed, and reliability across technology nodes.