IdVgs

IdVgs, short for Id versus Vgs, is the transfer characteristic of a field-effect transistor. It describes how the drain current Id responds to gate-source voltage Vgs when the drain-source voltage Vds is held fixed. The IdVgs curve is a primary tool for device characterization, providing the threshold or turn-on behavior, transconductance, and the potential for amplification.

In enhancement-mode MOSFETs, Id is essentially zero for Vgs below the threshold Vth and rises as Vgs increases above Vth. In the conventional long-channel MOSFET model, in saturation Id ≈ 0.5 μn Cox (W/L) (Vgs − Vth)^2 (1 + λVds), where μn is mobility, Cox is oxide capacitance per area, W/L is the channel aspect ratio, and λ accounts for channel-length modulation. In the triode region, Id ≈ μn Cox (W/L)[ (Vgs − Vth) Vds − Vds^2/2 ]. Real devices show deviations due to short-channel effects, velocity saturation, mobility degradation, and DIBL. Subthreshold conduction produces nonzero Id for Vgs below Vth, with an exponential-like dependence Id ∝ exp((Vgs − Vth)/nVt).

Measurement and use: To obtain IdVgs, Vds is fixed and Vgs is swept while recording Id. The curve yields the threshold voltage, transconductance gm = dId/dVgs, body-effect, and parameters used in compact models. Temperature, device geometry, and fabrication process shift the curve, affecting gm and Vth. In circuit design, the IdVgs transfer curve informs biasing, switching thresholds, and small-signal performance, and is embedded in device models for simulation.