drainström

Drainström, or drain current, is the electric current that flows from the drain terminal to the source terminal of a field-effect transistor (FET) or a similar device. In most FETs, the drain current is controlled by the gate-source voltage (Vgs) and, to a lesser extent, by the drain-source voltage (Vds).

In a MOSFET, when Vgs exceeds a threshold voltage Vth, an electron or hole conduction channel forms between source and drain, allowing a current Id to flow. In the saturation region (Vds larger than Vgs − Vth), Id increases slowly with Vgs and is roughly Id ≈ (1/2) μn Cox (W/L) (Vgs − Vth)^2, with corrections for channel-length modulation (factor 1 + λVds). In the triode or linear region (small Vds), Id ≈ μn Cox (W/L) [ (Vgs − Vth)Vds − Vds^2/2 ].

In a JFET or depletion-mode device, Id is controlled by the reverse gate-source bias; Ids follows a simple model like Id ≈ Idss [1 − Vgs/|Vp|]^2 for an n-channel depletion device, and Id decreases as Vgs becomes more negative.

Drain current is a central parameter for determining the operating point (Q-point) of amplifiers and switching circuits. It is measured in amperes and depends on temperature, carrier mobility, oxide thickness, and device geometry. Understanding drainström is essential for circuit analysis, design, and performance prediction.

In Swedish technical literature, drainström is the standard term for this parameter, and the conceptual framework follows the same principles as in English-language texts.