drain

In a field effect transistor, the drain is the terminal where the majority charge carriers (electrons in an N-channel device, holes in a P-channel device) exit the active channel. Current flows from the source, through the channel controlled by the gate voltage, and out through the drain. The drain connects to the circuit's load or the next stage of amplification.

The drain differs fundamentally from the source in that it receives the modulated current and typically sits at a higher potential in N-channel devices. In analog amplifier circuits, the drain often connects to a load resistor or active load that converts the drain current into a voltage signal. The drain voltage is what changes when the gate signal varies, making it the point where amplification becomes visible to the rest of the circuit.

Drain current, abbreviated ID, is the primary output parameter of a FET. Maximum drain current capacity, specified in the transistor datasheet, limits how much current the device can safely handle before degradation or failure. Drain-source voltage (VDS) must also stay within rated limits to avoid breakdown across that junction. In switching applications, the drain carries the full load current during the on-state.

The drain terminal's physical location on the package varies by device type. In dual in-line packages (DIPs) and surface mount small-outline integrated circuits (SOICs), the drain pin position follows standard pinouts that must be verified against the manufacturer's datasheet. Confusing drain and source pins is a common assembly error that either prevents a circuit from functioning or damages the FET under bias.

The term drain originated from the analogy to vacuum tubes, where electrons were extracted from a hot cathode and collected at a positively charged plate called the plate or anode. In FET terminology, this electrode was renamed the drain because current flows out of the channel like water draining away, while the source supplies it. This naming convention stuck across all field effect transistor types: JFETs, MOSFETs, and modern enhancement-mode and depletion-mode variants.