Q.Write the function of drain and gate.

Every JFET has three terminals, each with a clear job. Learning their functions is a standard one- and two-mark exam expectation.

• Source (S) — the terminal through which the majority charge carriers enter the channel. It supplies the carriers that take part in conduction.
• Drain (D) — the terminal through which the carriers leave the channel. It collects the carriers delivered by the source; the current entering here is the drain current $I_D$.
• Gate (G) — the controlling terminal. When a suitable (reverse-bias) potential is applied to the gate, the electric field it produces widens or narrows the channel and so controls the flow of carriers through it. Because the gate junction is reverse biased, the gate itself draws almost no current.

In operation a voltage applied across the drain and source drives the carriers from source to drain, forming the device current, while the gate voltage regulates how much current flows. This division of labour — one terminal to inject carriers, one to collect them, and one to control the flow with a field — is the essence of how a field effect transistor works, and it mirrors the roles of the emitter, collector and base in a bipolar transistor while using a voltage rather than a current to control the output.