Field Effect Transistors (FET/JFET/MOSFET) — A/L Physics

Field Effect Transistor (FET) A voltage-controlled semiconductor device where output current is controlled by an electric field applied via a gate terminal. Unlike BJTs (current-controlled), FETs have extremely high input impedance (MΩ–GΩ), drawing negligible gate current. This makes them ideal for voltage amplification stages and low-power switching. Intuition: Think of a water tap where the handle (gate voltage) controls flow without needing water to leak through the handle itself.

Junction FET (JFET) A FET where the gate-channel junction is a reverse-biased pn junction; the gate voltage modulates the channel width. In an n-channel JFET, applying a negative gate-source voltage ( V_GS ) narrows the conductive channel, reducing drain current ( I_D ). At a sufficiently negative V_GS (pinch-off voltage V_P ), the channel is fully depleted and I_D cuts off. Example: Used in analog switches and low-noise preamplifiers.