/ˈdʒiː ˌɛf ˈiː ˈtiː/
noun — "the voltage-controlled transistor for analog circuits."
JFET (Junction Field-Effect Transistor) is a type of field-effect transistor (FET) that uses a reverse-biased p-n junction to control current flow through a semiconductor channel. Unlike bipolar junction transistors (BJTs), JFETs are voltage-controlled devices, offering high input impedance, low noise, and simple operation, making them ideal for amplifiers, switches, and analog signal processing.
Technically, JFET consists of a channel of n-type or p-type material with gate terminals forming a p-n junction on either side. Applying a reverse-bias voltage to the gate reduces the channel width, controlling the current between the source and drain. When the gate-source voltage reaches the pinch-off threshold, the channel closes, and current flow stops. JFETs are predominantly used in low-noise preamplifiers, buffering stages, and analog switches because of their predictable characteristics and minimal signal distortion.
Key characteristics of JFET include:
- Voltage-controlled: input voltage regulates current, not input current.
- High input impedance: reduces loading on previous stages.
- Low noise: suitable for sensitive analog applications like RF amplifiers.
- Simple design: fewer components needed compared to other transistor types.
- Pinch-off behavior: predictable current control through gate voltage modulation.
In practical workflows, JFETs are implemented in audio preamplifiers, RF front-end circuits, voltage-controlled resistors, and analog switching networks. Engineers leverage their high input impedance and linear characteristics to maintain signal integrity while controlling current flow precisely with minimal power consumption. Often, JFETs are paired with CMOS or bipolar stages in hybrid analog-digital designs.
Conceptually, JFET is like a water valve controlled by pressure: adjusting the gate voltage is equivalent to turning the valve, regulating the flow without consuming much energy.
Intuition anchor: JFET offers precise, low-noise current control, making it a staple of analog and RF electronics.