JFET

A junction field-effect transistor (JFET) is a voltage-controlled semiconductor device in which current from the drain to the source is modulated by the voltage applied to a gate terminal that forms a reverse-biased PN junction with the conducting channel. The gate controls the width of the narrow channel, changing its effective cross-section and thus the drain current. Because the gate-channel junction is reverse-biased, gate current is very small, giving the JFET a high input impedance.

Structure and types: JFETs are depletion-type devices. The channel carries either electrons (n-channel JFET) or holes (p-channel JFET). The gate is formed by PN junctions surrounding the channel; in most devices the two gates are connected together to a single gate terminal. Applying a reverse bias between gate and source widens the depletion region and reduces the channel cross-section, lowering the drain current Ids. For an n-channel JFET, positive Vgs increases Ids, while for a p-channel JFET, negative Vgs does so (relative to the source). The pinch-off voltage Vp marks the point where the channel is effectively closed and Ids becomes minimal.

Electrical characteristics: In normal operation, Ids is a function of Vgs and is relatively insensitive to Vds in the saturation region, making JFETs useful for linear amplification and impedance buffering. The transconductance gm = dIds/dVgs indicates how effectively the gate controls the channel. Simple models describe Ids ≈ Idss [1 − Vgs/Vp]^2 for certain ranges, with Idss the maximum drain current at Vgs = 0. Gate current is very small under reverse bias, contributing to the JFET’s high input impedance.

Applications: JFETs are used in analog signal amplification, impedance conversion, buffers, and front-end stages of operational amplifiers, as well as in RF and audio circuits where low noise and high input impedance are desirable. They compete with MOSFETs in some applications but offer different biasing and noise characteristics.