10 - Junction field effect transistor

Abstract

The Junction Field Effect Transistor (JFET) works by the control of the conducting channel, by the potential of the nonconducting gate. In N-channel JFET, the reverse bias is applied to the gate, which increases the width of the depletion layer, thus decreasing the width of the channel, and eventually closing the channel. Thus, while for the MOS the channel is normally absent, this requires the gate-to-channel voltage (Vgs) to be greater than the threshold voltage (Vth) for its creation. However, for the JFET, the channel is normally present, requiring Vgs < Vth for its closure. The JFET preceded the Metal-Oxide Silicon Field-Effect Transistor (MOSFET) by a few years, and there are now a few applications where either the bipolar or MOS device does not give a better performance. However, there remain some with better performance—namely, the long-tailed pair at the input of the TL081 bi-fet operational amplifier that is used in most of the experimental work, and the JFET audio power amplifier, used in the popular 500W car blaster.