Introduction

Every electronics student meets the bipolar junction transistor (BJT) first, where the output current is steered by an input current. The field effect transistor (FET) works on a completely different idea: an electric field set up by a small input voltage widens or narrows a conducting channel and so controls the current flowing through the device. Because the controlling terminal draws almost no current, the FET has a very high input resistance, switches fast and adds little noise — which is why it sits at the heart of amplifiers, switches and the integrated circuits that fill modern gadgets.

In this opening chapter of Karnataka 2nd PUC Electronics you will build the FET from the ground up: how a junction FET (JFET) is constructed, what its source, drain and gate terminals do, how a reverse-biased depletion region pinches the channel, and how the drain and transfer characteristic curves reveal parameters such as drain resistance, transconductance and amplification factor. These fundamentals are asked repeatedly in the II PUC board exam, and they prepare you for the MOSFET and amplifier chapters that follow. Work through the figures and the Shockley relation carefully — they are the tools you will reuse for the rest of the course.