Field effect transistors

We can find Field effect transistors in 2 basic forms; junction gate & insulated gate. The gate source junction of a junction gate field effect transistor (JFET) is well a reverse-biased p-n junction.The gate connection of an insulated gate field effect transistor(IGFET), on the other end is insulated from the channel and charge is capacitively coupled to the channel. To keep matter simple, only JFET devices are considered in this tutorial. Diagram below presents the basic structure of an n-channel JFET.

JFET transistors is made up of a channel of p-type or n-type material surrounded by material of the opposite polarity. The ends of the channel (where conduction takes place) form electrodes known as the source & drain. Effective width of the channel (where conduction takes place) is controlled by a charge placed on the third (gate) electrode. The effective resistance between the source and drain is consequently determined by the voltage present at the gate. (The plus (+) signs in diagram are used to point out a region of deep doping thus n+ simply indicates a heavily doped n-type region.) JFETs present a very much higher input resistance when compared with bipolar transistors. For example, the input resistance of a bipolar transistor operating in common-emitter mode is usually around 2.5k OHM.

A JFET transistor operating in the same common source mode would typically display an input resistance of 100M ohm! This characteristic makes JFET devices ideal for use in applications where a very high input resistance is desirable.

As with bi-polar transistors, the characteristics of a FET are frequently accessible in the form of a set of graphs relating voltage and current present at the terminals of transistor.