The Junction Field Effect Transistor or JFET

Junction Field Effect Transistor or JFET

The Junction Field Effect Transistor (JFET) is another type of FET, a basic structure of which may consist of a semiconductor bar with ohmic contacts at the end and heavily doped regions on its opposite sides. If the semiconductor bar is made of n-type material, then it is an n-channel JFET. The JFET is p-channel if the bar is made of p-type material.

The terminals at the ends of the bar correspond to the source and drain of the JFET. The heavily doped regions on the sides of the bar are connected to serve as the gate of the JFET. Needless to say, the gate regions are doped to be of opposite type with respect to the channel, so that a p-n junction is formed between the channel and the gate regions.

By applying a voltage across the source and the drain of a JFET, current consisting of majority carriers (electrons for an n-channel and holes for a p-channel) is caused to flow through the channel. The current flowing through the channel is controlled by applying a gate voltage V_gs that reverse biases the p-n junction formed by the gate with respect to the source. The higher the Vgs is, the more the p-n junction is reverse-biased, and the wider the depletion region across the channel becomes. The wider depletion region results in a narrower channel, consequently constricting the flow of current through the channel. Varying Vgs therefore varies the current through the channel for any given voltage across the source and the drain.

Figure 1. Structure of a single-ended-geometry junction FET

The JFET structure described above is no longer practical to use because of the difficulty with having to diffuse dopants from two opposite sides of a bar. Most JFETs built onto IC's nowadays involve single-ended geometries that require doping for the gate from only one side of the channel, i.e., the surface of the wafer. This is achieved by building the JFET on an epitaxially grown channel over a doped substrate that acts as the second gate.

The current through the channel of a MOSFET or JFET consists of only the majority carriers, which is why FETs are referred to also as unipolar transistors.

HOME