Field Effect Transistor or JFET
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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
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
By applying a
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 Vgs 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
wider depletion region results in a
consequently constricting the flow of current through the channel.
therefore varies the
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
The current through the
channel of a MOSFET or JFET consists of only the
which is why FETs are referred to also as
What is a Semiconductor?; p-n Junction;
Bipolar Transistor; MOSFET;
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