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Since Jfet is a Unipolar voltage controlled device and current flows into base terminal moreover the main current flows from drain to source

Category Engineering Forum in Electrical
asked Jun 12 in Electrical by Apurvi Pandey | 25 views

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Junction field effect transistor can be made by fabricated with either an n channel Or P channel. For fabricating an n channel jfet, first a narrow bar of ntype material semiconductor material is taken and then two ptype junctions are diffused on opposite sides of its  middle part. These two pn junctions firm two P-N diodes or gates and the area between these gates is called channel. (Shown in fig 1) 

Working of jfet 

Gates are always reverse biased 

The source terminal is always connected to that end of the drain supply which provides the necessary charge carriers

Now three case rises:

When V*gs=0 and V*ds=0

The depletion regions around the p junctions are of equal thickness and symmetrical(shown in fig 2) means channel remain of same length. 

When V*gs=0 and V*ds is increased from zero

In the present case external bias V*gs=0.hence gate channel reverse bias is provided by V*ds alone. Since the value of V*ds keeps decreasing (due to progressive drop along the channel)  as we go from D to S, the gate -channel bias also decrease accordingly. It has maximum value in the drain gate region and minimum in the source gate region. Hence, depletion regions penetrate more deeply into the channel in the drain gate region than in the source gate region. This explains why the depletion regions become wedge shaped (shown in fig 3) 

When V*ds =0 and V*gs is decreased from zero

In this case, as V*gs is made more and more negative, the gate reverse bias increases which increases the thickness of the depletion regions. (Shown in fig 4) image

answered Jun 17 by Suhanibaru