Junction Field Effect | Electronic devices | EC

by / Friday, 20 October 2017 / Published in Technical Corner

Real Life Application:

In saturation region JFET will act as an amplifier. Input impedance offered by this device is very high so we can use as input component of voltage amplifier. Packing density is higher than BJTs.

Explanation:

Source: Terminal which will provide majority charge carrier to device is known as Source.

Drain: Terminal which will take out majority charge carrier from device is known as drain.

Gate: Terminal which will control the conduction of the device is known as gate.

Case 1:  When Vgd=0 and Vsd>0.

As we will increase Vsd, voltage slowly decrease from Source to drain.  Due to which bottom side of junction will become more reverse bias. So depletion region towards drain side will be more than source side and depletion region will become wedge shaped. Depletion width will increase w.r.t. Vsd and cross-sectional area will decrease.

Voltage at which available area for conduction will become zero and current will saturate due to high electric field within short sticking width is known as Pinch-Off voltage.

Case 2:  When Vgd<0 and Vsd=0.

As we will increase Vgd then both parasitic diodes will be reverse biased and depletion width will keep on increasing with Vgd. Due to which complete area will depleted by depletion region and no charge carrier will be available over long distance from source to drain. So for any value Vgd also current will be zero. This value of Vgd for which current is becoming zero is known as Pinch-Off.

Triode region:

As we have seen that before pinch-off cross-sectional area of the device can be altered by applied voltage due to which resistance offered by the device will also get change. So device will act as Voltage variable resistor. This mode is known as triode region of JFET.

Saturation region:

As we have seen that if we will keep on increasing value of Vsd, depletion region of both side will keep on increasing and about to touch to each other but because of high electric field it is not possible. Due to which current will saturate and it will attain a constant value. This region is known as Saturation region.

Current voltage relation of JFET is given by:

Idss = Maximum value of current offered by device when gate to drain voltage is zero.

Vp = Pinch-Off voltage

Idss = Maximum value of current offered by device when gate to drain voltage is zero.

Vp = Pinch-Off voltage

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