Electronics Principles V11

Clive W. Humphris

FIELD EFFECT TRANSISTORS: Basic N Channel FET.  


JFET is short for 'Junction-Gate Field Effect Transistor' normally referred to as FETs. FETs are controlled by an electric field (voltage) as opposed to an electric current as is the case for a bi-polar transistor. The channel is made of n-type material and the gate p-type. Their operation is very similar to that of a thermionic valve.

As the gate voltage is made more negative (gate to channel junction is reverse biased) the depletion region widens. This is an area where positive holes are filled with negative electrons, thereby removing the current carriers through cancellation. Note the shape of the depletion region, tapering is caused by the voltage drop along the channel, where the width will be greater closer to the drain due to the greater reverse bias.

As VGG is made more negative a point is reached when ID ceases to flow, known as 'pinch off'. By holding VGG at 0V, then current through the device can be controlled by voltage VDD. Behaving as fixed value resistor. Having a reverse biased input means the input impedance is extremely high. Its this high impedance that can destroy the device if static charges come in contact with an unprotected device.

Also available are p-type FETs where the channel is made of p type semiconductor material. Voltages for this device are simply a reversal of those for the n-type. Also available are IGFETS, insulated gate field effect transistors, called MOSFETS, these have an insulated gate just a few microns thick, between the channel and the gate. This reduces any leakage current, thereby increasing the input impedance even more up to hundreds of megohms.

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