Junction field effect transistors

As with bipolar junction transistors field effect transistors come in two types called n channel and p channel. The emitter, base and collector terms are replaced by source, gate and drain. The channel allows electron movements through it unhindered by any pn junction. For reasons that I will explain below the channel is of lightly doped material whilst the gate material is heavily doped.

An n channel JFET is illustrated below. In its no voltage state natural depletion zones exist at the pn junctions. They have been created by the movement of lightly bound electrons (they are not free as many describe) from n material into electron desiring locations in the p material. The light doping of the n channel material mens the width of the more energy stable depletion zone created is mainly resident in the n material.

If we now apply a voltage supply twixt gate and source with source positive we push electrons into the p material and pull electrons out of the n material. In doing so we extend the depletion zones in those materials but again, and for reasons explained above, the extensions are much into the n channel.

The more negative we make the gate relative to the source the more we increase the depletion zone and the narrower the channel becomes. It is important to note that these electron moves that build the depletion zone do not constitute a noticeable current flow. At a certain level of negative voltage on the gate the depletion zone will pinch off the channel and close it to any would be current flow. This voltage is called the “pinch off” voltage.

If we apply a voltage between drain and source with the channel open current flows along the channel whose value is largely determined by how open the channel is. Remember resistance to flow increases as area of flow increases. It means the device gate voltage controls the channel current.

The voltage between drain and source also influences depletion zone shape. The pulling of electrons into the drain is decreasing the lightly bound electron n channel density and the pushing of electrons into the source is increasing that density. The net effect is to cause the tapering of the depletion zone as in the above diagram.

The electrical symbols used for FET’s clearly distinguish them from bipolar junction transistors. They use less power and dissipate less heat than BJT’s and can be made more compact than them. As such their technology is much used in integrated circuits.

The p channel FET behavior is much the same. It requires different voltage connections and its channel is less conductive than that of a n channel FET.

Whilst in an n channel FET lightly bound electron movements happen because they come under pressure from electrons in motion in a p channel FET the loosely bound electrons have to be drawn into the more fixed structure points that are energy desiring holes. In so doing they vacate a point in space that has an energy desire and which can draw in other electrons. Others describe this as a movement of positive holes. I prefer to think in terms of energy desires and electron motions.

As with n channel FETS a sufficiently high but now positive gate voltage will produce a depletion zone that closes the channel and also as with n channel FETS the level of the current flow between drain and source adds to the depletion zone width.

It is now not unreasonable to ask if, when the gate voltage is less than “pinch off”, the drain current level can deliver a “pinch off” condition? The answer is no because the mere act of closing the channel diminishes the current flowing through it and so reduces the pressure to close the channel. The result is that this type of transistor has a controlled saturation current that varies with gate voltage.

The performance diagram above shows how for each gate to source voltage there is limited linear relationship between drain voltage and current. Any drain to source voltage increase after that causes a channel closure and increase resistance so as to hold the current at a saturation level. The performance diagram also shows how the transistor will fail and breakdown if the drain voltage continues to increase.

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