MOSFETS

The bipolar transistor that we have looked at is a current controlled device.  The current that flows from the collector to the emitter is governed by the current flowing from the base to the emitter.  The base-emitter voltage is about 0.7 V, while the current is 10 mA, or more.

  There is a family of devices called field effect transistors that are voltage-controlled devices.  This means that the output current is controlled by the voltage.  The input current is very small indeed, about 1 pA (1 × 10^-12 A).  This allows the output current to governed by the tiny currents produced by transducers such as microphones.

A field effect transistor consists of a bar of material with a metal contact at each end.  One end is called the source and the other is called the drain.  There is a third terminal called the gate.  The general arrangement is shown below:

The junction gate field effect transistor is shown above.  The gate is reverse-biased, i.e. negative with respect to the source.

The depletion layer is a region between the n-type material (which has electrons as the majority charge carrier) and the p-type material (which has holes as the majority charge carriers).  In a depletion layer, some of the holes migrate into the n-type of the material, and some of the electrons migrate into the p-type material.  This results in the material becoming more of an insulator.

If the gate is made more negative, the depletion layer gets wider and the channel gets narrower.  This reduces the current.  Conversely if the gate gets less negative, the depletion layer is reduced, allowing a bigger current to flow.  In effect it’s a bit like controlling the flow of water in a rubber tube by squeezing it or releasing it.

The depletion layer is changed by altering the voltage, which alters the electric field.  This is why this kind of transistor is called a field effect transistor.

There are two main kinds of FET:

·        Junction gate field effect transistor (JUGFET or JFET), which we have mentioned above.

·        Metal oxide silicon field effect transistor (MOSFET), which we will look at in this section of work.

The general structure of a MOSFET is rather different to the JUGFET, and it is shown below.

  This MOSFET has an n – channel, a thin layer of n – type material, which means that the majority charge carriers are electrons.  It is surrounded by a thin layer of silicon oxide on one side and p – type material on the other side.  If we put a voltage positive with respect to the gate, holes in the p – type material are repelled by the electric field and this increases the width of the n – channel.  So a bigger conventional current flows from the drain to the source.  (Remember that electron flow is in the opposite direction to conventional current, so the electrons travel from the source to the drain.)

We describe the increase of the width of the n-channel as the enhancement mode.  The narrowing of the channel as shown in the JUGFET is called depletion mode.  MOSFETS are available in n-channel or p-channel types.  Each type is available in depletion or enhancement mode.

This circuit shows the action of an n-channel enhancement mode MOSFET.

			Figure 85 MOSFET action

 

It allows us to draw a characteristic curve, which is shown below

We need to note the following about this graph:

·        The drain source voltage is fixed.

·        From a threshold voltage of 2.0 V, the drain current increases linearly with the gate source voltage.

·        The gain of any FET is measured using its transconductance.

Gain = DI_D

            DV_GS

[DI_D – change in drain current caused by the change DV_GS.  Units are milliamps per volt (mA/V), or milliseimens (mS)*]

·        Transconductance can be measured by working out the gradient of the graph.

We can use MOSFETs where we have a source of voltage that can provide very little current.  Consider this circuit which is a touch sensor:

  This circuit works as there is a voltage at the gate of the MOSFET.  This happens because the finger can conduct a small electric current.

The general characteristics for a MOSFET are:

·        The transconductance is about 1 to 10 mA/V

·        The input resistance is very high, about 10 ¹² W.

·        The output resistance is about the same as a bipolar transistor.  The actual value depends on the type.  For a signal MOSFET it would be in the range 10 to 50 kW, while in a power MOSFET it would be somewhat lower.

MOSFETs can be used for:

·        Transducer drivers for high power devices like motors and light bulbs, since they give a large current output for a very tiny current input.  So a MOSFET can act as the interface between an integrated circuit that can give only a tiny current, and the motor that takes a big current.

·        In complimentary pairs they are used in hi-fi power amplifiers.  They produce less distortion as they are more linear than bipolar transistors.

·        Integrated circuits, as they can be made very compact.

We have seen how a MOSFET can be used as a switch.  There are advantages and disadvantages when compared to the bipolar transistor as a switch:

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