The Bipolar Junction Transistor (BJT).

NPN (and PNP) transistors.

An NPN transistor can crudely be described, as being made up of a layer of P type semiconductor, "sandwiched" between two pieces of N type semiconductor. (A PNP transistor has the opposite arrangement). In this section we will describe the operating principles of an NPN transistor. (The PNP has the same principle of operation, except that the role of the charge carriers is interchanged and the applied voltages have opposite polarity).

The term bipolar, refers to the fact that in this type of transistor the current flow involves two types of majority charge carriers, (i.e. electrons and holes).

The figure below shows a diagram for an NPN transistor along with the circuit symbol. The three different regions of the transistor are called the emitter, base and collector. It is very important to note that the diagram does not reflect the actual comparative dimensions of each region/layer within the transistor. In fact the difference in dimensions along with the difference in doping levels between each region, are critical to the function of the device.

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Forward biased of the base emitter junction.

If an external voltage is applied to forward bias the base emitter junction, then as expected current will flow across it. However the emitter is much more heavily doped than the base region. Therefore the current that flows in the base (Ib), (and across the base emitter junction ) is limited by the lower doping of the base region.

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Reverse biased base collector junction.

It can be seen that the applied polarity causes the base collector junction to be reversed biased. The base current still flows due to the forward biased base emitter junction. This consist of holes flowing in the base and electrons flowing in the emitter, which recombine at the junction. However a large number of the electrons from the emitter, are swept across the base by the positive voltage of the collector, before they get chance to encounter and then recombine with the holes. This is because of the very low density of holes in the base region and the fact that it is extremely thin. The net result is that a large number of electrons that are injected into the emitter, are "swept across" the base by the positive voltage of the collector, before they can encounter and recombine with a hole. This causes a large current to flow between the collector and emitter.

Operation of the transistor.

Above we have described the function of the transistor, in terms of the factors affecting the behaviour of mobile charge carriers in the base, emitter and collector regions. We will now consider an overview of the operation of the transistor in terms of conventional current flow.

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The base emitter voltage Vbe causes a current (Ib) to flow into the base of the transistor. This current enables the voltage between the collector and emitter (Vce), to produce a current (Ic,) that flows into the collector. Due to the different doping levels (as described above) and the larger potential of Vce, the collector current Ic is much larger than the base current Ib.
In accordance with Kirchhoff's current law the current flowing out of the emitter (Ie), will be the sum of the base and collector current (i.e. Ie = Ib + Ic).
The net result is that in a correctly biased transistor, a small base current Ib causes a much larger collector current Ic to flow.
(We can think of Ib as effectively controlling the resistance "experienced" by Vce)

Keeping Vce constant, then if Ib is increased Ic increases (upto a maximum value reached when the transistor is "fully conducting" (i.e. when it presents negligible resistance to Vce)). If Ib is reduced then Ic reduces (until Ib eventually reduces Ic to zero ).

This leads to two applications for the transistor.

  1. The transistor as a switching device .
    The base current can be switched from zero to a value that causes the transistor to fully conduct. This will cause the resistance "seen" by Vce to change from being effectively open circuit to closed circuit. i.e. The transistor will act like a switch (controlled by Ib and controlling Ic).
  2. The transistor as a signal amplifier.
    With the base current set to the mid point of its operating range, a variation of the base current will cause a proportional variation in the collector current, providing the base current does not approach its upper and lower operating limits. If we go outside this range, then the change in Ic will no longer be proportional the change in Ib . Eventually we could reach the point when the transistor has been switched fully on or off as described above, so further changes in Ib, cannot cause any further change in Ic.

PNP transistor.

The diagrams below show the biasing arrangement and circuit symbol for a PNP transistor.

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