Viewed from 'outside' the transistor we can describe its behaviour in terms of a Current Gain, Beta. This is defined in terms of the ratio of the number of electrons which manage to cross the transistor to those which get caught. We can now treat the transistor as a Current Amplifier since when we put 'in' (i.e. into the Base) a current, IB, we get 'out' (i.e. from the Emitter and Collector) a current, IC or IB, which is much larger and whose value we can control by altering IB.



This leads us to the 'conventional' view of Bipolar Transistors as they are represented in most electronics books. The diagram above shows two changes: We now refer the Collector potential to the Emitter (VCE) rather than to the Base, and we now represent the current in conventional terms - passing from positive to negative. Since the precise Collector voltage doesn't have much effect on the currents, moving the place we reference it from isn't very important and this new view is more convenient in practice. Changing to conventional (positive to negative) current flow allows us to fit in with the normal view of electronics. Note also that, for simplicity, we can normally assume that the values of the Collector and Emitter currents are essentially identical since they only differ by a percent or so.

By remembering that the Base-Emitter is a forward baissed diode junction and taking the above description into account we can formulate two rough 'rules of thumb' for the behaviour of a Bipolar Transistor when using it as an amplifier, etc: Neither of these rules is strictly true in practice (so don't tell any solid-state physicists about them!), but they allow us to get a suprisingly long way in understanding, analysing, and designing circuits which use Bipolar Transistors!

Characteristic Curves of a Bipolar Transistor.


The only other important thing we often have to know is that the PNP transistor is a sort of 'mirror image' of the 'NPN'. Since it reverses the roles of the free electrons and holes it works in a similar way, but with the voltages and currents reversed. So in diagrams like those shown on this page, if we replaced the NPN transistor with a PNP we could just reverse the polarities of the applied voltages and the directions of the current arrows and get the right results!



Where those names came from...



Now we've seen how a Bipolar Transistor works we can understand why its three sections have the names they've been given. So far as the transistor is concerned,







Well, the Base was given that name from the way early Bipolar Transistors were made. This started with a basic piece of semiconductor and then 're-doped' two bits of it to create the Emitter and Collector. The result of this process is shown in the above illustration. From this picture we can see that the 'Base' was given its name because it is electrically a part of the large piece of semiconductor material we started with. The name 'Base' reminded early transistor makers that this was the mechanical and electronic starting point of the manufacturing process. To build an NPN transistor you start with a lump of P-type material and re-dope two parts near each other to make them N-type. To build a PNP transistor you do it the other way around. These days Bipolar Transistors are made lots of different ways, so the name 'Base' no longer tells us much about how a specific device was made. The name is a sort of historical legacy...

Ideally, you need to get the thickness of the unmodified Base region just right. Too thin, and the Base would essentially vanish. The Emitter and Collector would then form a continuous piece of semiconductor, so current would flow between them whatever the base potential. Too thick, and electrons entering the Base from the Emitter wouldn't notice the Collector as it would be too far away. So then, the current would all be between the Emitter and the Base, and there'd be no Emitter-Collector current. For these reasons the precise design of a Bipolar transistor is much more complex than we've described here, but it still works in the way we've seen.
Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
using HTMLEdit3 on a StrongARM powered RISCOS machine.
University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.