BJTs are very versatile and very simple devices. They can be used for amplification and switching in a variety of configuratons, each with uses and drawbacks.
BJTs are so named because they consist of junctions between the two kinds of semiconductor (n and p type) and have one pn junction and one np junction (i.e. the other way around). Compare this to a unipolar junction transistor,which has only one junction. The construction of a BJT is very simple - it is three alternating layers of semiconductor. The diagram below show the basic construction, but they are not to scale.
The NPN transistor is made of layers in that order:
It is equivalent to two back-to-back diodes, but connecting two diodes like this will not work, as the thicknesses and geometry of each layer is important, as are effects from the interaction of the two outer layers.
The symbol for this kind of transistor is shown to the right. It can be drawn with or without the circle. With is technically more correct, but it is not necessary. A useful mnemonic for remembering which way the arrow points on an NPN transistor is:
- "Not Pointing iN".
The arrow is always drawn on the emitter - it actually means which way (conventional) current flows in the emitter.
The PNP transistor is the same, except the order of the layers is reversed:
It is equivalent to two nose-to-nose diodes, but as before, just connecting two diodes will not work. Generally, a PNP transistor is identical to an NPN transistor, but with all currents reversed.
The symbol is the same, but with the arrow reversed. Note that the emitter of PNP transistors often face upwards.
We will not really consider the PNP transistor at first - it is essentially the same as the more common NPN transistor, but reversed. We will look at applications for it later.
How the BJT works depends on the bias of each junction. Each junction is like a diode, and in some ways behaves accordingly. Since there are two junctions, each of which can be biased in two ways (forward or reverse), there are four possible modes for the BJT. The following table lists them. The modes are the same for NPN and PNP.
The Active mode is generally used for amplification, and the Cut-Off and Saturation modes are used for switching and binary logic (when you just want 1 or 0).
The working of a BJT will be explained on the next page, BJTs in Active Mode.