Basics of Electronics: Transistor Time!

Woo! Transistors! Drool, drool. Ok, my childish reactions apart, transistors are the backbone of electronics as we know it. They represent a control structure, where you can control one part of the circuit with another (as a master-slave relationship). As an overview, a transistor has 3 terminals (I'm talking about BiJunction Transistors' (BJT) here), which are called the Emitter, Base, and Collector (EBC). By changing the potential difference between the Base and Emitter, the current between the Emitter and Collector can be varied. I'm going to use acronyms for EBC's from now on, because I'm lazy.

Generic NPN transistor diagram below:

Ok, we'll discuss the NPN (negative-positive-negative) transistor right now. The PNP transistor is nearly the same, except the polarities are reversed. So the NPN transistor consists of three regions, a N junction (see previous article for junctions), a P junction and a N junction. They are the Collector, Base and Emitter respectively. If you want to, you can look at the transistor as two diodes, with the P junction fused. You can take two PN diodes and try to make a transistor out of it, but it'd be really crappy, because the P junction must be very very thin for reasons we shall soon see.

Let's connect up our transistor to a battery, and try and understand how it works:

Firstly, note that the C is connected in reverse bias (i.e. the positive end is connected to it), with respect to the EC connection. The BE is in forward bias though. Remember the depletion zone (from the previous article) right? Well, its here too. By varying the potential difference between the BE, we are able to change the 'thickness' of the depletion layer. That allows more current to flow. Initially, the layer is pretty thick, and electrons can't flow from the E to the C, so there is no current in the EC circuit. Now, as we increase the potential between BE, the layer thins down. So now electrons are allowed to flow into the B region.

Remember that the EC in reverse bias? That means that the C can't give electrons because of the gap, but it does mean that it is very very eager to take electrons. Thus, it sucks up the electrons that enter the B region. The B region must be very thin so that the electrons are not absorbed by the B or taken away from the circuit.

There you have it folks, by varying the BE potential, you vary the depletion layer. This allows a current into the B region, which is taken up again by the C region (so the Collector collects the current). Through a model, the relation between the V(BE) and I(EC) is Ice is proportional to exp(Vbe) (exp is e power). There are other components called FET (field emmission transistors), which vary as Ice proportional to Vbe^2.

In the next article, we'll discuss Op-amps, Comparators and Logic Gates

(Did I catch yous in time for the PBs?)