Transistors –[page 1] Background & Basics
September 7, 2011
Where to begin…
I decided to write this paper when I began to realise that many of the people in our robotics group did not have a clear, working knowledge of transistors and how they function.
Some of this first page may look familiar, but with added drawings. There was a question about why the leads of a transistor are called Collector, Emitter and Base and to what those terms relate.
Another of our brother LMRans, Jassper, answered in part: "The Collector "Collects" electrons. The Emitter "Emits" electrons. The Base controls the electron flow between the Emitter and Collector. Like a "Military base Unit" controlling where the jets go and how many."
What Jassper said about emitting and collecting electrons is basically correct, but only applies to NPNs, and not PNPs, since they are wired electrically reverse to an NPN.
As to why these terms were picked, we have to go back to the 1940s & 1950s. In those days, it was common to think, not of electrons flowing from negative to positive, but electrons leapfrogging, producing the effect that holes (positive charges) flow from positive to negative. In reality, current flow is made up of flow in both directions, but it does not matter in practice as long as you are consistent within your own minds as to how you think of the current flowing.
[ In practice, "Hole flow" is the predominant effect. Unless you are speaking of something like lightning, it is extremely unlikely for the same electron to make it all the way from one end of a circuit to the other through all those atoms making up the wires of a circuit. Normally, an electron jumps from one atom to another, leaving a hole or positive charge in the atom it left. This in turn captures another electron from yet another atom, leaving a hole there. Even though there is electron flow from one layer to another within semiconductor junctions, it is the holes or positive charges that tend to "flow" from one end of a circuit to another.]
Since I am a 1940s kid, I (originally) learned electricity as current flowing from positive to negative. Also, the first transistors were made as PNPs. NPNs were a bit harder to make at first, until what was actually happening at the atomic level began to be understood better. That is why the arrows on transistors and diodes point the direction they do, from positive to negative. (If you are one of those who only think of electrons moving from negative to positive, then you must always consider the current is moving reverse to the way the arrows point. Consequently, even though advanced electronics has taught me that current is flowing in both directions at once (holes from plus to minus and electrons from minus to plus) I still find it easier to think in terms of positive to negative flow, so the arrows always point the right direction for me, but as I said before, it does not matter which way you think of it as long as you remain consistant in your own mind. Being consistent within your own thoughts is the key to understanding.
Now the first transistors started with a single germanium crystal, which they labelled the 'base' because it was physically the base of the device. The other two leads were made from tiny gold wire contacts (called "cat's whiskers" in those days) touching spots on the crystal. These were originally placed by hand, since touching different spots on the crystal gave better or poorer contact & current flow. Next, they found that if they tried to pass current from one of the point-contacts to the other one, this current could be controlled --increased or decreased -- by a slight change in the field charge on the base crystal. A small current flowing through the base would block a larger current trying to flow between collector and emitter, since for the current to pass from one point contact to the other, it had to go through the base to get to its destination.
As to calling either the emitter or the collector 'ground', this is an incorrect view, since it is possible to have different basic configurations with emitter grounded, collector grounded, or even base grounded. These three types of circuits give different output results, different amplification factors, different output impedances and have different input impedances, and so on. (See the next drawing:)
(Crap, I just realised I forgot to put the LMR logo on these three drawings... Oh, well, they are simple drawings anyway...)
I intend to make this longer, but truthfully there are tonnes of material on transistors (if you weighed the books written on the subject), and I will hardly make a dent. I will add more later on, either on this page or I'll make a second page. Anyway, I want my fellow LMRans to understand transistors enough to be able to throw together simple working circuits when they want to. Consequently, I plan to add more later regarding actual circuit design. You do not need to understand the physics of exactly what is happening inside a transistor to make working circuits. All you need to know is how to read a specification sheet (spec sheet) enough to know what to do.
This series will start out assuming that you already understand things like Ohm's Law and basic electricity. If you don't, you should brush up on all that. There is a bit to understand if you are a complete novice to electricity and electronics.
Add notes below to tell me what sort of things are unclear to you and I can tailor my future pages to answer those questions. (I will answer the novice questions first and we can build up to the more complicated later on.)
Continued at: http://letsmakerobots.com/node/28927