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How does a transistor work?

Thousands of textbooks have been written to explain electronics and we haven’t found a single one that can explain the operation of a transistor. Let’s see if we can do better. Here is a picture of a plumbing transistor. We are the network plumbers after all. Our transistor runs on water current. You see there are three entrance/exits which we have labelled “B” (Base), “C” (Collector) and “E” (Emitter) for convenience. These also happen to be the names used for the three connections of a transistor.

We provide a reservoir of water for “C” (the “power supply voltage”) but it can’t move because there’s a plunger in the way which is blocking the outlet to “E”. The reservoir of water is called the “supply voltage” otherwise known as the rail or HT in electronics circles. If we increase the amount of water sufficiently, it will burst our transistor just the same as if we increase the voltage to a real transistor. We don’t want to do this, so we keep that “supply voltage” at a safe level.

If we pour water current into “B” this current flows along the “Base” pipe and pushes the plunger upwards, allowing more water to flow from “C” to “E”. Some of the water from “B” also joins it and flows away. If we pour even more water into “B”, the plunger moves up further and more water current flows from “C” to “E”.

So basically, a tiny amount of current flowing into “B” allows a correspondingly and proportionally larger current to flow from “C” to “E” so we have an “amplification effect”. We can control a BIG flow of current with a SMALL flow of current. If we continually change the small amount of water flowing into “B” then we cause corresponding changes in the LARGE amount of water flowing from “C” to “E”. For example, if we measure the current flow in litres/minute: Suppose 1 litre/minute flowing into “B” allows 100 litre/minute to flow from “C” to “E” then we can say that the transistor has a “gain” or “amplification” factor of 100 times. In a real transistor we measure current in thousandths of an Ampere or “milliamps”. So 1mA flowing into “B” would allow 100mA to flow from “C” to “E”.

2. The amount of current that can flow from “C” to “E” is limited by the “pipe diameter”. So, no matter how much current we push into “B”, there will be a point beyond which we can’t get any more current flow from “C” to “E”. The only way to solve this problem is to use a larger transistor. A “power transistor”.

3. The transistor can be used to switch the current flow on and off. If we put sufficient current into “B” the transistor will allow the maximum amount of current to flow from “C” to “E”. The transistor is switched fully “on”. If the current into “B” is reduced to the point where it can no longer lift the black plunger thing, the transistor will be “off”. Only the small “leakage” current from “B” will be flowing. To turn it fully off, we must stop all current flowing into “B”. This is how a transistor can be thought of as a switch.

In a real transistor, any restriction to the current flow causes heat to be produced. This happens with air or water in other things: for example, your bicycle pump becomes hot near the valve when you pump air through it. A transistor must be kept cool or it will melt. It runs coolest when it is fully OFF and fully ON. When it is fully ON there is very little restriction so, even though a lot of current is flowing, only a small amount of heat is produced. When it is fully OFF, provided we can stop the base leakage, then NO heat is produced. If a transistor is half on then quite a lot of current is flowing through a restricted gap and heat is produced. To help get rid of this heat, the transistor might be clamped to a metal plate which draws the heat away and radiates it to the air. Such a plate is called a “heat sink”. It often has fins to increase its surface area and, thereby, improve its efficiency.

In a nutshell this is the premise behind the transistor. It is most often used as an amplifier although it has many many more uses. In every processor in use in every computer/router/smartphone/tablet today there are millions of these transistors which have been integrated into a chip in order to form a circuit but at the heart of it, each of these microscopic transistors is effectively doing what is described above. We will go on to discuss the integration of these transistors into integrated circuits or “chips” in a future post but for now you might like to read a little more here.


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