We normally think of something as either a conductor of electricity or an insulator.
Transistors are semi-conductors which means that their ability to conduct electricity can be controlled. There’s an older video I made that explains DC analysis of transistors that you can watch if you visit Facebook.com/ElectronicThinking.
Let’s examine conductors and insulators first. Every time you use a cloth to pick up something hot, you’re using an insulator. In this case a heat insulator. And the reason we use metal skillets and pots is because metal conducts heat.
It all comes to this. A conductor allows something to move from one place to another and an insulator stops this movement. Silicon and a few other elements are a bit different though. Silicon is normally an insulator of electricity. But it can be turned into a conductor by adding impurities. This is called doping and it involves hitting the silicon with some fast moving elements such as boron which then embed themselves inside the silicon.
In order for anything to conduct electricity, it has to contain ions which are positive or negative charged atoms. By carefully choosing what elements to inject into the silicon and how many, we can introduce atoms that can either easily supply extra electrons or accept electrons. It’s the electrons that move around. That’s the charge. By injecting boron for example, it’ll pull an electron from a nearby silicon atom which creates a positive charged silicon ion. And by injecting arsenic, it’ll provide an extra electron to a nearby silicon atom creating a negative charged silicon ion.
There are two basic types called NPN and PNP transistors. They consist of three alternating doped sections of silicon which is why there are two types. Each transistor has three connections each leading to one of the N or P sections. The connections are called the emitter, base, and collector. Because of how the sections are made, the transistor will initially not conduct any current from the emitter to the collector. But by raising the voltage at the base above a certain threshold until current starts to flow through the base, it has the effect of causing a larger amount of current to flow through the emitter and collector. This allows a transistor to operate as an amplifier.
Listen to the full episode to learn how transistors can operate as switches and why this is needed by computers. Or you can also read the full transcript below.
Transcript
Transistors are semi-conductors which means that their ability to conduct electricity can be controlled. A full explanation of how this is done is probably too difficult to explain with audio only. There’s an older video I made that explains DC analysis of transistors that you can watch if you visit Facebook.com/electronicthinking.
I used to make videos explaining electrical engineering and some of those are still available on Facebook if you go directly to the Electronic Thinking page. What I’ll describe here are the concepts and how transistors relate to computer programming.
Let’s examine conductors and insulators first. Every time you use a cloth to pick up something hot, you’re using an insulator. In this case a heat insulator. And the reason we use metal skillets and pots is because metal conducts heat.
It all comes to this. A conductor allows something to move from one place to another and an insulator stops this movement. Normally a material such as metal or wood or glass or plastic will be either a conductor or an insulator or somewhere in-between. It doesn’t change. It is possible for a material like metal to be a good conductor of both heat and electricity but an insulator of other things like light. We haven’t yet figured out how to make transparent aluminum. The point is when deciding whether something is a conductor or an insulator, you have to ask what’s being moved.
Silicon and a few other elements are a bit different though. Silicon is normally an insulator of electricity. But it can be turned into a conductor by adding impurities. This is called doping and it involves hitting the silicon with some fast moving elements such as boron which then embed themselves inside the silicon. Sort of like how a bullet embeds itself in its target.
Another good example is water. People always say to be careful with electricity when around water. But is water a conductor or an insulator of electricity? Water itself is a very poor conductor of electricity. It’s a very good insulator. But most water is not pure and contains minerals and salt that allow it to conduct electricity. The reason you need to be careful of water around electricity is because it can soak through your clothing, touch your skin and seep into the pores and wrinkles of your skin which then provides a good path for current to flow. Much less resistance than trying to go through dry socks into the ground. And as the resistance goes down, the amount of current can increase.
So although water can be made to conduct electricity, it can’t be controlled. The water molecules are free to move around and so are the impurities. But silicon is a rigid material with a well defined structure so any impurities we put into it will remain fixed in place.
In order for anything to conduct electricity, it has to contain ions which are positive or negative charged atoms. By carefully choosing what elements to inject into the silicon and how many, we can introduce atoms that can either easily supply extra electrons or accept electrons. It’s the electrons that move around. That’s the charge. By injecting boron for example, it’ll pull an electron from a nearby silicon atom which creates a positive charged silicon ion. And by injecting arsenic, it’ll provide an extra electron to a nearby silicon atom creating a negative charged silicon ion.
If you’re thinking, wait a minute, isn’t arsenic poisonous, well, yes. Integrated circuits are full of dangerous material. And making them is also dangerous. I used to work in a factory that made integrated circuits and we had to be very careful with the arsenic. It would get into anything that came into contact with it. Even the oil in the vacuum pumps would get contaminated with arsenic and required special handling. Arsenic doesn’t need to be eaten to cause problems. Our skin is able to absorb arsenic so touching oil containing arsenic is not a healthy activity.
Back to transistors, there are two basic types called NPN and PNP transistors. They consist of three alternating doped sections of silicon which is why there are two types. Either negative, positive, negative, or you can have positive, negative, positive. There’s a little more to it than this but that’s the basic configuration.
Some transistors will be designed to handle more power and some might be designed for speed. The transistors included in integrated circuits are microscopic which is why we can fit billions of them onto a single chip. But sometimes, especially for high power transistors, there’ll be just a single larger transistor packaged all by itself.
Each transistor no matter how big has three connections each leading to one of the N or P sections. The connections are called the emitter, base, and collector. Because of how the sections are made, the transistor will initially not conduct any current from the emitter to the collector. But by raising the voltage at the base above a certain threshold until current starts to flow through the base, it has the effect of causing a larger amount of current to flow through the emitter and collector. This allows a transistor to operate as an amplifier.
Think of it like taking a small picture and scaling it up to a larger picture. As long as the larger picture fits on the paper, you’ll get an accurate but bigger picture. But if you try to make the picture too big, then it can’t fit on the paper anymore. This is called saturation.
Have you ever tried talking or signing into a microphone playing over a loudspeaker? It amplifies your voice and makes it louder. But talk too loud or start screaming into the microphone or even turn the volume knob too far and the sound coming out of the speaker will get distorted and sound bad. This is saturation. The output tried to exceed the limits.
Normally for amplifiers, saturation is bad. Nobody wants to listen to distorted music. But computers love saturation. Remember that everything inside a computer is binary, on or off, ones and zeros, high and low voltages. A good way to achieve this is to make sure that every transistor is saturated so that it’s either all the way on or all the way off.
Before ending this episode, I’ll try to briefly explain how transistors work. You know a little of how they’re made now and how they can amplify or saturate signals but how does this actually work? What is it about these positive or negative doped sections of silicon?
The secret comes down to the junctions between the positive and negative sections. This is called the PN junction. A transistor has two junctions. I’ll just explain how a single junction works here. If you apply a positive voltage to the P side of a PN junction, then because similar charges repel each other, it will cause the charge inside the P section to spread out. The important point is that the charge is pushed towards the junction line. And applying a negative voltage to the N side has the same effect inside the N section with the charge also being pushed towards the junction. When this happens, then current can flow through the PN junction because there’s a continuous path of charge carriers across the entire PN junction.
What happens if you reverse the voltages? By applying a negative voltage to the P side, it has the effect of attracting the charge inside the P section toward the negative voltage. And by applying a positive voltage to the N side, the same thing happens. The result is a gap along the PN junction where the charge has been pulled away. Current can’t flow through the silicon when there’s no extra or missing electrons to carry the charge.
The reason silicon is called a semiconductor is because the impurities can have their charge carriers moved around based on voltages applied resulting in an increase of decrease in the conductivity. Sometimes it’ll conduct and sometimes it’ll insulate. Just like turning a switch on or off.