Posted January 18, 2018
What does a transistor actually do?
The electronic components inside a pocket-style analog hearing aid.
Photo: Compact hearing aids were among the first applications for transistors—and this one dates from about the late 1950s or 1960s. About the size of a pack of playing cards, it was designed to be worn in or on a jacket pocket. There's a microphone on the other side of the case that picks up ambient sounds. You can clearly see the four little back transistors inside, amplifying those sounds and then shooting them out to the little loudspeaker that sits in your ear.
A transistor is really simple—and really complex. Let's start with the simple part. A transistor is a miniature electronic component that can do two different jobs. It can work either as an amplifier or a switch:
When it works as an amplifier, it takes in a tiny electric current at one end (an input current) and produces a much bigger electric current (an output current) at the other. In other words, it's a kind of current booster. That comes in really useful in things like hearing aids, one of the first things people used transistors for. A hearing aid has a tiny microphone in it that picks up sounds from the world around you and turns them into fluctuating electric currents. These are fed into a transistor that boosts them and powers a tiny loudspeaker, so you hear a much louder version of the sounds around you. William Shockley, one of the inventors of the transistor, once explained transistor-amplifiers to a student in a more humorous way: "If you take a bale of hay and tie it to the tail of a mule and then strike a match and set the bale of hay on fire, and if you then compare the energy expended shortly thereafter by the mule with the energy expended by yourself in the striking of the match, you will understand the concept of amplification."
Transistors can also work as switches. A tiny electric current flowing through one part of a transistor can make a much bigger current flow through another part of it. In other words, the small current switches on the larger one. This is essentially how all computer chips work. For example, a memory chip contains hundreds of millions or even billions of transistors, each of which can be switched on or off individually. Since each transistor can be in two distinct states, it can store two different numbers, zero and one. With billions of transistors, a chip can store billions of zeros and ones, and almost as many ordinary numbers and letters (or characters, as we call them). More about this in a moment.
The great thing about old-style machines was that you could take them apart to figure out how they worked. It was never too hard, with a bit of pushing and poking, to discover which bit did what and how one thing led to another. But electronics is entirely different. It's all about using electrons to control electricity. An electron is a minute particle inside an atom. It's so small, it weighs just under 0.000000000000000000000000000001 kg! The most advanced transistors work by controlling the movements of individual electrons, so you can imagine just how small they are. In a modern computer chip, the size of a fingernail, you'll probably find between 500 million and two billion separate transistors. There's no chance of taking a transistor apart to find out how it works, so we have to understand it with theory and imagination instead. First off, it helps if we know what a transistor is made from.
How is a transistor made?
A silicon wafer
Photo: A wafer of silicon. Photo by courtesy of NASA Glenn Research Center (NASA-GRC).
Transistors are made from silicon, a chemical element found in sand, which does not normally conduct electricity (it doesn't allow electrons to flow through it easily). Silicon is a semiconductor, which means it's neither really a conductor (something like a metal that lets electricity flow) nor an insulator (something like plastic that stops electricity flowing). If we treat silicon with impurities (a process known as doping), we can make it behave in a different way. If we dope silicon with the chemical elements arsenic, phosphorus, or antimony, the silicon gains some extra "free" electrons—ones that can carry an electric current—so electrons will flow out of it more naturally. Because electrons have a negative charge, silicon treated this way is called n-type (negative type). We can also dope silicon with other impurities such as boron, gallium, and aluminum. Silicon treated this way has fewer of those "free" electrons, so the electrons in nearby materials will tend to flow into it. We call this sort of silicon p-type (positive type).
Quickly, in passing, it's important to note that neither n-type or p-type silicon actually has a charge in itself: both are electrically neutral. It's true that n-type silicon has extra "free" electrons that increase its conductivity, while p-type silicon has fewer of those free electrons, which helps to increase its conductivity in the opposite way. In each case, the extra conductivity comes from having added neutral (uncharged) atoms of impurities to silicon that was neutral to start with—and we can't create electrical charges out of thin air! A more detailed explanation would need me to introduce an idea called band theory, which is a little bit beyond the scope of this article. All we need to remember is that "extra electrons" means extra free electrons—ones that can freely move about and help to carry an electric current.
Silicon sandwiches
We now have two different types of silicon. If we put them together in layers, making sandwiches of p-type and n-type material, we can make different kinds of electronic components that work in all kinds of ways.
Suppose we join a piece of n-type silicon to a piece of p-type silicon and put electrical contacts on either side. Exciting and useful things start to happen at the junction between the two materials. If we turn on the current, we can make electrons flow through the junction from the n-type side to the p-type side and out through the circuit. This happens because the lack of electrons on the p-type side of the junction pulls electrons over from the n-type side and vice-versa. But if we reverse the current, the electrons won't flow at all. What we've made here is called a diode (or rectifier). It's an electronic component that lets current flow through it in only one direction. It's useful if you want to turn alternating (two-way) electric current into direct (one-way) current. Diodes can also be made so they give off light when electricity flows through them. You might have seen these light-emitting diodes (LEDs) on pocket calculators and electronic displays on hi-fi stereo equipment.
The electronic components inside a pocket-style analog hearing aid.
Photo: Compact hearing aids were among the first applications for transistors—and this one dates from about the late 1950s or 1960s. About the size of a pack of playing cards, it was designed to be worn in or on a jacket pocket. There's a microphone on the other side of the case that picks up ambient sounds. You can clearly see the four little back transistors inside, amplifying those sounds and then shooting them out to the little loudspeaker that sits in your ear.
A transistor is really simple—and really complex. Let's start with the simple part. A transistor is a miniature electronic component that can do two different jobs. It can work either as an amplifier or a switch:
When it works as an amplifier, it takes in a tiny electric current at one end (an input current) and produces a much bigger electric current (an output current) at the other. In other words, it's a kind of current booster. That comes in really useful in things like hearing aids, one of the first things people used transistors for. A hearing aid has a tiny microphone in it that picks up sounds from the world around you and turns them into fluctuating electric currents. These are fed into a transistor that boosts them and powers a tiny loudspeaker, so you hear a much louder version of the sounds around you. William Shockley, one of the inventors of the transistor, once explained transistor-amplifiers to a student in a more humorous way: "If you take a bale of hay and tie it to the tail of a mule and then strike a match and set the bale of hay on fire, and if you then compare the energy expended shortly thereafter by the mule with the energy expended by yourself in the striking of the match, you will understand the concept of amplification."
Transistors can also work as switches. A tiny electric current flowing through one part of a transistor can make a much bigger current flow through another part of it. In other words, the small current switches on the larger one. This is essentially how all computer chips work. For example, a memory chip contains hundreds of millions or even billions of transistors, each of which can be switched on or off individually. Since each transistor can be in two distinct states, it can store two different numbers, zero and one. With billions of transistors, a chip can store billions of zeros and ones, and almost as many ordinary numbers and letters (or characters, as we call them). More about this in a moment.
The great thing about old-style machines was that you could take them apart to figure out how they worked. It was never too hard, with a bit of pushing and poking, to discover which bit did what and how one thing led to another. But electronics is entirely different. It's all about using electrons to control electricity. An electron is a minute particle inside an atom. It's so small, it weighs just under 0.000000000000000000000000000001 kg! The most advanced transistors work by controlling the movements of individual electrons, so you can imagine just how small they are. In a modern computer chip, the size of a fingernail, you'll probably find between 500 million and two billion separate transistors. There's no chance of taking a transistor apart to find out how it works, so we have to understand it with theory and imagination instead. First off, it helps if we know what a transistor is made from.
How is a transistor made?
A silicon wafer
Photo: A wafer of silicon. Photo by courtesy of NASA Glenn Research Center (NASA-GRC).
Transistors are made from silicon, a chemical element found in sand, which does not normally conduct electricity (it doesn't allow electrons to flow through it easily). Silicon is a semiconductor, which means it's neither really a conductor (something like a metal that lets electricity flow) nor an insulator (something like plastic that stops electricity flowing). If we treat silicon with impurities (a process known as doping), we can make it behave in a different way. If we dope silicon with the chemical elements arsenic, phosphorus, or antimony, the silicon gains some extra "free" electrons—ones that can carry an electric current—so electrons will flow out of it more naturally. Because electrons have a negative charge, silicon treated this way is called n-type (negative type). We can also dope silicon with other impurities such as boron, gallium, and aluminum. Silicon treated this way has fewer of those "free" electrons, so the electrons in nearby materials will tend to flow into it. We call this sort of silicon p-type (positive type).
Quickly, in passing, it's important to note that neither n-type or p-type silicon actually has a charge in itself: both are electrically neutral. It's true that n-type silicon has extra "free" electrons that increase its conductivity, while p-type silicon has fewer of those free electrons, which helps to increase its conductivity in the opposite way. In each case, the extra conductivity comes from having added neutral (uncharged) atoms of impurities to silicon that was neutral to start with—and we can't create electrical charges out of thin air! A more detailed explanation would need me to introduce an idea called band theory, which is a little bit beyond the scope of this article. All we need to remember is that "extra electrons" means extra free electrons—ones that can freely move about and help to carry an electric current.
Silicon sandwiches
We now have two different types of silicon. If we put them together in layers, making sandwiches of p-type and n-type material, we can make different kinds of electronic components that work in all kinds of ways.
Suppose we join a piece of n-type silicon to a piece of p-type silicon and put electrical contacts on either side. Exciting and useful things start to happen at the junction between the two materials. If we turn on the current, we can make electrons flow through the junction from the n-type side to the p-type side and out through the circuit. This happens because the lack of electrons on the p-type side of the junction pulls electrons over from the n-type side and vice-versa. But if we reverse the current, the electrons won't flow at all. What we've made here is called a diode (or rectifier). It's an electronic component that lets current flow through it in only one direction. It's useful if you want to turn alternating (two-way) electric current into direct (one-way) current. Diodes can also be made so they give off light when electricity flows through them. You might have seen these light-emitting diodes (LEDs) on pocket calculators and electronic displays on hi-fi stereo equipment.
