Transistors Changed Everything. Here’s How They Work

Last week I revealed the sneaky physics behind the “joule thief,” an electrical circuit that lets you squeeze more energy out of seemingly dead batteries. The key, it turned out, was a clever pairing of a transformer and a transistor. But the transistor deserves its own headline, because it's maybe only the most important invention of the 20th century.

Life would be completely different without transistors. For starters, there would be no personal computers or cell phones, so no Amazon, no video games, no dating apps, no messaging, no streaming, no social media, no Apple Pay or Google Maps, and no AI (hmm). Cars contain billions of transistors. They're really everywhere. So what is a transistor?

The best way to understand this pivotal technology is to follow the evolutionary trail, all the way back to the electric relays used in telegraphs in the 1800s.

Electric Relays

Telegraph lines were basically long electrical circuits. And you probably never thought about it before, but they ran on batteries—primitive, low-voltage batteries that filled an entire closet at the telegraph station—because there was no electrical grid back then.

The electric relay, invented in 1835, was a key component in these systems. Basically, it was a switch that turned an electrical current on and off—sort of like the wall switch that you use to turn a light on. But lacking fingers, the relay used a second electric current to flip the switch. Why would you use a current to turn on a current? That's a good question.

Suppose you wanted to turn on a light in another town 50 miles away. Well, you’d string real long wires on utility poles. Then you could toggle the light on and off, and even use a code with dots and dashes to send text messages. But there was a problem: The longer the wire, the more resistance it had, so not enough current would get through to deliver an intelligible signal.

The solution? Split the circuit into two 25-mile lengths and connect them with a relay. When you closed the switch on the first circuit, it operated the relay, sending the same pattern of current from a second battery through the next circuit and turning on the light. Here's a diagram:

Of course, the telegraph used a buzzer instead of a light, but same idea. Anyway, relays are still widely used today, and the idea of using a current to turn on another current opened up all kinds of possibilities. In cars, for example, it lets you use low-power dashboard controls to switch on high-power circuits that operate things like starters, headlights, or air conditioners.

How does the relay work? Basically, it's just an electromagnet. Here's a simple diagram, along with a real-life example:

Inside there's a coil of wire wrapped around an iron core. When a current flows through this control wire, it creates a magnetic field. This pulls down the metal switch on top, which contacts the output wire. You've heard relays. When your oven’s thermostat turns the heating element on and off, it's what makes that audible click.

Although relays are cool and useful, they are basic on/off switches. We have something else that’s sort of like a variable relay—the vacuum tube, invented around 1905, which made possible old-time radios.

Vacuum Tube

You can think of a vacuum tube as a modified light bulb. An incandescent light is just a thin wire that gets super hot when current runs through it, so that it glows. That filament is put inside a glass container, and the air is pumped out so the filament doesn't burn. (That's the whole purpose of the bulb—to keep out oxygen.)

But there's something else that happens, which you can't see: When the filament gets super hot, electrons are ejected from it. Since a flow of electrons is an electric current, you can use these thermal electrons in a way that's similar to a relay. Here's a diagram of a rudimentary vacuum tube:

Electrons travel across the bulb to a collector plate, which creates the output current. Now, it might seem like a silly way to do that. But if we add another wire (a grid) between the filament and the collector, we can control this output current. A negative voltage on the control will push the electrons back away from the collector to reduce the output current. Putting a positive voltage on the grid increases the flow of output current.

So this is again a current switch, and just like the relay, it's controlled by a different wire. But there are two big differences: First, there's no mechanical contact, which means the output current can change much faster. Second, the output current is not just on or off; it can vary with the strength of the control voltage.

This is what made the first audio amplifiers possible. If you had a weak signal from a distant radio station, it would not produce enough current to drive a speaker so that you could hear anything. But if you fed that signal into the control voltage in a vacuum tube, you could get an output that's much stronger yet maintains the same pattern (like music) as the original signal.

But wait! There's something else you could do with vacuum tubes—you could build a computer. Yes, early computers were just a bunch of vacuum tubes controlled by other vacuum tubes, creating logic gates. You used an input signal that is either 1 volt or 0 volts. An AND gate had two signal inputs and one output. If both inputs are 1 volt, it output 1 volt. Otherwise it gave 0 volts. An OR gate would output 1 volt if either of the inputs was 1 volt.

Actually, you could have built a computer with electric relays. But relays are much slower than vacuum tubes, and all that clicking and clacking would have been maddening. Vacuum tubes were silent, purely electronic components, with no moving parts, and that was a game changer.

Transistor

Still, there were three problems with vacuum tubes. They used a lot of power, so early computers ran hot, required massive cooling systems, and were insanely expensive to operate. Second, the tubes were fragile and easily burned out, so the computers required constant maintenance. (Literally, like full-time crews that spent their days locating and replacing dead tubes.) Finally, the tubes were just big. Those early computers, like ENIAC in 1945, filled entire rooms.

The transistor, invented at Bell Labs in 1947, fixed all that by using semiconductors. That's a word you hear all the time, but what is a semiconductor? Well, you know how some materials (like copper) conduct electricity, while others (like rubber) are insulators? Well, a semiconductor (like silicon) can switch between being one or the other.

There are two types of semiconductor: If you add extra electrons to silicon, you get an n-type semiconductor; take away electrons and you get a p-type. Electrons, of course, have negative charges, so the missing electrons act like positive charges, and we call those “electron holes.”

A transistor is created by combining different types of semiconductors. Here’s an example called an NPN transistor:

The two n-type semiconductors are separated by a p-type. This prevents electrons from moving from the input region (the “source”) to the output region (the “drain”). However, if a voltage is applied to the control, or gate, then electrons can move to the output. Yes, once again, it's basically a switch, where one current turns on a second current—but now with much finer control.

Best of all, they could be miniaturized—and boy oh boy oh boy, were they ever. Floor-standing radio consoles were replaced by “transistor radios,” carried everywhere by teens in the 1950s and ’60s. They contained six to 10 transistors. Today, the iPhone 17 Pro contains up to 30 billion transistors. That is crazy, but it's what makes our computer-based world run.