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.”