Wikijunior:How Things Work/Transistor
Who invented it?
The first transistor was invented by a person called Julius Edgar Lilienfield, but he didn't build one or write any papers on it, so everybody ignored it. The transistor as we know it today was invented by two scientists, called William Shockley and John Bardeen. The transistor was named by John Pierce, who joined the words transfer and varistor.
How does it get power?
The transistor gets power from electricity, which it gets through its 'legs'.
How does it work?
The very first transistor used a crystal made of Germanium, a metal-like substance. The Germanium crystal usually stops power from going through it, however an interesting property was found if it had three 'legs'. These three legs are called the 'gate' (or 'base'), the 'source' (or 'collector') and the 'drain' (or 'emitter'). When electricity is sent through the gate, it lets electricity flow between the source and the drain. This happens because the gate can 'inject' some electrons into the Germanium, allowing electricity to go through it.
Modern transistors use silicon. Silicon works better than Germanium crystals, but it has to be very pure to work at all. Any semiconductor (a substance which can be 'injected' with electrons, like Germanium or Silicon) will work, but Silicon is the best one we can use.
You can still buy transistors with three "legs", but they're far too big to fit enough into a computer. Instead, lots of very small transistors on one computer chip are used. These can join together without needing big metal wires.
How dangerous is it?
Because transistors don't need much power, a single one cannot electrocute you much.
What does it do?
Transistors are very useful. They can be used as switches, or a lot of them can be linked together to make a microchip, which controls your computer.
How has it changed the world?
Transistors have changed the world a lot. Without transistors, we wouldn't have small computers (only giant computers, the size of a room), and you wouldn't be able to read this. Computers control almost everything today, and we owe all that to the transistor. You need a lot of them though; a computer chip can need billions (thousands of millions) of them!
What idea(s) and/or inventions had to be developed before it could be created
Transistors could never have been made without a good understanding of electricity. You can read more about electricity here. It was only in 1800 that the first batteries were made. Before then there weren't good electricity supplies, so no-one could use a transistor.
We also had to learn how to make good-quality, carefully controlled semiconductors. These are materials that are half-way between conducting electricity, like metal, and blocking it, like plastic. This is part of the reason why Julius Edgar Lilienfield's ideas about transistors weren't very practical in 1925.
Before transistors were invented, Surface Physics, which explains why transistors work, had to be discovered.