"Transistor", Shockley, Bell Labs, 1948
Smaller than vacuum tubes, more reliable, switches faster.
(not used in computers until about 1958, tricky)
Suppose a modern CPU were made out of vacuum tubes? Something like 10 cubic centimetres times 10 million,... compare to size of lecture hall?
In a way, the transistor is to the vacuum tube as the vacuum tube was to the mechanical storage mechanisms of the Babbage devices and their predecessors. Further electronics advances continue to help to make computers smaller, cheaper, and more reliable; but the use of the transistor was the last fundamental change in the technological basis for the building of computing devices... so far.