Have you ever lifted a vintage guitar amp? They are massive and heavy, primarily because until recently they've been powered by vacuum tubes. Can you imagine how heavy your Walkman would be if had vacuum tubes -- or how bulky, or how hot?

  The invention that replaced the vacuum tube was developed by three AT&T researchers. They called it a "transistor" and it revolutionized the electronics industry. It also allowed people to take music out of their homes and into their cars, or along for a morning jog. Digital audio would be impossible without the transistor, as would CD players, boom boxes, the VDT you're looking at now, the computer that controls it....

[More information about the transistor.]