Electrons travel through the vacuum tube toward the collector plate, creating an output current. While this may seem like a simple process, the current can be controlled by placing a third wire, called a control grid, between the filament and collector. Applying a negative voltage to the grid repels electrons and reduces the output current. Applying a positive voltage attracts more electrons and increases current flow.
A vacuum tube therefore works as an electronic current switch, much like a relay controlled by a separate wire. However, vacuum tubes have two important advantages over mechanical relays. First, they have no moving contacts, allowing the output current to change much faster. Second, the current is not limited to simply on or off; it can vary according to the strength of the control voltage.
This variable current made the first electronic audio amplifiers possible. A weak signal from a distant radio station might not provide enough power to drive a speaker. By applying that signal to the tube’s control grid, the vacuum tube could produce a much stronger output while preserving the original pattern of the signal, including speech or music.
Vacuum tubes could also be used to build early computers. These machines connected vacuum tubes together to create logic gates. Digital systems used input signals representing either 1 volt or 0 volts. An AND gate produced a 1-volt output only when both inputs were 1 volt. If either input was 0 volts, the output was 0 volts. An OR gate produced a 1-volt output when at least one input was 1 volt.
Computers could also be built with electromechanical relays. However, relays operated much more slowly than vacuum tubes and often produced noticeable clicking or rattling sounds. Vacuum tubes were fully electronic, silent, and free of moving parts—a major breakthrough in the development of modern computing.
Transistors
Despite their advantages, vacuum tubes had three major drawbacks. They consumed large amounts of electricity, causing early computers to generate significant heat and require complex cooling systems. They also needed frequent maintenance because the fragile tubes could burn out easily. In some cases, entire teams were needed to locate and replace failed tubes. Finally, vacuum tubes were physically large. The 1945 ENIAC, for example, filled an entire room.
The transistor, invented at Bell Laboratories in 1947, solved many of these problems by using semiconductor materials. A semiconductor has electrical properties between those of a conductor and an insulator. Metals such as copper conduct electricity easily, while materials such as rubber resist electrical flow. Semiconductors such as silicon can be precisely controlled to either conduct or restrict electricity, making them ideal for electronic switches and amplifiers.
There are two primary types of semiconductor material: n-type and p-type. Adding extra electrons to silicon creates an n-type semiconductor. Removing an electron creates a p-type semiconductor. Because electrons carry a negative charge, the empty space left behind behaves like a positive charge. This space is called an electron hole, or simply a hole.
Source: www.wired.com