Inside every electronic device, from televisions to air traffic control systems, there are small circuits continually switching on and off telling the device what to do. In a computer, for instance, combinations of switching circuits represent all the millions of numbers, and other data the computer handles. Switching devices Electronic systems use a variety of devices for switching the flow of electricity on and off, including resistors and capacitors. But the most important switching devices are transistors. Transistors are made using semiconductors, such as germanium and silicon, whose ability to conduct electricity rises with temperature.
Most transistors are kinds of triodes.
Triodes and diodes were devices originally developed to amplify radio signals. Diodes were valves with two electrodes that allowed current to pass only one way. A triode has a third electrode which can be used to switch in a much bigger current, so amplifying a faint signal dramatically. Most transistors are now incorporated into integrated circuits. This means that all the circuitry is packed inside a single tiny sliver or chip of silicon.