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The transistor is a fundamental building block in modern electronic systems. It was smaller, cheaper, and more reliable than the vacuum tubes that were used in the early computers. It is a three-terminal, solid-state electronic device, and it can control electric current or voltage between two of the terminals by applying an electric current or voltage to the third terminal. The three-terminal transistor enables an electric switch to be made which can be controlled by another electric switch. Complicated logic circuits may be built up by cascading these switches (switches that control switches that control switches, and so on).