A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit.
Functions as a switch with an "on" state and an "off" state.
Functions as a current amplifier to either amplify or rectify the incoming electrical signal, affecting either the current, the voltage, or both.
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Most modern electronics use transistors, especially the kind of stuff that goes into rockets and modern automobiles.