= Transistor
{wiki}
A <transistor> is a <semiconductor> device in which an input controls an output <electric current>. Used with <biasing> and an <electrical load>, it can form an <electronic amplifier> or a switch. Its <power gain> comes from an external supply: the input <signal> controls the conversion of supplied <energy> into output <signal> <power>.
The <point-contact transistor> makes the idea tangible: two nearby contacts interact through one piece of <semiconductor>. A <bipolar junction transistor> instead uses two <p-n junctions> and a thin intervening region. The distinction matters both for the history and for choosing a physical model.
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