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A diode (also called a rectifier) is an electronic device which allows current to pass through it in one direction, but not in the other.

The earliest diodes were vacuum tubes, but roughly contemporaneous were primitive semiconductor devices called 'cat's whiskers diodes', formed by placing a metal contact onto a piece of semiconducting material, and moving the whisker around until it hit a place that produced the desired diode effect. Tubes were used for 'serious' applications, since the early semiconductor ones were very unreliable.

The physics behind the early semiconductor diodes was not actually understood, and research into how and why they worked was a key part of the semiconductor 'revolution' in electronics that led to the invention of the transistor.