English: A carborundum crystal detector used in early vacuum tube radio receivers, from a 1928 radio magazine. An early semiconductor diode, its function was to rectify the radio signal, extracting the audio (sound) signal from the radio frequency carrier wave. Crystal detectors, first used in unpowered crystal radios in the first decades of the 20th century, were also used in early vacuum tube radios during the 1920s because they were more sensitive than the triode grid leak detectors also used. The detector, inside the cartridge at top, consisted of a piece of carborundum (silicon carbide) with a spring-loaded contact pressing against it. Carborundum was used because it did not require the delicate "cat's whisker" wire contact that other crystal detectors such as galena did, so it did not require adjustment and could be mounted in a sealed cartridge. The carborundum diode had a wide band gap and required a DC bias voltage of several volts across it to reach full sensitivity. This module consists of the diode (top), a bias battery (bottom), and a potentiometer (variable resistor) (center), to adjust the bias. It was mounted on the front panel of the receiver and the potentiometer was adjusted until the radio station was heard loudest from the speaker.