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Semiconductor Diode Construction
A semiconductor diode consists of a piece of silicon doped with impurities so that one half of the crystal has an excess of positive charge carriers and the other half has an excess of negative charge carriers; these two regions are called the "P" and the "N" regions respectively. The point at which they join each other is called a "PN junction." The negative charge carriers (electrons) in the N region of the PN junction are attracted to the positive charge carriers (holes) in the P region, and vice versa. When electrons encounter holes, they merge to form a neutral region in the PN junction; this is called the "charged space region."
Forward and Reverse Biasing
The charged space region prevents current flowing through the PN junction. If the P region is held at a higher voltage than the N region, the charged space region increases in thickness and the PN junction is said to be "reverse biased." If the N region is held at a higher voltage than the P region, the charged space region shrinks and the PN junction is said to be "forward biased." At a certain small forward voltage -- usually around 0.7 V -- the charged space region disappears completely, allowing current to flow through the PN junction. This property of the PN junction is responsible for the diode only allowing current to flow in one direction.
Point-Contact Diode Construction
Point-contact diodes work on the same principle as do standard PN semiconductor diodes. In a point-contact diode, a block of semiconductor with an excess of negative charge carriers is created. A sharp point of a metal is placed on this block of semiconductor; this metal must be from group 3 of the periodic table. Some of the metal migrates into the block of semiconductor, creating a P region within the semiconductor; a charged space region forms between the majority N region and the small P region. The point-contact diode behaves similarly to a standard semiconductor diode.
Uses of Point-Contact Diodes
Point-contact diodes are also called "cat's whisker" diodes or "crystal detectors." Such diodes were used in early radio receivers; these radios used the amplitude of a particular radio frequency to transmit information. The cat's whisker diode was needed to process the radio signal so that it could be interpreted by a audio speaker. Radios of this type were used between 1906 and 1945. Point-contact diodes are now obsolete and are only used in historical replications of old-fashioned radios.