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A semiconductor device which conducts electric current run in one direction only This is the simplest kind of semiconductor device it has two terminals and a single PN junction One diode can be used as a half-wave rectifier or four as a full-wave rectifier Electron tube evacuated glass or metal envelope containing two electrodes a cathode and an anode It is used as a rectifier and as a detector in electronic circuits such as radio and television receivers When a positive voltage is applied to the anode or plate electrons emitted from the heated cathode flow to the plate and return to the cathode through an external load If a negative voltage is applied to the plate electrons cannot escape from the cathode and no plate current flows Thus a diode permits electrons to flow from the cathode to the plate but not from plate to cathode If an alternating voltage is applied to the plate current flows only during the time when the plate is positive The alternating voltage is said to be rectified or converted to dc direct current In the indirectly heated cathode type of tube shown in the illustration the electron emitter consists of a metallic cylinder usually nickel coated with an electron emitter such as a mixture of barium and strontium oxides The heat is provided by a coil of wire heater located inside the sleeve but insulated from it In the directly heated cathode right the heater wire itself serves as the source of electrons and is referred to as the filament Solid-state rectifiers which have largely replaced the vacuum type are also frequently referred to as diodes Heres a simple guide to understanding diode characteristics and their use in circuit design Although diodes have been around for many years few hobbyists truly understand how the devices operate nor are they aware of the many applications to which these little globs of grease are suited and so they are
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