A diode is a solid-state device that allows current to flow in only one direction, a process known as rectification. Diodes are a fundamental component of electrical circuits. They are also used to form other components, such as the bipolar transistor which uses two diodes in series.
History of the Diode
Thermionic rectifiers were discovered in 1873 by Frederick Guthrie, and later rediscovered by Thomas Edison in 1880, while crystal rectifiers were discovered in 1874 by Karl Braun. It wasn’t until 1919 that rectifiers were renamed diodes by William Eccles, although power diodes are still called rectifiers today. The name diode comes from the Greek for “two path” (di and odos).
How a Diode Works
A junction area, known as the depletion layer, forms around the boundary between two different semiconductors. The boundary is usually created by doping one half of a silicon substrate with a chemical. The substrate remains conductive but the junction is non-conductive, due to the potential difference created when charge carriers (electrons and holes) diffuse through the boundary. When a voltage of reverse polarity to this potential difference is applied, the charge carriers join and current flows, a process known as recombination.
Schottky and Zener diodes are slightly different. Schottky diodes have a metal and semiconductor junction. This allows fast switching between the conducting and non-conducting states because there is no recovery time, unlike regular diodes which need time to change. A Zener diode works like a regular diode, but also allows current to flow in the other direction if the voltage exceeds the “Zener voltage”.
A diode has two terminals: cathode and anode. On circuit diagrams, the diode symbol is an arrow head and a perpendicular line. The arrow represents the anode, and the line represents the cathode, as it does on the diode case. Current flows through the diode when the cathode is made negative, and the anode is made positive. However, the current stops if the polarity is reversed.
Types of Diodes
There are many different types, including light-emitting diodes, and peltier diodes. The major development in recent years has been the organic light-emitting diode (OLED). They are made from plastic, and are used to make thin video screens which have better visual quality than LCD or plasma screens.
Light-emitting diodes (LEDs) produce light in a process called electroluminescence. When an electron meets a hole at the junction, it drops to a lower energy level and releases a photon of a certain wavelength. These photons are the light emitted by the LED, and their color (wavelength) depends on the materials used in the diode. These materials include gallium nitride which produces green light, and diamond which produces ultraviolet light. A special type of LED is the laser diode, used in CD/DVD players and optical fiber networks. The photodiode behaves in the opposite way to an LED, by creating a current when photons are absorbed from light striking it’s surface.
Peltier diodes absorb heat on one side of the junction, and emit it from the other side. This transfer effect allows them to be used as thermoelectric heat pumps. However, they have such low efficiency (under 10%) that they are only used when the benefits of a solid state device justify their inefficiency. They are commonly used to cool other electronic components where mechanical cooling would be impractical.