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    Diode(from ancient Greek δις - two and -od - from the end -od of the term electrode; literally "two-electrode"; root -od comes from ancient Greek ὁδός "path") - an electronic element with different conductivity in depending on the direction of the electric current. Diode electrodes are called anode and cathode.
A semiconductor diode consists of either p-type and n-type semiconductors (semiconductors with different types of impurity conductivity), or semiconductor and metal (Schottky diode). The contact between semiconductors is called a pn junction and conducts current in one direction (has one-way conductivity).

Special types of diodes:

• Zener diodes (Zener diodes) are diodes operating in the mode of (reversible) breakdown of the pn junction (see the reverse branch of the current-voltage characteristic). Used for voltage stabilization.
• Tunnel diodes (Leo Esaki diodes) are diodes that use quantum mechanical effects. On the current-voltage characteristic, they have a region of the so-called "negative resistance". They are used as amplifiers, generators, etc.
• Inverted diodes are diodes that have a much lower on-state voltage drop than conventional diodes. The principle of operation of such diodes is based on the tunneling effect.
• Point diodes are diodes characterized by a low p-n-junction capacitance and the presence on the reverse branch of the current-voltage characteristic of a section with a negative differential resistance. Previously, they were used in microwave technology (due to the low capacity of the p-n-junction) and were used in generators and amplifiers (due to the presence on the reverse branch of the current-voltage characteristic of a section with negative differential resistance).
• Varicaps (John Geumm diodes) are diodes with a large capacity when the pn junction is locked, depending on the applied reverse voltage. They are used as variable capacitors.
• LEDs (Henry Round diodes) are diodes that differ from ordinary diodes in that they emit light in the visible range (and not in the infrared) when electrons and holes recombine in the pn junction. LEDs are also produced with radiation in the infrared range, and more recently - in the ultraviolet.
• Semiconductor lasers are diodes that are similar in design to LEDs, but have an optical resonator. Emit coherent light.
• Photodiodes are diodes that are exposed to light.
• A solar cell is a diode similar to a photodiode but operates without bias. Light falling on the pn junction causes the movement of electrons and the generation of current.
• Gunn diodes are diodes used for generating and converting frequencies in the microwave range.
• The Schottky diode is a low voltage drop diode with direct power on.
• An avalanche diode is a diode whose principle of operation is based on an avalanche breakdown (see the reverse section of the current-voltage characteristic). It is used to protect circuits from overvoltage.
• Avalanche-transit diode is a diode, the principle of operation of which is based on avalanche multiplication of charge carriers. It is used to generate oscillations in microwave technology.
• Magnetodiode is a diode, the current-voltage characteristic of which significantly depends on the value of the magnetic field induction and the location of its vector relative to the plane of the p-n-junction.
• Stabilizers are diodes, during the operation of which a section of the branch of the current-voltage characteristic is used, corresponding to the "forward voltage" on the diode.
• A mixing diode is a diode designed to multiply two high-frequency signals.
• pin diode - a diode with a lower capacitance due to the presence between heavily doped p- and n-type semiconductors of a material characterized by its own conductivity. Used in microwave technology, power electronics, as a photodetector.

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