The most commonly used diode for special purpose is:

Explanation

Special Purpose P-N Junction Diodes includes:Zener DiodeLight-Emitting DiodePhotodiodeA solar cell or photovoltaic devicesLight-emitting diodes, photodiodes, and photovoltaic devices are known as optoelectronic junction devices. Light-Emitting Diode (LED)Light-Emitting Diode is an electric component that emits light when the electric current passes through it. Light-Emitting Diode (LED) is a specific type of diode having similar characteristics as the p-n junction diode. When electricity passes through the LED, the electrons recombine with holes emitting light in the process.The symbol of the diode is shown below. The diode is encased with a clear cover to view the emitted light. The LED can be seen in the picture below.When the LED is forward biased, the electrons flow from the N-type to the P-type. Holes are moved from p to n layer. Due to forward bias, at the boundary, the concentration of the minority carriers increases. At the junction, majority carriers recombine with excess minority carriers. When the minority carriers recombine with the majority carriers, energy is released in the form of photons. Photons with energy less or equal than the bandgap are emitted. The intensity of the light increases when the current reaches maximum.The VI-characteristic of a LED is as shown in the figure below.LEDs can emit red, green, orange, yellow, and blue light. LEDs are widely used as decorative lamps, used in television remote, optical communication, automotive, and display devices.Light-Emitting Diodes require less power to operate. They adapt to instant on-off switching capability. They are economical and offer long-life durability.PhotodiodesPhotodiodes are special p-n junction diodes operated in reverse bias. They are mainly designed for detecting optical signals. Photodiodes feature a transparent window which allows light to fall on it. Photodiodes are also addressed with various names like a light detector, photodetector, and photo-sensor. Silicon, Germanium and Indium gallium arsenide are some of the photodiode materials. The symbol of the photodiode is shown below.The real-time image of the photodiode is as shown below. Photodiodes are made of photosensitive semiconducting material to absorb the light which falls on it. Photodiodes are operated under reverse bias conditions below the breakdown voltage.When the energy of the photons which strike (hv) is greater than the energy gap (Eg) of the semiconductor material in the photodiode, electron-hole pairs are created near the depletion region.Electrons and holes are separated before they combine in the presence of an electric field. When electric current passes through it, electrons are accumulated in the n-side and holes in the p-side. EMF is generated when electrons and holes move in the photodiode. current flows through the device when external load is connected. The magnitude of the photocurrent generated is directly proportional to the intensity of incident light.In reverse bias condition, as the intensity of the light increases, the current also increases. Photodiodes are used in logic circuits, solar cell panels, detection circuits, character recognition circuits and optical communication. Photodiodes are used in smoke detectors and fire detectors.