Light to Electricity

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There is an enormous amount of light around us. The light from the sun carries energy. Hence we can clearly categorize it as a renewable source of energy. With the advancement of science and technology, the device has been discovered which is capable of transferring light energy into electrical energy/ electrical current. Such devices are called Photodiodes.

It is a semiconductor device which is capable of converting light into electric current. When a light photon is absorbed by the photodiode, an electric current of equal energy is generated. The generated current is stored for various activities. A typical photodiode consists of built-in lenses, optical filters with varied surface area(Large/small). The increase in surface area results in reducing response time. The traditional solar cells which are commercially/domestically used to generate electricity is an example for large area photodiode.

Photodiodes are very similar to other semiconductor diodes except for the fact that these are designed to absorb the photons from the definite electromagnetic range(UV/X-Ray/Visible) These diodes use PIN Junction in the place of p-n junction, this not only increases the response time but also capable of working in reverse biased condition.

The prime working principle involved here is the photoelectric effect; when a light photon with sufficient energy strikes photosensitive surface an electron-hole pair is generated. When this setup is subjected to a potential difference, these generated electrons and holes move towards the corresponding cathode and anode resulting in photocurrent. There are two types of currents involved; the photocurrent and the dark current(the current that is produced in the absence of light). The total current through a photodiode is the sum of both. The dark current is minimised as to maximise the sensitivity of photosensors. The resultant current is independent of the angle of incidence of light photons.

There are mainly two modes of operation; Photovoltaic mode and Photoconductive mode. The photodiodes, when operated in Photovoltaic mode or zero bias mode, the voltage is generated by restricting the flow of photocurrent beyond the device. This mode employs the photovoltaic effect; the current and voltage are developed by incorporating physical and chemical phenomenon across the device when exposed to light. This is bias for the traditional solar cell which is enfolded under large area photodiode.

When operated in Photoconductive mode, the photodiodes are under reverse biased condition. This additional reverse bias widens the depletion region, eventually resulting in the reduction of response time as well as junction capacitance which Is the ability of any system to store electric charge, unit of capacitance is farad. This condition also increases the dark current in the system without effecting photocurrent. The photocurrent is directly proportional to irradiance or illuminance, for a definite spectral distribution. This mode is faster at the cost of electronic noise. Other than these two modes, there is avalanche mode of operation and many more which are used only to achieve specific performance.

The application of this miraculous electronic component is immense. These are embedded at every scale from consumer electronics like- smoke detectors, CD player, Television, Various sensors, Cameras, LED’s, Remote control devices etc to Astronomy, Laser rangefinding, Spectroscopy, Night vision equipment etc.

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Marcus Haney

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