Theory
A photoconductive type diode or a photodiode is a semiconductor diode which depends for its operation on the inner photoelectric effect. The incident light controls the reverse current of the photo diode. When the photons of energy greater than the energy gap of the devices material are absorbed in the device hole electron pair are generated the conduction of the diode increases and the reverse current built up. This is the current mode of operation the current-voltage characteristic cureve for this mode of operation is shown in the figure the operating voltage of photodiode is usually 10-30v. The Dark current does not exceed to 10-20µA for germanium devices and 1-2µA for silicon devices.
A photoconductive type diode or a photodiode is a semiconductor diode which depends for its operation on the inner photoelectric effect. The incident light controls the reverse current of the photo diode. When the photons of energy greater than the energy gap of the devices material are absorbed in the device hole electron pair are generated the conduction of the diode increases and the reverse current built up. This is the current mode of operation the current-voltage characteristic cureve for this mode of operation is shown in the figure the operating voltage of photodiode is usually 10-30v. The Dark current does not exceed to 10-20µA for germanium devices and 1-2µA for silicon devices.
Principle of operation of Photoconductive type of diode
The reversed biased P-N junction is the only way to operate a photosensitive device. When the photon is absorbed in a semiconductor a hole-electron pair is formed and swept across the junction by electric effectand developed across the depletion region. A photo current results owing results to a separated electon hole pair. The minority carrer electon in the p region goes to the n side and a hole in N region goes to the P-side. Separation of photon generated hole-electron pair is more likely to occur when the pair is formed ion the region where there is electric field. The flow of photocurrent in external circuit is proportional to the illumination.
The distribution of the electric field in a semiconductor diode is not uniform. In the region of P-type diffusion and N-type diffusion, the field is much weaker than it is in centre region known as the depletion region. For the best result photodiode should be made so as to allow the greatest number of photons to be absorbed in the depletion region. The photons should not be absorbed until they have penetrated as far as the depletion region, the should be absorbed before penetrating the depletion region.
The depth at which a photon will penetrate before it is absorbed is the function of its wavelength. Short wavelength photons are absorbed near the surface. Those of longer wavelength may penetrate entire thickness of the crystal. Therefore if a photodiode is to have an broad spectrum of wavelength it should have a very thin P-layer to allow the photons of short wavelength to penetrate, as well as thick depletion region to maximize photo current from the longer wavelength photons. The thickness of depletion regions depends on the resistivity of the region to be depleted and on the reverse bias. A depletion region exists at room temperature even if no reverse bias is applied because of the built in field produced by the minority carriers across the junction. Reverse biasing aids this built-in field and expands the depletion region(at room temperature).
The distribution of the electric field in a semiconductor diode is not uniform. In the region of P-type diffusion and N-type diffusion, the field is much weaker than it is in centre region known as the depletion region. For the best result photodiode should be made so as to allow the greatest number of photons to be absorbed in the depletion region. The photons should not be absorbed until they have penetrated as far as the depletion region, the should be absorbed before penetrating the depletion region.
The depth at which a photon will penetrate before it is absorbed is the function of its wavelength. Short wavelength photons are absorbed near the surface. Those of longer wavelength may penetrate entire thickness of the crystal. Therefore if a photodiode is to have an broad spectrum of wavelength it should have a very thin P-layer to allow the photons of short wavelength to penetrate, as well as thick depletion region to maximize photo current from the longer wavelength photons. The thickness of depletion regions depends on the resistivity of the region to be depleted and on the reverse bias. A depletion region exists at room temperature even if no reverse bias is applied because of the built in field produced by the minority carriers across the junction. Reverse biasing aids this built-in field and expands the depletion region(at room temperature).
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