The Diode is composed of a PN junction, and the photodiode can convert the optical signal into an electrical signal, as shown below:
Usually, the covalent bond is ionized when the PN junction is illuminated with light. This creates holes and electron pairs. The photocurrent is generated due to the generation of electron-hole teams. When photons with energy exceeding 1.1 eV hit the Diode, electron-hole pairs will be formed. When a photon enters the depleted region of the Diode, it hits the atom with high energy. This results in the release of electrons from the atomic structure. After the electrons are released, free electrons and holes are generated. In general, electrons are negatively charged, and holes are positively charged. The depleted energy will have a built-in electric field. Because of this electric field, the electron-hole pair is far away from the PN junction. Therefore, the holes move toward the anode, and the electrons move toward the cathode to generate a photocurrent.
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The material of the photodiode determines many of its characteristics. The essential characteristic is the wave of light that the photodiode responds to, and the other is the noise level, both of which depend mainly on the materials used in the photodiode. Different materials use different responses to wavelengths because only photons with sufficient energy can excite electrons in the material’s band gap and generate significant power to generate current from the photodiode.
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Although the wavelength sensitivity of materials is significant, another parameter that may significantly impact the performance of photodiodes is the noise level generated. Because of their more significant band gap, silicon photodiodes produce less noise than germanium photodiodes. However, it is also necessary to consider the wavelength of the photodiode, and the germanium photodiode must be used for wavelengths longer than 1000 nm.
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