Definition: A semiconductor device that detects light with a p-n or p-i-n structure. Photodiodes are often used as photodetectors. Such devices contain a p-n junction and usually have an intrinsic layer between the n and p layers. Devices with intrinsic layers are called PIN-type photodiodes. The depletion layer or the intrinsic layer absorbs light and generates electron-hole pairs, which contribute to the photocurrent. Over a wide power range, the photocurrent is strictly proportional to the absorbed light intensity. Operating mode Photodiodes can operate in two different modes: Photovoltaic mode: Similar to a solar cell, the voltage produced by a photodiode irradiated by light can be measured. However, the relationship between voltage and optical power is nonlinear, and the dynamic range is relatively small. And it can't reach peak speeds either. Photoconductive mode: At this point a reverse voltage is applied to the diode (ie, the diode is non-conductive at this voltage in the absence of incident light) and the resulting photocurrent is measured. (It is sufficient to keep the voltage close to 0.) The dependence of the photocurrent on the optical power is very linear, and its magnitude is six orders of magnitude or more larger than the optical power, e.g., for a silicon p-i-n with an active area of several mm2 For photodiodes, the latter ranges from a few nanowatts to tens of milliwatts. The magnitude of the reverse voltage has almost no effect on the photocurrent and has a weak effect on the dark current (in the absence of light), but the higher the voltage, the faster the response and the faster the device heats up. Common amplifiers (also called transimpedance amplifiers) are often used for pre-amplification of photodiodes. This amplifier keeps the voltage constant (eg, close to 0, or some adjustable negative number) so that the photodiode operates in photoconductive mode. And current amplifiers generally have good noise properties, and the sensitivity and bandwidth of the amplifier can be better balanced than a simple loop consisting of a resistor and a voltage amplifier. Some commercial amplifier setups use many different sensitivity settings to make the measurement power very flexible in the laboratory, so you can get a large dynamic range, low noise, some have built-in displays, adjustable bias voltage and signal offset, can be tuned filters, etc. Semiconductor material: Typical photodiode materials are: Silicon (Si): small dark current, fast speed, high sensitivity in the 400-1000nm range (highest in the 800-900nm range). Germanium (Ge): high dark current, slow speed due to large parasitic capacitance, high sensitivity in the range of 900-1600nm (highest in the range of 1400-1500nm). Indium Gallium Arsenide Phosphorus (InGaAsP): Expensive, low dark current, fast, high sensitivity in the 1000-1350nm range (highest in the 1100-1300nm range). Indium Gallium Arsenide (InGaAs): Expensive, low dark current, fast, high sensitivity in the 900-1700nm range (highest in the 1300-1600nm range) The wavelength range described above can be greatly exceeded if a model with a wider spectral response is used. key properties: The most important properties of photodiodes are: Responsivity, which is photocurrent divided by optical power, is related to quantum efficiency and depends on wavelength Active area, i.e. light sensitive area. Maximum allowable current (usually limited by saturation effects). Dark current (exists in photoconductive mode, very important for detecting very low light intensities). Speed, or bandwidth, is related to rise and fall times and is affected by permittivity.
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