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Photodiodes belong to a large family of semiconductor p-n junction devices. They are sometimes referred to as photosensors or photodetectors. These elements are used to convert the energy of photons, or light energy, into an electric current, unlike LEDs (Light-Emitting Diodes), which draw an electric current to produce it. The first application that comes to mind is photovoltaic panels, but for this purpose there is a special family of photodiodes called solar cells. Another application of photosensors is to use them in dusk to dawn sensors, i.e., lamps that automatically light up at dusk, or in sensors that work together with a laser diode, shining directly on photodiode junction. Such a solution can be found, for example, in laser barrier curtains used in industry, thanks to which they are able to detect any interruption of the laser beam, and thus inform the control system, e.g., of the body reaching the hazardous zone, for example, in the vicinity of a working industrial robot. They are also used in meters and control or switching circuits, but also as infrared light detectors, distance meters, or for fibre optic communication.
are typically used as detectors. By using this type of element in the electronic circuit, the user can measure light Intensity using photodiode. For this to happen, the p-n junction of the photosensor in the system must be reverse biased. This means that the intrinsic region of the junction will be enlarged and the diffusion of majority carriers into the region of the opposite doping type will be hindered. However, if photons of sufficiently high energy are incident on the p-n junction of the photodiode, they are able to knock out electrons from the valence band to the conduction band. If this happens far enough away from the p-n junction, the electron and the hole will have enough time to recombine. However, if this occurs close enough to the p-n junction, the electron will be attracted to the positive terminal and the hole will be attracted to the negative terminal. A current will then flow through a simple electrical circuit using a photodiode. If more photons fall on the photosensor per unit time, more hole-electron pairs will be created, so that the element will generate a current of greater intensity. Thanks to this principle of operation, the photodiode (or in fact the measuring system that uses it as a sensor) is able to measure the intensity of the light falling on it.
When talking about photodiodes, you can often find the characteristics of this type of components, which are plotted on the current diagram, given in amperes[A], depending on the voltage, which is given in volts [V]. As photodiodes are usually reverse biased, and the current produced by it flows in the reverse direction, both values will be presented as negative, and the most important part of the graph will be in the third quadrant of the coordinate system. Such a graph shows an increase in the current intensity with an increase in the voltage difference between the photodiode terminals, but only in a very narrow range, counting from the zero potential difference, and then this value stabilizes very quickly. A further increase in the voltage difference has only a minimal effect on the increase of the current flowing through the diode in the reverse direction because the increase in the current intensity depends mainly on the intensity of light incident on the p-n junction. For this reason, one can notice several lines determining the current waveforms, depending on the voltage applied on its terminals and different intensities of the light incident on p-n junction. For photodiodes the dependence of the current flowing through them on the light intensity is almost linear. When no light falls on the photodiode junction, a negligible current can still be observed, which is referred to as black current. This is because of the thermal energy which, when supplied to the electrons in the valence band, is able to knock them out into the conduction band in small amounts.
The photodiode can also work in the photovoltaic mode. Then no external voltage is applied to the p-n junction. The flow of current, and in fact a minority carrier, then occurs as a result of exposure of the photodiode to light.
A special variation of photodiode using the p-n junction is the PIN photodiode. It has a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. Thus, the wide intrinsic region means the diode will have a relatively low capacitance and inertia. This results in a very quick response to changes in light intensity.
Another type of photodiode is an avalanche photodiode. Its design and principle of operation is almost identical to a standard photodiode. The difference is that it is applied with a very high reverse bias voltage. As a result of photons falling on the photodiode junction, electrons move to the conduction band, and are accelerated to such an extent that they knock other bound electrons free. This is the so-called avalanche effect. In this way, even the smallest amount of light falling on photosensor junction can be detected.
Photodiodes are characterized by several key parameters that should be considered when choosing the right element for a given application. The first of these is breakdown voltage. It usually ranges from a few to several dozen volts. Exceeding this parameter may damage the photodiode. It is also worth considering the magnitude of the dark current, which is given in nanoamperes [nA]. This is a very small value but may play a significant role in some applications. Another parameter that must not be ignored is the wavelength range, as well as the wavelength at the maximum sensitivity, for which the change in the value of the current produced by the photodiode, depending on the intensity of the light incident on p-n junction, will be the greatest. Failure to select this parameter for a given application or for elements already used in the system, may lead to a situation in which the diode will not "see" the signal of the desired wavelength, and the entire system may not work properly. Some diodes may have an infrared filter to respond only to wavelengths that are invisible to the human eye. Also, of importance is viewing angle, which can vary from 10°, e.g., for sensors working with a laser diode, to as much as 150° for dusk to dawn sensors. The result of the product of the amount of current produced by the photodiode and the voltage applied to the p-n junction is the value of optical power, which is also often provided by manufacturers. Its basic unit is the Watt [W].
Photodiodes, like most other electronic components, can be found in the form of surface-mounted components, the so-called SMD and through-hole components, the so-called. THT. This also affects the type of housing in which it will be installed. Some of the most popular are, for example: 0805, DIL (Dual In Line), 3 mm, or TO5.
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