Varicaps, LEDs, Photodiodes, Optocouplers - Electronics


Recall that when applying reverse voltage pn - the structure is likened to a capacitor whose plates are p - and n-regions separated by a dielectric (a transition almost free of charge carriers). The resulting barrier capacitance can be used as a capacitor in electronic equipment. Varicaps - are semiconductor diodes whose operation is based on the phenomenon of the barrier capacitance of a locked pn - transition. Since the size of the p-n-junction region depends on the value of the reverse voltage applied to it, the value of the barrier capacitance also varies with this voltage.

External reverse voltage, pulling electrons deep into the n-region, and holes - deep into the p-region, widens the p-n junction and changes the barrier capacitance. The main characteristic of the varicap is the dependence of its capacity on the value of the reverse voltage - volt-farad characteristic. The main parameters of the varicaps are the nominal capacitance and the range of its variation, as well as the permissible reverse voltage and power. Varicaps are used to electrically adjust the oscillatory circuits in the radio equipment.

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Based on the phenomena occurring in the

transition when a forward current flows through it, it is possible to obtain semiconductor devices capable of generating optical radiation. Such devices are semiconductor LEDs. The operation of the LEDs is based on injection electroluminescence, ie. generation of optical radiation in the p-n junction, which is under direct external stress. Under the influence of external energy, the electrons in the atoms go into an excited state with a higher energy level W 2, called the metastable excitation level. When these electrons return from the metastable level W 2 to the initial W 1, photons emitted with a wavelength determined by the relation :

The advantages of semiconductor light-emitting diodes include a high efficiency in comparison with incandescent lamps, a relatively narrow spectrum of radiation and a good directional pattern, high speed and low supply voltage. All this provides convenience of matching with integrated microcircuits, high reliability, durability and manufacturability. The emission spectrum, and hence its color, depends on the semiconductor material used. LEDs are manufactured not on the basis of silicon or germanium, like most semiconductor devices, but on the basis of gallium arsenide-gallium arsenide. The brightness of the glow is proportional to the direct current of the LED. A current of several milliamperes is already sufficient for a clear indication. Light-emitting diodes are made both in the form of separate indicators, and in the form of seven-segment or dot matrixes. Seven segmented matrices consist of seven luminous stripes - segments, from which it is possible to synthesize the image of any digit from 0 to 9 (such matrices are used, for example, in electronic clocks with digital indication). In dot matrixes, the image is formed from luminous points. On the basis of point matrices, it is possible to synthesize an image not only of a digit, but of any indicative sign (letter, special symbol, etc.).


The simplest photodiode is a conventional semiconductor diode (see Figure 1.4, i), which allows the effect of optical radiation on the pn junction. In the equilibrium state, when the radiation flux is completely absent, the carrier concentration, the potential distribution, and the energy band diagram of the photodiode completely correspond to the usual p-n structure.

When the radiation is exposed in a direction perpendicular to the p -n-transition plane, as a result of absorption of photons with an energy greater than the band gap, electron-hole pairs. These electrons and holes are called photocarriers. In the diffusion of photocarriers into the interior of the n-region, the bulk of the electrons and holes do not have time to recombine and reaches the boundary of the pn junction. Here the photocarriers are separated by the electric zero of the pn junction, where the holes go over into the p-region, and the electrons can not overcome the transition and accumulate at the boundary of the pn junction and the n-region.

Thus, the current through the p-n-junction is due to the drift of the minority carriers-holes. The drift current of photocarriers is called photocurrent I f. Photo-carriers-holes-charge the p-region positively with respect to the n-region, and photocarriers-electrons-charge the n-region negatively with respect to the p-region. The resulting potential difference is called the photo EMF - E f. The generated current in the photodiode is reverse, it is directed from the cathode to the anode. And its value is the greater, the more illumination.

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Photodiodes can be used to generate electrical energy. Thus, solar cells are made on the basis of photodiodes with a large area of ​​the pn junction.


LEDs and photodiodes are often used in the Pars. In this case, they are placed in one housing in such a way that the photosensitive area of ​​the photodiode is located opposite the emitting area of ​​the LED. Semiconductor devices using bunches & quot; LED-photodiode & quot ;, are called optocouplers (Figure 1.7). They are widely used in electronic equipment for galvanic isolation of input and output circuits.


Fig. 1.7. Optocoupler:

1 - LED; 2 - Photodiode

The input and output circuits in such devices are not electrically connected in any way, because the signal is transmitted through optical radiation.

The use of optocouplers in electronic computing devices is one of the main methods of increasing the noise immunity of equipment.

The main carrier of interference in radio electronic equipment is the case. The case is used as one of the poles of the power supply, so connecting various power devices to it leads to the formation of short-time impulse noise during switching of high-current circuits. At the same time, for the transmission of information purely electrically between the source and receiver of information - there must be electrical connection to the hull. If power circuits are connected to the same housing, interference caused by switching in these circuits leads to malfunctions of other devices connected to the housing.

The transmission of information with the help of optocouplers makes it possible to decouple the electrical supply circuits of the source and information receiver, since the information carrier is electrically neutral optical radiation. Thus, the devices may have different housings, i. E. are galvanically isolated and not subject to interference.

In addition to protection from interference, galvanic decoupling based on optocouplers allows solving another problem - the joint operation of devices under different potentials. Any, even a small, potential difference does not allow purely electrically connecting different devices, since this will lead to their failure. Signal transmission in the optocoupler is possible, even if the LED and photodiode circuits are under different voltages (in some optocouplers up to 500 V). Thus, devices, information related to the use of optocouplers, can be under different potentials.

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