Photodiodes are light-sensitive semiconductor elements. Their main function is to transform the luminous flux into an electrical signal. Such semiconductors are used in various devices, the operation of which is based on the use of light fluxes.
How photodiodes work
The basis of the action of photodiode elements is the internal photoelectric effect. It consists in the appearance in a semiconductor under the influence of a light flux of nonequilibrium electrons and holes (i.e. atoms with space for electrons), which form a photoelectromotive force.
- When light hits the p-n transition, light quanta are absorbed with the formation of photocarriers
- Photo carriers located in the n region approach the boundary at which they are separated under the influence of the electric field
- Holes move to zone p, and electrons collect in zone n or near the boundary
- The holes charge the p-region positively, and electrons charge the n-band negatively. A potential difference is formed
- The higher the illumination, the greater the reverse current
If the semiconductor is in the dark, then its properties are similar to a conventional diode. If you ring the tester in the absence of lighting, the results will be similar to testing a conventional diode. In the forward direction there will be little resistance, in the opposite direction the arrow will remain at zero.
Photodiode circuit
Operating modes
Photodiodes are divided according to the mode of operation.
Photo generator mode
It is carried out without a power supply. Photogenerators, which are components of solar panels, are also called "solar cells". Their function is to convert solar energy into electrical energy. The most common photogenerators are based on silicon - cheap, widespread, well-studied. They have a low cost, but their efficiency reaches only 20%. Film elements are more progressive.
Photo conversion mode
The power supply to the circuit is connected with reverse polarity, the photodiode in this case serves as a light sensor.
main parameters
The properties of photodiodes determine the following characteristics:
- Volt-ampere. Determines the change in the value of the light current in accordance with the changing voltage with a stable light flux and dark current
- Spectral. Characterizes the influence of the light wavelength on the photocurrent
- The time constant is the period during which the current responds to an increase in dimming or illumination by 63% of the set value
- Sensitivity threshold - the minimum luminous flux to which the diode reacts
- Dark resistance - an indicator characteristic of a semiconductor in the absence of light
- Inertia
What does a photodiode consist of?
Varieties of photodiodes
P-i-n
These semiconductors are characterized by the presence in the pn junction zone of a section with its own conductivity and a significant value of resistance. When the light flux hits this section, pairs of holes and electrons appear. The electric field in this area is constant, there is no space charge. This auxiliary layer expands the operating frequency range of the semiconductor. According to their functional purpose, p-i-n-photodiodes are divided into detector, mixing, parametric, limiting, multiplying, tuning and others.
Avalanche
This species is highly sensitive. Its function is to convert the luminous flux into an electrical signal, amplified using the avalanche multiplication effect. It can be used in conditions of low luminous flux. Avalanche photodiodes use superlattices to reduce signal transmission noise.
Schottky barrier
It consists of a metal and a semiconductor, around the junction of which an electric field is created. The main difference from conventional p-i-n-type photodiodes is the use of basic, not additional charge carriers.
Heterostructure
It is formed from two semiconductors with different bandgaps. The layer between them is called heterogeneous. By selecting such semiconductors, it is possible to create a device that operates over the full wavelength range. Its disadvantage is the high complexity of manufacturing.
Applications of photodiodes
- Optoelectronic integrated circuits. Semiconductors provide optical communication, which guarantees efficient isolation of power and control circuits while maintaining functional communication.
- Multi-element photodetectors - scanistors, photosensitive devices, photodiode arrays. An optoelectric element is able to perceive not only the brightness characteristic of an object and its change in time, but also to create a complete visual image.
Other areas of use: fiber optic lines, laser rangefinders, positron emission tomography installations.
Other related materials
Anatoly Melnik
Specialist in the field of radio electronics and electronic components. Parts selection consultant at RadioElement.
When reproducing a photographic phonogram, the signal source is a photodiode. It can operate in photovoltaic or photodiode mode. Connection diagram photodiode, working in photovoltaic mode, to the input of a transistor amplifier is shown in Fig. 45, and.In this mode, the photodiode operates without a power supply. Under the influence of light in the n-type region, covalent bonds are destroyed, and the released electrons accumulate in this region, charging it negatively, and holes are drawn into the p-type region, charging it positively. Thus, a potential difference is created between the anode and cathode - photo-EMF E f.With a constant luminous flux in rest mode, under the action of this EMF, a constant current flows from the region rto the area pthrough a load resistor R nf. When playing a phonogram, the light flux pulsates, therefore, the photo-EMF and the current in the photodiode circuit pulsate. Variable voltage component at the load R nfis the voltage of the input signal, which is transmitted through the capacitor C c to the base of the transistor. The variable component of the photodiode current branches out: part passes through the resistor R nf and the other part through the capacitor C c and the emitter junction of the transistor.
The photodiode operation in the photovoltaic mode is used in mobile sound-reproducing equipment of the K3VP-I0 and K3VP-14 types.
When the photodiode operates in the photodiode mode (Fig. 45, b), a constant voltage is supplied to it from the power source, which is the reverse voltage of the electron-hole junction. In the absence of a light flux, a very small current flows through the photodiode - this is a dark current. Under the influence of light, the reverse resistance of the p - n - junction sharply decreases and the current through the photodiode increases.
In the absence of a signal, the light flux remains constant and a constant current flows through the photodiode. It goes from the plus of the power supply through the load resistance, photodiode R nf and photodiode to the minus of the power supply. In the mode of reproducing the signal recorded on the phonogram, the luminous flux and current of the photodiode, as in the first mode, pulsate, and the alternating current component creates an input signal at the load and at the amplifier input.Figure: 45 Schemes for switching on the photodiode: a - in the photovoltaic mode;
b - in photodiode mode
In the photodiode mode, the sensitivity of the photodiode is increased compared to the photovoltaic mode, and the input the noise signal increases; the internal resistance of the AC photodiode also increases.
The photodiode operation in the photodiode mode is used in stationary transistor equipment of the "Sound T ".
Photodiodes installed in photocells on film projectors of different posts can have a scatter of parameters, and in particular, an unequal sensitivity, which leads to unequal response of posts. So that when a movie is shown, the sound volume does not change when switching from post to post, the photo-cell provides for the regulation of the photodiode response. The regulation circuit (Fig. 46) allows a variable resistor Rreduce the signal coming from the given photodiode to inputamplifier. In the upper position of the resistor slider R3chain resistance R1, R3, C1,connected in parallel with the photodiode, the maximum, therefore the input signal is the largest. As you move the slider down, the resistance R3shorts more and more, the total resistance of the chain R1, R3,Сl decreases, its shunting effect increases, and the signal at the amplifier input decreases. Such a circuit for switching on a photodiode of the FDK155 type is used in sound reproducing equipment of the "Sound T2-25.50 ".
The connection line of the photodiode to the amplifier input must be shielded, as for other signal sources.
Photodiodes used in cinema equipment have a sensitivity of the order of 4-6 mA / lmand give an input signal current of 1-2 μA.
Fig. 46 Photodiode recoil control circuit
Questions for self-test:
1. What is called an input circuit, and what types of input circuits are there?
2. Draw and explain the pickup connection diagrams.
3. Draw and explain the circuits for turning on the microphone.
4.Why should the input circuits be shielded and the input transformer balanced? In the meantime, there is no need to worry about it. ”
5. Why is the pickup connected to the amplifier input most often through a voltage divider, and an input transformer is used to turn on the microphone and magnetic head in high-quality equipment?
6. Draw and explain the circuits for switching on the photodiode.
Photodiodes, which are used in various devices and devices, occupy a special place in electrical engineering. A photodiode is a semiconductor element similar in properties to a simple diode. Its reverse current directly depends on the intensity of the light flux incident on it. Most often, semiconductor elements with a pn junction are used as a photodiode.
Device and principle of operation
A photodiode is included in many electronic devices. Therefore, he gained wide popularity. A conventional LED is a pn junction diode whose conductivity depends on the light incident on it. In the dark, the photodiode has the characteristics of a conventional diode.
1
- semiconductor junction.
2
- positive pole.
3
- photosensitive layer.
4
- negative pole.
When the light flux acts on the transition plane, photons are absorbed with an energy exceeding the limiting value, therefore, pairs of charge carriers - photocarriers are formed in the n-region.
When photocarriers are mixed in the depth of the "n" region, the main part of the carriers does not have time to recombine and passes to the pn boundary. At the transition, photocarriers are divided by an electric field. In this case, the holes go to the "p" region, and the electrons are not able to go through the transition, therefore they accumulate near the pn transition boundary, as well as the "n" region.
The reverse current of the diode increases when exposed to light. The amount by which the reverse current rises is called photocurrent.
Photocarriers in the form of holes carry out a positive charge of the "p" region in relation to the "n" region. In turn, the electrons produce a negative charge in the "n" region relative to the "p" region. The resulting potential difference is called the photoelectromotive force, and is designated "E f". The electric current generated in the photodiode is reversed and is directed from the cathode to the anode. Moreover, its value depends on the amount of illumination.
Operating modes
Photodiodes can operate in the following modes:
- Photo generator mode ... Without connecting the source of electricity.
- Photoconverter mode ... With external power supply connection.
In work photogenerator photodiodes are used in place of a power source that converts sunlight into electrical energy. Such photogenerators are called solar cells. They are the main parts of solar cells used in various devices, including spacecraft.
The efficiency of silicon-based solar cells is 20%; this parameter is much higher for film cells. An important property of solar cells is the dependence of the output power on the weight and area of \u200b\u200bthe sensitive layer. These properties reach values \u200b\u200bof 200 W / kg and 1 kW / m 2.
When the photodiode functions as photoconverter , the voltage source E is connected to the circuit with reverse polarity. In this case, reverse graphs of the current-voltage characteristics are used at different illumination levels.
The voltage and current at the load R n are determined on the graph by the intersections of the characteristics of the photodiode and the load line, which corresponds to the resistor R n. In the dark, the photodiode is equivalent to an ordinary diode in its action. The current in the dark mode for silicon diodes ranges from 1 to 3 microamperes, for germanium from 10 to 30 microamperes.
Types of photodiodes
There are several different types of photodiodes that have their own advantages.
p – i – n photodiode
In the pn region, this diode has a section with high resistance and its own conductivity. When exposed to light, pairs of holes and electrons appear. The electric field in this zone has a constant value, there is no space charge.
This auxiliary layer significantly reduces the capacitance of the barrier layer and is not voltage dependent. This expands the operating frequency band of the diodes. As a result, the speed rises sharply, and the frequency reaches 10 10 hertz. The increased resistance of this layer significantly reduces the operating current in the absence of lighting. For the light flux to be able to penetrate the p-layer, it should not be thick.
Avalanche photodiodes
This type of diode is a highly sensitive semiconductor that converts lighting into an electric current signal using a photoelectric effect. In other words, these are photodetectors that amplify the signal due to the effect of avalanche multiplication.
1
- ohmic contacts 2
- antireflection coating Avalanche photodiodes are more sensitive than other photodetectors. This makes it possible to use them for low light powers.
Superlattices are used in the design of avalanche photodiodes. Their essence lies in the fact that significant differences in the impact ionization of carriers lead to a drop in noise.
Another advantage of using similar structures is the localization of avalanche reproduction. It also reduces interference. In a superlattice, the thickness of the layers ranges from 100 to 500 angstroms.
Operating principle
At a reverse voltage close to the value of the avalanche breakdown, the photocurrent increases sharply due to impact ionization of charge carriers. The action consists in the fact that the energy of the electron increases from the external field and can exceed the ionization limit of the substance, as a result of which the meeting of this electron with an electron from the valence band will lead to the appearance of a new pair of an electron and a hole. The charge carriers of this pair will be accelerated by the field and can facilitate the formation of new charge carriers.
Characteristics
The properties of such light diodes can be described by some dependencies.
Volt-ampere
This characteristic is the dependence of the current strength at a constant light flux on the voltage.
I - current M - multiplication factor U - voltage Light
This property is the dependence of the diode current on lighting. As the light flux increases, the photocurrent increases.
Spectral
This property is the dependence of the diode current on the light wavelength, and is the width of the boundary zone.
Time constant
This is the time during which the photocurrent of the diode changes after the light is supplied in comparison with the steady-state value.
Dark Resistance
This is the resistance value of the diode in the dark.
Inertia
Factors affecting this characteristic:
- Diffusion time of nonequilibrium charge carriers.
- Travel time along the pn junction.
- The recharge period of the pn junction barrier capacity.
Scope of application
Photodiodes are the main components of many optoelectronic devices.
Integrated microcircuits (optoelectronic)
The photodiode can have a significant operating speed, but the current amplification factor is not more than unity. Due to the optical connection, the microcircuits have significant advantages: ideal galvanic isolation of control circuits from powerful power circuits. At the same time, a functional connection is maintained between them.
Photo detectors with multiple elements
These devices in the form of a photodiode array, a scanistor, are new progressive electronic devices. Their optoelectronic eye with a photodiode can create a reaction to the spatial and brightness properties of objects. In other words, he can see his full visual image.
The number of light-sensitive cells is very large. Therefore, in addition to questions of speed and sensitivity, it is necessary to read information. All photodetectors with multiple photocells are scanning systems, that is, devices that allow you to analyze the investigated space by sequential element-by-element viewing.
Photodiodes are also widely used in fiber optic lines, laser rangefinders. Recently, such light emitting diodes have been used in positron emission tomography.
Currently, there are samples of photosensitive arrays consisting of avalanche photodiodes. Their effectiveness and scope depends on some factors.
The most influencing factors were:
- The total leakage current generated by adding noise and current in the absence of light.
- Quantum efficiency, which determines the fraction of incident quanta that lead to the emergence of current and charge carriers.
The photodiode is actively used in modern electronic devices, from the name it becomes clear that the device is a construction using a semiconductor, so let's consider what a photodiode is A photodiode is a semiconductor diode that has the property of one-sided conductivity when exposed to optical radiation. A photodiode is a semiconductor crystal, usually with an electron-hole junction (PN). It is equipped with two metal leads and is mounted in a plastic or metal case.
There are two modes of photodiode operation.
1) photodiode - when a constant current source is contained in the external circuit of the photodiode, which creates a reverse bias at the junction and a gate when there is no such source. In the photodiode mode, the photodiode, like the photoresistor, is used to control the current. The photocurrent of a photodiode strongly depends on the intensity of the incident radiation and does not depend on the bias voltage.
2) Valve mode - when a photodiode, like a photocell, is used as an EMF generator.
The main parameters of the photodiode are the sensitivity threshold, the noise level, the spectral sensitivity range is in the range from 0.3 to 15 microns (micrometers), the inertia is the recovery time of the photocurrent. There are also photodiodes with a straight structure. The photodiode is a component in many optoelectronic devices. ... photodiodes and photodetectors are widely used in opron pairs, receivers for video and audio signals. It is widely used to receive signal from laser diodes in CD and DVD drives.
The signal from the laser diode, which contains encoded information, first enters the photodiode, which in these devices has a complex design, then, after decryption, the information goes to the central processor, where, after processing, it turns into an audio or video signal. All modern floppy drives work on this principle. Photodiodes are also used in various security devices, in infrared motion and presence sensors. Another review for a novice radio amateur has come to an end, good luck in the world of radio electronics - AKA.
Theory for beginners
Discuss the article PHOTODIODES
radioskot.ru
|
Photodiodes are light-sensitive semiconductor elements. Their main function is to transform the luminous flux into an electrical signal. Such semiconductors are used in various devices, the operation of which is based on the use of light fluxes.
How photodiodes workThe basis of the action of photodiode elements is the internal photoelectric effect. It consists in the appearance in a semiconductor under the influence of a light flux of nonequilibrium electrons and holes (i.e. atoms with space for electrons), which form a photoelectromotive force.
If the semiconductor is in the dark, then its properties are similar to a conventional diode. If you ring the tester in the absence of lighting, the results will be similar to testing a conventional diode. In the forward direction there will be little resistance, in the opposite direction the arrow will remain at zero. Photodiode circuit
Operating modesPhotodiodes are divided according to the mode of operation. Photo generator modeIt is carried out without a power supply. Photogenerators, which are components of solar panels, are also called "solar cells". Their function is to convert solar energy into electrical energy. The most common photogenerators are based on silicon - cheap, widespread, well-studied. They have a low cost, but their efficiency reaches only 20%. Film elements are more progressive. Photo conversion modeThe power supply to the circuit is connected with reverse polarity, the photodiode in this case serves as a light sensor. main parametersThe properties of photodiodes determine the following characteristics:
What does a photodiode consist of?
Varieties of photodiodesP-i-nThese semiconductors are characterized by the presence in the pn junction zone of a section with its own conductivity and a significant value of resistance. When the light flux hits this section, pairs of holes and electrons appear. The electric field in this area is constant, there is no space charge. This auxiliary layer expands the operating frequency range of the semiconductor. According to their functional purpose, p-i-n-photodiodes are divided into detector, mixing, parametric, limiting, multiplying, tuning and others. AvalancheThis species is highly sensitive. Its function is to convert the luminous flux into an electrical signal, amplified using the avalanche multiplication effect. It can be used in conditions of low luminous flux. Avalanche photodiodes use superlattices to reduce signal transmission noise. Schottky barrierIt consists of a metal and a semiconductor, around the junction of which an electric field is created. The main difference from conventional p-i-n-type photodiodes is the use of basic, not additional charge carriers. HeterostructureIt is formed from two semiconductors with different bandgaps. The layer between them is called heterogeneous. By selecting such semiconductors, it is possible to create a device that operates over the full wavelength range. Its disadvantage is the high complexity of manufacturing. Applications of photodiodes
Other areas of use: fiber optic lines, laser rangefinders, positron emission tomography installations. |
www.radioelementy.ru
Photodiodes
It is customary to call a photodiode a semiconductor device with one p-n junction, the current-voltage characteristic of which depends on the light acting on it.
The conventional graphic designation, structure and appearance of the photodiode are shown in Fig. 17.6.
Figure: 17.6. Photodiode:
a - conventional graphic designation; b - structure; c - appearance
The simplest photodiode is a conventional semiconductor diode, in which it is possible to influence the pn junction by optical radiation. In the equilibrium state, when the radiation flux is completely absent, the concentration of carriers, the potential distribution and the energy band diagram of the photodiode completely correspond to the usual p-n junction (see Fig. 1.3).
When exposed to radiation in the direction perpendicular to the plane of the p-n junction, as a result of the absorption of photons with an energy greater than the band gap, electron-hole pairs appear in the n region. These electrons and holes are called photocarriers. During diffusion of photocarriers deep into the n region, the majority of electrons and holes do not have time to recombine and reaches the pn junction boundary. Here the photocarriers are separated by the electric field of the p-n junction, and the holes pass into the p region, and electrons cannot overcome the transition field, and accumulate at the border of the p-n junction and the n region. Τᴀᴋᴎᴍ ᴏϬᴩᴀᴈᴏᴍ, the current through the p-n junction is due to the drift of non-basic carriers - holes. The drift current of a photocarrier is usually called a photocurrent.
Photocarriers - holes charge the p region positively with respect to the n region, and photocarriers - electrons - the n region negatively with respect to the p region. The resulting potential difference is usually called the photo EMF Ef. The generated current in the photodiode is reverse, it is directed from the cathode to the anode, and its value is the greater, the greater the illumination.
Photodiodes can operate in one of two modes - with an external source of electrical energy (converter mode), or without an external source of electrical energy (generator mode).
When the photodiode is operating in the converter mode, a reverse voltage is applied to it (Fig. 17.7, a). The reverse branches of the I - V characteristic of the photodiode are used at different levels of illumination Ф, Ф1, Ф2 (Fig. 17.7, b).
Taking into account the dependence on the illumination level, the reverse current of the photodiode changes, and the voltage across the load resistor changes. In railroad automation systems according to this scheme, a germanium photosensor is included in the devices for detecting a heated axle box (germanium is sensitive to infrared rays, and silicon is sensitive to visible light).
| and) | b) |
Figure: 17.7. Photodiode operation in photoconverter mode:
a - connection diagram; b - volt-ampere characteristics
Photodiodes operating in generator mode are used as power sources that convert solar energy into electrical energy. Οʜᴎ are called solar cells and are part of solar cells. The output voltage of a solar cell is highly dependent on the light level. To obtain a stable voltage in the load, the solar battery is used in conjunction with the battery. The diagram of the solar-storage battery is shown in Fig. 17.8.
Figure: 17.8. Schematic diagram of a solar-storage battery
At maximum illumination, the solar battery powers the load and charges the battery. Placed on ref.rf In the dark, the load is powered only from the battery, and so that the battery does not discharge to the solar battery, the VD1 diode is installed in the circuit.
The efficiency of silicon solar cells is about 20%. Important technical parameters of solar cells are the ratio of their output power to the mass and area occupied by the solar cell. These parameters reach values \u200b\u200bof 200 W / kg and 1 kW / m2, respectively.
More detailed information about photodiodes is given in the literature.
Read also
Lecture 14 Photoresistors are called semiconductor devices, the principle of which is based on a change in the resistance of a semiconductor under the influence of light radiation. Figure 7.31 shows the device of a photoresistor, consisting of a dielectric substrate 1, ... [read more].
Photodiodes are semiconductor diodes in which the amount of reverse current is controlled by light. The photodiode is designed in such a way that it provides light access to the - transition. In the absence of a luminous flux in a photodiode with a reverse voltage ... [read more].
Figure: 9. Photodiode in photoresistor mode Photodiode in photoresistor mode and its I – V characteristic are shown in Fig. 9. A reverse voltage is applied to the photodiode from the EMF source, so its junction is closed. If the flux is zero, then the reverse current through the photodiode is approximately ... [read more].
A photodiode is a semiconductor photovoltaic device with an internal photoelectric effect that displays the process of converting light energy into electrical energy. The internal photoelectric effect is that under the action of the energy of light radiation in the p - n - junction ... [read more].
A photodiode is a photovoltaic detector with an electron-hole junction, the irradiation of which with light causes an increase in the reverse current. The material of the semiconductor of the photodiode is usually silicon, silver sulfide, thallium sulfide or gallium arsenide .... [read more].
Photodetectors. In planar and projection scanners, charge-coupled devices (CCDs) are used, and in drum scanners, photomultiplier tubes and photodiodes are used. Sometimes it's the other way around. The operation of the CCD is based on the property of MOS-structure capacitors (metal-oxide -... [read more].
The photodiode has the structure of a conventional pn junction. The reverse current of the photodiode depends on the light level. Photodiodes are housed in a metal case with a transparent window. A conventional graphic image of a photodiode and its equivalent circuit are shown in Figure 3.11. In fig.3.12 ... [read more].
referatwork.ru
Photodiodes | Technique and Programs
The principle of the photodiode
A semiconductor photodiode is a semiconductor diode whose reverse current depends on the illumination.
Usually, semiconductor diodes with a pn junction, which is biased in the opposite direction by an external power source, are used as a photodiode. When light quanta are absorbed in the pn junction or in the areas adjacent to it, new charge carriers are formed. Minority charge carriers, which have arisen in the regions adjacent to the pn junction at a distance not exceeding the diffusion length, diffuse into the pn junction and pass * through it under the action of an electric field. That is, the reverse current increases when illuminated. The absorption of quanta directly in the pn junction leads to similar results. The amount by which the reverse current increases is called the photocurrent.
Photodiode characteristics
The properties of a photodiode can be characterized by the following characteristics:
The current-voltage characteristic of the photodiode is the dependence of the light current at a constant light flux and dark current 1m on the voltage.
The light characteristic of the photodiode is due to the dependence of the photocurrent on the illumination. With increasing illumination, the photocurrent increases.
The spectral characteristic of a photodiode is the dependence of the photocurrent on the wavelength of the incident light on the photodiode. It is determined for long wavelengths by the band gap, and at short wavelengths by a large absorption coefficient and an increase in the effect of surface recombination of charge carriers with decreasing wavelength of light quanta. That is, the short-wavelength sensitivity limit depends on the base thickness and on the surface recombination rate. The position of the maximum in the spectral characteristic of the photodiode strongly depends on the degree of increase in the absorption coefficient.
The time constant is the time during which the photodiode photocurrent changes after illumination or after the photodiode is darkened by a factor of e (63%) with respect to the steady-state value.
Dark resistance is the resistance of the photodiode in the absence of illumination.
Integral sensitivity is determined by the formula:
where 1ph - photocurrent, Ф - illumination.
Inertia
There are three physical factors that affect persistence:
1. Time of diffusion or drift of nonequilibrium carriers through the base t;
2. The time of flight through the pn transition t,;
3. The recharge time of the barrier capacity of the pn junction, characterized by the time constant RC6ap.
The thickness of the pn junction, which depends on the reverse voltage and the concentration of impurities in the base, is usually less than 5 μm, which means that m is 0.1 ns. RC6ap is determined by the barrier capacitance of the pn junction, which depends on the voltage and resistance of the base of the photodiode at a low load resistance in the external circuit. RC6ap is typically a few nanoseconds.
Photodiode efficiency and power calculation
The efficiency is calculated by the formula:
where Rosv is the illumination power; I - current strength;
U is the voltage across the photodiode.
The calculation of the photodiode power is illustrated in Fig. 2.12 and table 2.1.
Figure: 2.12. Dependence of the photodiode power on voltage and current
The maximum power of the photodiode corresponds to the maximum area of \u200b\u200bthe given rectangle.
Table 2.1. Power versus efficiency
| Illumination power, mW | Current strength, mA | Voltage, V | |
Application of a photodiode in oltoelectronics
The photodiode is an integral element in many complex optoelectronic devices:
Optoelectronic integrated circuits.
The photodiode can be fast, but its photocurrent gain does not exceed unity. Due to the presence of optical communication, optoelectronic integrated circuits have a number of significant advantages, namely: almost perfect galvanic isolation of control circuits from power ones while maintaining a strong functional connection between them.
Multi-element photodetectors.
These devices (scanistor, photodiode array with MOS transistor control, charge-coupled photosensitive devices and others) are among the most rapidly developing and progressive electronic products. An optoelectric “eye” based on a photodiode is capable of responding not only to the brightness-temporal, but also to the spatial characteristics of an object, that is, to perceive its complete visual image.
The number of photosensitive cells in the device is quite large, therefore, in addition to all the problems of a discrete photodetector (sensitivity, speed, spectral region), it is necessary to solve the problem of reading information. All multi-element photodetectors are scanning systems, that is, devices that allow you to analyze the space under investigation by sequential viewing (elementwise decomposition).
How is the perception of images?
The distribution of the brightness of the observation object is converted into an optical image and focuses on the photosensitive surface. Here light energy is converted into electrical energy, and the response of each element (current, charge, voltage) is proportional to its illumination. The luminance pattern is converted into an electrical relief. The scanning circuit performs periodic sequential polling of each element and reads the information it contains. Then, at the output of the device, we get a sequence of video pulses in which the perceived image is encoded.
When creating multi-element photodetectors, an attempt is made to ensure that they perform best the conversion and scanning functions. Optocouplers.
An optoelectronic device is called an optoelectronic device, in which there is a source and a receiver of radiation with one or another type of optical connection between them, structurally combined and placed in one housing. There is no electrical (galvanic) connection between the control circuit (the current in which is small, of the order of several mA), where the emitter is connected, and the executive circuit, in which the photodetector operates, and control information is transmitted by means of light radiation.
This property of an optoelectronic pair (and in some types of optocouplers there are several even optically unconnected optocouplers) turned out to be indispensable in those electronic nodes where it is necessary to eliminate as much as possible the effect of output electrical circuits on the input. All discrete elements (transistors, thyristors, microcircuits that are switching assemblies, or microcircuits with an output that allows switching a high power load) control and executive circuits are electrically connected to each other. This is often unacceptable if a high voltage load is switched. In addition, the resulting feedback inevitably leads to the appearance of additional interference.
Structurally, the photodetector is usually mounted on the bottom of the case, and the emitter - in the upper part. The gap between the emitter and the photodetector is filled with an immersion material - most often this role is played by polymer optical glue. This material acts as a lens that focuses radiation onto the sensitive layer of the photodetector. The immersion material is covered from the outside with a special film that reflects the light rays inward to prevent the radiation from scattering outside the working area of \u200b\u200bthe photodetector.
The role of emitters in optocouplers, as a rule, is performed by gallium arsenide LEDs. Photosensitive elements in optocouplers can be photodiodes (AOD series optocouplers ...), phototransistors, phototrinistors (AOU series optocouplers.,.) And highly integrated photo relay circuits. In a diode optocoupler, for example, a silicon-based photodiode is used as a photodetector element, and an infrared emitting diode serves as an emitter. The maximum of the spectral characteristic of the diode radiation falls on a wavelength of about 1 μm. Diode optocouplers are used in photodiode and photogenerator modes.
Transistor optocouplers (AOT series ...) have some advantages over diode ones. The collector current of the bipolar transistor is controlled both optically (by acting on the LED) and electrically along the base circuit (in this case, the operation of the phototransistor in the absence of radiation from the control LED of the optocoupler is practically no different from the operation of an ordinary silicon transistor). The field-effect transistor is controlled through a gate circuit.
In addition, the phototransistor can operate in the key and amplifying modes, and the photodiode only in the key. Optocouplers with composite transistors (for example, AOT1YUB), have the highest gain (like a conventional node on a composite transistor), can switch the voltage and current of sufficiently large values \u200b\u200band, according to these parameters, are inferior only to thyristor optocouplers and optoelectronic relays of the type KR293KP2 - KR293KP4, which adapted for switching high-voltage and high-current circuits. New optoelectronic relays of the K449 and K294 series have appeared in retail today. The K449 series allows switching voltages up to 400 V at currents up to 150 mA. Such microcircuits in a four-pin compact DIP-4 package are replacing low-power electromagnetic relays and have a lot of advantages over relays (quiet operation, reliability, durability, lack of mechanical contacts, wide operating voltage range). In addition, their affordable price is due to the fact that there is no need to use precious metals (in the relay, they cover the switching contacts).
In resistor optocouplers (for example, OEP-1) and emitters are electric mini-incandescent lamps, also placed in one housing.
According to GOST, the graphic designations of optocouplers are assigned a conditional code - the Latin letter U, followed by the serial number of the device in the circuit.
Chapter 3 of the book describes devices and devices that illustrate the use of optocouplers.
Application of photodetectors
Any optoelectronic device contains a photodetector unit. And in most modern optoelectronic devices, the photodiode forms the basis of the photodetector.
In comparison with other, more complex photodetectors, they have the greatest stability of temperature characteristics and the best operational properties.
The main disadvantage that is usually pointed out is the lack of amplification. But it is rather arbitrary. In almost every optoelectronic device, the photodetector operates on one or another matching electronic circuit. And the introduction of an amplifying stage into it is much simpler and more expedient than imparting amplification functions unusual to a photodetector.
High information capacity of the optical channel due to the fact that the frequency of light vibrations (about 1015 Hz) is 103 ... 104 times higher than in the mastered radio-technical range. The small value of the wavelength of light vibrations provides a high achievable density of information recording in optical storage devices (up to 108 bit / cm2).
Sharp directivity (accuracy) of light radiation, due to the fact that the angular divergence of the beam is proportional to the wavelength and can be less than one minute. This allows you to concentrately and with low losses transfer electrical energy to any area of \u200b\u200bspace.
Possibility of double - temporal and spatial - modulation of the light beam. Since the source and the receiver in optoelectronics are not electrically connected to each other, and the connection between them is carried out only by means of a light beam (electrically neutral photons), they do not affect each other. And therefore, in an optoelectronic device, the flow of information is transmitted only in one direction - from the source to the receiver. The channels, through which optical radiation propagates, do not affect each other and are practically insensitive to electromagnetic interference, which determines their high noise immunity.
An important feature of photodiodes is their high speed. They can operate at frequencies up to several MHz. usually made from germanium or silicon.
The photodiode is a potentially broadband receiver. This explains its widespread use and popularity.
IR spectrum
An infrared emitting diode (IR diode) is a semiconductor diode that, when a direct current flows through it, emits electromagnetic energy in the infrared region of the spectrum.
Unlike the spectrum of radiation visible to the human eye (which, for example, produces a conventional light-emitting diode based on gallium phosphide), infrared radiation cannot be perceived by the human eye, but is recorded using special devices that are sensitive to this radiation spectrum. Among the popular photodetecting diodes of the IR spectrum, photosensitive devices MDK-1, FD263-01 and the like can be noted.
The spectral characteristics of IR emitting diodes have a pronounced maximum in the wavelength interval 0.87 ... 0.96 μm. The radiation efficiency and efficiency of these devices are higher than that of light-emitting diodes.
On the basis of IR diodes (which in electronic structures occupy an important place as transmitters of IR spectrum pulses), fiber-optic lines (favorably distinguished by their speed and noise immunity), versatile electronic household units and, of course, electronic security units are designed. This has its own advantage, because The IR beam is invisible to the human eye and in some cases (provided that several multidirectional IR beams are used), it is impossible to visually determine the presence of the security device itself until it switches to the "alarm" mode). Experiences in the field of production and maintenance of security systems based on IR emitters still allow us to give some recommendation on determining the operating state of IR emitters.
If you look closely at the emitting surface of the IR diode (for example, AL147A, AL156A), when a control signal is applied to it, you will notice a faint red glow. The light spectrum of this glow is close to the color of the eyes of albino animals (rats, hamsters, etc.). In the dark, the IR glow is even more pronounced. It should be noted that it is undesirable from a medical point of view to peer into a device emitting IR light energy for a long time.
In addition to security systems, IR emitting diodes are currently used in alarm key fobs for cars, various wireless signal transmitters for a distance. For example, by connecting a modulated low-frequency signal from an amplifier to the transmitter, you can listen to audio information using an IR receiver at a certain distance (depending on the radiation power and terrain), telephone conversations can also be broadcast over a distance. This method is less effective today, but it is still an alternative to a home radiotelephone. The most popular (in everyday life) application of IR emitting diodes are remote controls for various household appliances.
As any radio amateur can easily be convinced by opening the cover of the remote control, the electronic circuit of this device is not complicated and can be repeated without any problems. In amateur radio designs, some of which are described in the third chapter of this book, electronic devices with IR emitting and receiving devices are much simpler than industrial devices.
The parameters that determine the static modes of operation of IR diodes (forward and reverse maximum allowable voltage, forward current, etc.) are similar to the parameters of photodiodes. The main specific parameters by which they are identified for IR diodes are:
Radiation power - Rizl - radiation flux of a certain spectral composition emitted by a diode. A characteristic of a diode as a source of IR radiation is the watt-ampere characteristic - the dependence of the radiation power in W (milliwatts) on the forward current flowing through the diode. The radiation pattern of the diode shows the decrease in the radiation power depending on the angle between the radiation direction and the optical axis of the device. Modern IR diodes differ between highly directional and diffuse radiation.
When designing electronic components, it should be borne in mind that the transmission range of the IR signal directly depends on the tilt angle (alignment of the transmitting and receiving parts of the device) and the power of the IR diode. When interchanging IR diodes, this radiation power parameter must be taken into account. Some reference data on domestic IR diodes are given in table. 2.2.
Data on the interchangeability of foreign and domestic devices are given in the appendix. Today, the most popular types of IR diodes among radio amateurs are devices of the AL 156 and AL147 model range. They are optimal in terms of versatility and cost.
Pulsed radiation power - Rizl im - the amplitude of the radiation flux measured at a given pulse of forward current through the diode.
The width of the radiation spectrum is the wavelength interval in which the spectral power density of the radiation is half the maximum.
The maximum permissible direct impulse current is 1pr im (IR diodes are mainly used in a pulsed mode of operation).
Table 2.2. Infrared emitting diodes
| Radiation power, mW | Wavelength, μm | Spectrum width, μm | Device voltage, V | Radiation angle, degrees |
|
| there is no data | there is no data |
||||
The rise time of the radiation pulse tHaprad is the time interval during which the diode radiation power increases from 10 to 100% of the maximum value.
The tcnM3J1 pulse decay time parameter is similar to the previous one.
Duty cycle - Q - the ratio of the period of the pulsed oscillations to the pulse duration.
The electronic components proposed for repetition (Chapter 3 of this book) are based on the principle of transmitting and receiving a modulated IR signal. But not only in this form, you can use the principle of operation of the IR diode. Such opto-relays can also work in response to the reflection of rays (the photodetector is placed next to the emitter). This principle is embodied in electronic units that respond to the approach of an object or person to a combined transceiver unit, which can also serve as a sensor in security systems.
There are infinitely many options for using IR diodes and devices based on them, and they are limited only by the effectiveness of the creative approach of the radio amateur.
nauchebe.net
A photodiode is ... What is a Photodiode?
Photodiode FD-10-100 active area-10x10 mm² FD1604 (active area of \u200b\u200bthe cell 1.2x4mm2 - 16 pcs) Designation on the diagramsA photodiode is an optical radiation receiver that converts light hitting its photosensitive area into an electric charge due to processes in the pn junction.
A photodiode, the work of which is based on the photovoltaic effect (separation of electrons and holes in the p- and n-region, due to which the charge and EMF are formed) is called a solar cell. In addition to p-n photodiodes, there are p-i-n photodiodes, in which there is a layer of undoped semiconductor i between the p- and n- layers. p-n and p-i-n photodiodes only convert light into electric current, but do not amplify it, unlike avalanche photodiodes and phototransistors.
Description
Block diagram of a photodiode. 1 - semiconductor crystal; 2 - contacts; 3 - conclusions; Φ - flux of electromagnetic radiation; E - constant current source; RH - load.Principle of operation:
When exposed to radiation quanta in the base, free carriers are generated, which rush to the pn-junction boundary. The base width (n-region) is made such that the holes do not have time to recombine before going over to the p-region. The photodiode current is determined by the current of minority carriers - the drift current. The speed of the photodiode is determined by the rate of carrier separation by the field of the p-n-junction and the capacity of the p-n-junction Cp-n
The photodiode can operate in two modes:
- photovoltaic - no external voltage
- photodiode - with external reverse voltage
Features:
- simplicity of manufacturing technology and structure
- combination of high photosensitivity and speed
- low base resistance
- low inertia
Parameters and characteristics of photodiodes
Parameters:
- the sensitivity reflects the change in the electrical state at the output of the photodiode when a single optical signal is applied to the input. Quantitatively, the sensitivity is measured by the ratio of the change in the electrical characteristic taken at the output of the photodetector to the luminous flux or radiation flux that caused it. ; - current sensitivity for luminous flux; - voltaic energy flow sensitivity
- noise In addition to the useful signal, a chaotic signal with a random amplitude and spectrum appears at the output of the photodiode - the noise of the photodiode. It does not allow the registration of useful signals as small as desired. Photodiode noise is made up of semiconductor material noise and photon noise.
Specifications:
- current-voltage characteristic (VAC) dependence of the output voltage on the input current.
- spectral characteristics dependence of the photocurrent on the wavelength of the incident light on the photodiode. It is determined from the side of long wavelengths by the bandgap, at small wavelengths by a large absorption coefficient and an increase in the effect of surface recombination of charge carriers with a decrease in the wavelength of light quanta. That is, the short-wavelength sensitivity limit depends on the base thickness and on the surface recombination rate. The position of the maximum in the spectral characteristic of the photodiode strongly depends on the degree of increase in the absorption coefficient.
- light characteristics The dependence of the photocurrent on the illumination corresponds to the direct proportionality of the photocurrent on the illumination. This is due to the fact that the thickness of the base of the photodiode is much less than the diffusion length of minority charge carriers. That is, almost all minority charge carriers arising in the base take part in the formation of the photocurrent.
- the time constant is the time during which the photodiode photocurrent changes after illumination or after darkening the photodiode by a factor of e (63%) with respect to the steady-state value.
- dark resistance resistance of the photodiode in the absence of illumination.
- inertia
Classification
- In the p-i-n structure, the middle i-region is enclosed between two regions of opposite conductivity. At a sufficiently high voltage, it penetrates the i-region, and free carriers, which appeared due to photons during irradiation, are accelerated by the electric field of p-n junctions. This results in gains in performance and sensitivity. The increase in the response rate in the p-i-n photodiode is due to the fact that the diffusion process is replaced by the drift of electric charges in a strong electric field. Already at Urev≈0.1V p-i-n the photodiode has an advantage in speed.
- Schottky photodiode (Schottky barrier photodiode) Metal-semiconductor structure. When the structure is formed, part of the electrons will transfer from the metal to the p-type semiconductor.
- Avalanche photodiode
- The structure uses an avalanche breakdown. It arises when the energy of photocarriers exceeds the energy of formation of electron-hole pairs. Very sensitive. To estimate there is an avalanche multiplication coefficient: To implement avalanche multiplication, two conditions must be met: 1) The electric field of the space charge region must be large enough for the electron to gain energy over the mean free path, which is greater than the band gap: 2) The width of the space charge region must be substantially greater than the mean free path: The value of the internal amplification factors is M \u003d 10-100, depending on the type of photodiodes.
- A photodiode with a heterostructure A heterojunction is a layer that appears at the interface of two semiconductors with different bandgaps. One p + layer plays the role of a “receiving window”. The charges are generated in the central area. By selecting semiconductors with different bandgaps, the entire wavelength range can be covered. The disadvantage is the complexity of manufacturing.
Simple photodiode is a conventional semiconductor diode, which provides the ability to influence optical radiation on the p – n junction.
In a balanced state, when the radiation flux is 100% absent, the carrier concentration, potential distribution and energy band diagram of the photodiode are 100% consistent with an ordinary pn structure.
When exposed to radiation in a direction perpendicular to the plane of the p-n-junction, as a result of the absorption of photons with an energy greater than the width of the illegal zone, electron-hole pairs appear in the n-region. These electrons and holes are called photo carriers.
During diffusion of photocarriers deep into the n-region, the majority of electrons and holes do not have time to recombine and reach the p – n junction boundary. Here the photocarriers are divided by the electron field of the p – n junction, while the holes run over to the p-region, and electrons cannot overcome the transition field and accumulate at the boundary of the p – n-junction and the n-region.
Thus, the current through the p – n junction is justified by the drift of minority carriers - holes. The drift current of photocarriers is called photocurrent.
Photocarriers - holes charge the p-region positively with respect to the n-region, and photocarriers - electrons - the n-region negatively with respect to the p-region. The resulting potential difference is called the photo-emf Ef. The generated current in the photodiode is reverse, it is oriented from the cathode to the anode, and its value is the greater, the greater the illumination.
Photodiodes can operate in one of 2 modes - without an external source of electronic energy (photogenerator mode) or with an external source of electronic energy (photoconverter mode).
Photodiodes operating in the photogenerator mode are often used as power sources that modify the energy of solar radiation into electronic one. They are called solar cells and are part of solar cells used in spacecraft and satellites.
The efficiency of silicon solar cells is about 20%, while for film solar cells it can be much more important. The necessary technical parameters of solar cells are the ratio of their output power to the mass and area occupied by the solar cell. These characteristics achieve values \u200b\u200bof 200 W / kg and 1 kW / m2, respectively.
When the photodiode operates in the photoconverting mode, the power supply E is cut into the circuit in the blocking direction (Fig. 1, a). The reverse branches of the CVC of the photodiode are used at different illumination levels (Fig. 1b).
Figure: 1. Scheme for switching on the photodiode in the photoconversion mode: a - switching circuit, b - CVC of the photodiode.
The current and voltage across the load resistor Rн can be determined graphically by the crossing points of the I - V characteristic of the photodiode and the load band, corresponding to the resistance of the resistor Rн. In the absence of illumination, the photodiode operates as a conventional diode. The dark current for germanium photodiodes is 10 - 30 μA, for silicon photodiodes 1 - 3 μA.
If a reversible electronic breakdown is used in photodiodes, accompanied by an avalanche multiplication of charge carriers, as in semiconductor zener diodes, then the photocurrent, and, consequently, the sensitivity will increase significantly.
Sensitivity avalanche photodiodes may be several orders of magnitude larger than that of conventional photodiodes (for germanium - 200 - 300 times, for silicon - 104 - 106 times).
Avalanche photodiodes are fast-acting photovoltaic devices, their frequency spectrum can reach 10 GHz. The disadvantage of avalanche photodiodes is the higher noise level in comparison with ordinary photodiodes.
Figure: 2. Scheme of switching on the photoresistor (a), UGO (b), energy (c) and current-voltage (d) properties of the photoresistor.
Apart from photodiodes, photoresistors (Figure 2), phototransistors and photothyristors are used, which use an internal photoelectric effect. Their corresponding disadvantage is the highest inertia (cutoff operating frequency fgr
The design of the phototransistor is similar to an ordinary transistor, which has a window in the case through which the base can be illuminated. UGO phototransistor - a transistor with 2 arrows directed towards it.
LEDs and photodiodes are often used in pairs. With all this, they are placed in one housing in such a way that the photosensitive area of \u200b\u200bthe photodiode is located opposite the emitting area of \u200b\u200bthe LED. Semiconductor devices using LED-photodiode pairs are called optocouplers (Fig. 3).
Figure: 3. Optocoupler: 1 - LED, 2 - photodiode
The input and output circuits in such devices are not electrically connected in any way, since the signal is transmitted through optical radiation.
