Volt-ampere characteristics of the photodiode. Photodiodes properties, switching circuits, application

1. The energy characteristics of a photodiode relate the photocurrent to the light flux incident on the photodiode. The dependence of the photocurrent on the luminous flux when the photodiode operates in generator mode is strictly linear only when the photodiode is short-circuited. With increasing load resistance, the characteristics become more and more bent and, at higher values, have a pronounced saturation region (Fig. 3.12, a). When the photodiode operates in a circuit with an external voltage source, the energy characteristics are much closer to linear. As the applied voltage increases, the photocurrent increases slightly (Fig. 3.12, b). This is explained by the expansion of the -transition region and a decrease in the width of the base, as a result of which a smaller part of the charge carriers recombines in the base when moving towards the -transition.

2. The absolute and relative spectral characteristics of the photodiode are similar to the corresponding characteristics of the photoresistor and depend on the material of the photodiode and the introduced impurities (Fig. 3.12, c).

Spectral characteristics practically cover the entire visible (300-750 nm) and infrared regions of the spectrum.

4. The frequency response shows the change in integral sensitivity with changes in the brightness of the light flux with different radiation frequencies (Fig. 3.12, d). Sometimes the inertial properties of a photodiode are characterized by a cutoff frequency at which the integral sensitivity decreases by a factor of its static value.

Rice. 3.12. Energy characteristics of the photodiode in mode (a) and when working with an external source (b); relative spectral and frequency characteristics

The cutoff frequency of high-speed silicon photodiodes is on the order of Hz.

To improve performance and sensitivity, a number of photodiodes have been developed in recent years; with built-in electric field; based on Schottky barriers; avalanche photodiodes, etc.

In photodiodes with a built-in electric field, the base is obtained using a diffusion process. Due to the uneven distribution of impurity concentration, an internal electric field arises in it, which accelerates the movement of minority charge carriers towards the -transition.

Due to the overlap of the diffusion and drift motions of the photodiode, it increases slightly.

Photodiodes made on the basis have a significantly larger thickness of the region depleted of the majority charge carriers, since between the p- and -regions there is a -region with its own electrical conductivity. Significant stresses can be applied to the transition without the risk of breaking through it. As a result, a situation arises where light radiation is absorbed directly in a region depleted of major charge carriers, in which a high-intensity electric field is created. Electrons and holes appearing in the transition region during light irradiation are instantly transferred to the corresponding regions. As a result, the performance increases sharply and f reaches Hz.

Photodiodes based on the Schottky barrier are similar in performance. They are made of silicon, on the surface of which a transparent metal coating is applied from films of gold (µm) and zinc sulfide (01 to 0.05 microns), creating a Schottky barrier. Due to the minimal base resistance and the absence of processes of accumulation and resorption of excess charges, the response speed is quite high Hz).

Avalanche photodiodes use avalanche breakdown of a junction or Schottky barrier. They differ from conventional photodiodes in that the charge carriers generated as a result of light irradiation multiply avalanchely in the region of the -transition due to impact ionization. The choice of external voltage and circuit parameters ensures that an avalanche breakdown occurs only under light irradiation. This process leads to the fact that the current in the circuit increases in comparison with the current due to light generation and the thermal current of the transition by a factor of (M-coefficient of carrier avalanche multiplication.

The avalanche multiplication coefficient is described by the relationship

Sensors are completely different. They differ in the principle of action, the logic of their work and the physical phenomena and quantities to which they are capable of reacting. Light sensors are used not only in automatic lighting control equipment, they are used in a huge number of devices, ranging from power supplies to alarms and security systems.

Main types of photoelectronic devices. General information

A photodetector in a general sense is an electronic device that responds to changes in the light flux incident on its sensitive part. They may differ both in their structure and principle of operation. Let's look at them.

Photoresistors - change resistance when illuminated

A photoresistor is a photodevice that changes conductivity (resistance) depending on the amount of light incident on its surface. The more intense the sensitive area, the less resistance. Here is a schematic representation of it.

It consists of two metal electrodes, between which there is a semiconductor material. When light hits a semiconductor, charge carriers are released in it, which promotes the passage of current between the metal electrodes.

The energy of the light flux is spent on electrons overcoming the band gap and their transition to the conduction band. As a semiconductor for photoresistors, materials such as: Cadmium Sulfide, Lead Sulfide, Cadmium Selenite and others are used. The spectral characteristics of the photoresistor depend on the type of material.

Interesting:

The spectral characteristic contains information about which wavelengths (colors) of the light flux the photoresistor is most sensitive to. For some specimens, it is necessary to carefully select a light emitter of the appropriate wavelength in order to achieve the greatest sensitivity and operating efficiency.

The photoresistor is not intended to accurately measure illumination, but rather to determine the presence of light; according to its readings, one can determine whether the environment has become lighter or darker. The current-voltage characteristic of a photoresistor is as follows.

It shows the dependence of current on voltage at different values of luminous flux: F is darkness, and F3 is bright light. It's linear. Another important characteristic is sensitivity, it is measured in mA (μA)/(Lm*V). Which reflects how much current flows through the resistor, given a certain luminous flux and applied voltage.

Dark resistance is an active resistance in the complete absence of lighting, denoted Rt, and the characteristic Rt/Rsv is the factor of change in resistance from the state of the photoresistor in the complete absence of lighting to the maximum illuminated state and the minimum possible resistance, respectively.

Photoresistors have a significant drawback - their cutoff frequency. This value describes the maximum frequency of the sinusoidal signal with which you model the light flux, at which the sensitivity decreases by 1.41 times. In reference books this is reflected either by the frequency value or through the time constant. It reflects the speed of the devices, which usually takes tens of microseconds - 10^(-5) s. This does not allow it to be used where high performance is needed.

Photodiode - converts light into electrical charge

A photodiode is an element that converts light falling on a sensitive area into an electrical charge. This happens because during irradiation, various processes associated with the movement of charge carriers occur in the p-n junction.

If the conductivity of the photoresistor changes due to the movement of charge carriers in the semiconductor, then a charge is formed at the boundary of the p-n junction. It can operate in photoconverter and photogenerator mode.

Its structure is the same as a regular diode, but its body has a window for light to pass through. Externally, they come in various designs.

Photodiodes with a black body perceive only infrared radiation. The black coating is something similar to tinting. Filters the IR spectrum to exclude the possibility of triggering radiation of other spectra.

Photodiodes, like photoresistors, have a cutoff frequency, only here it is orders of magnitude higher and reaches 10 MHz, which allows for good performance. P-i-N photodiodes have high speed - 100 MHz-1 GHz, like diodes based on the Schottky barrier. Avalanche diodes have a cutoff frequency of about 1-10 GHz.

In photoconverter mode, such a diode works as a light-controlled switch; for this, it is connected to the circuit in forward bias. That is, the cathode is to a point with a more positive potential (towards plus), and the anode is to a more negative potential (towards minus).

When the diode is not illuminated by light, only reverse dark current Irev flows in the circuit (units and tens of μA), and when the diode is illuminated, a photocurrent is added to it, which depends only on the degree of illumination (tens of mA). The more light, the greater the current.

Photocurrent Iф is equal to:

where Sint is the integral sensitivity, Ф is the luminous flux.

Typical circuit for switching on a photodiode in photoconverter mode. Pay attention to how it is connected - in the opposite direction to the power source.

Another mode is generator. When light hits a photodiode, a voltage is generated at its terminals, and the short circuit currents in this mode are tens of amperes. This resembles, but has low power.

Phototransistors - open depending on the amount of incident light

A phototransistor is essentially one in which, instead of a base output, there is a window in the body for light to enter. The operating principle and reasons for this effect are similar to previous devices. Bipolar transistors are controlled by the amount of current flowing through the base, and phototransistors are similarly controlled by the amount of light.

Sometimes the UGO also displays the output of the base. In general, the voltage is applied to the phototransistor in the same way as to a regular one, and the second connection option is with a floating base, when the base pin remains unused.

Phototransistors are included in the circuit in a similar way.

Or swap the transistor and resistor, depending on what exactly you need. In the absence of light, a dark current flows through the transistor, which is formed from the base current, which you can set yourself.

Having set the required base current, you can set the sensitivity of the phototransistor by selecting its base resistor. This way, even the dimmest light can be captured.

In Soviet times, radio amateurs made phototransistors with their own hands - they made a window for light by cutting off part of the body of an ordinary transistor. Transistors like MP14-MP42 are excellent for this.

From the current-voltage characteristic, the dependence of the photocurrent on illumination is visible, while it is practically independent of the collector-emitter voltage.

In addition to bipolar phototransistors, there are also field-effect ones. Bipolar ones operate at frequencies of 10-100 kHz, while field ones are more sensitive. Their sensitivity reaches several Amps per Lumen, and the “faster” ones - up to 100 MHz. Field-effect transistors have an interesting feature: at maximum luminous flux values, the gate voltage has almost no effect on the drain current.

Application areas of photoelectronic devices

First of all, you should consider more familiar options for their use, for example, automatically turning on the light.

The circuit shown above is the simplest device for turning a load on and off at a certain light level. Photodiode FD320 When light hits it, it opens and a certain voltage drops across R1, when its value is sufficient to open transistor VT1 - it opens and opens another transistor - VT2. These two transistors are a two-stage current amplifier, necessary to power the relay coil K1.

Diode VD2 is needed to dampen the EMF self-induction that is formed when the coil is switched. One of the wires from the load is connected to the supply contact of the relay, the top one in the diagram (for alternating current - phase or zero).

We have normally closed and open contacts, they are needed either to select the circuit to be turned on, or to select to turn on or turn off the load from the network when the required illumination is achieved. Potentiometer R1 is needed to adjust the device to operate with the required amount of light. The greater the resistance, the less light is needed to turn on the circuit.

Variations of this circuit are used in most similar devices, adding a certain set of functions if necessary.

In addition to switching on the light load, such photodetectors are used in various control systems, for example, at metro turnstiles, photoresistors are often used to detect unauthorized (hare) crossing of the turnstile.

In a printing house, when a strip of paper breaks, the light hits the photodetector and thereby gives a signal to the operator about this. The emitter is on one side of the paper, and the photodetector is on the opposite side. When the paper is torn, light from the emitter reaches the photodetector.

In some types of alarm systems, an emitter and a photodetector are used as sensors for entering a room, while IR devices are used to prevent the radiation from being visible.

Regarding the IR spectrum, there is no mention of the TV receiver, which receives signals from the IR LED in the remote control when you change channels. The information is encoded in a special way and the TV understands what you need.

Information was previously transmitted in this way through the infrared ports of mobile phones. The transmission speed is limited both by the serial transmission method and by the operating principle of the device itself.

Computer mice also use technology related to photoelectronic devices.

Applications for signal transmission in electronic circuits

Optoelectronic devices are devices that combine an emitter and a photodetector in one housing, such as those described above. They are needed to connect two circuits of an electrical circuit.

This is necessary for galvanic isolation, fast signal transmission, as well as for connecting DC and AC circuits, as in the case of controlling a triac in a 220 V 5 V circuit with a signal from a microcontroller.

They have a conventional graphic designation that contains information about the type of elements used inside the optocoupler.

Let's look at a couple of examples of using such devices.

If you are designing a thyristor or triac converter you will encounter a problem. Firstly, if the transition at the control output breaks, a high potential will hit and the latter will fail. For this purpose, special drivers have been developed with an element called an optosimistor, for example MOC3041.

Switching stabilized power supplies require feedback. If we exclude galvanic isolation in this circuit, then if some components in the OS circuit fail, a high potential will arise on the output circuit and the connected equipment will fail, I’m not talking about the fact that you can get an electric shock.

In a specific example, you see the implementation of such an OS from the output circuit to the feedback (control) winding of the transistor using an optocoupler with the serial designation U1.

conclusions

Photo- and optoelectronics are very important sections in electronics, which have significantly improved the quality of equipment, its cost and reliability. Using an optocoupler, it is possible to eliminate the use of an isolating transformer in such circuits, which reduces weight and size parameters. In addition, some devices simply cannot be implemented without such elements.

Photodiodes are semiconductor elements that are photosensitive. Their main function is the transformation of light flux into an electrical signal. Such semiconductors are used as part of various devices, the functioning of which is based on the use of light fluxes.

The principle of operation of photodiodes

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 a pn junction, light quanta are absorbed to form photocarriers
Photocarriers located in the n region approach the boundary at which they are separated under the influence of the electric field
Holes move to the p zone, and electrons collect in the n zone or near the boundary
Holes charge the p-region positively, and electrons charge the n-zone 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. When the tester rings in the absence of lighting, the results will be similar to testing a conventional diode. In the forward direction there will be a small resistance, in the opposite direction the arrow will remain at zero.

Photodiode circuit

Operating modes

Photodiodes are divided according to their operating mode.

Photo generator mode

Performed without a power source. Photogenerators that are components of solar batteries are otherwise called “solar cells”. Their function is to convert solar energy into electrical energy. The most common photogenerators are based on silicon - cheap, widespread, and well studied. They have a low cost, but their efficiency reaches only 20%. Film elements are more progressive.

Photoconversion mode

The power source is connected to the circuit with reverse polarity, the photodiode in this case serves as a light sensor.

Main settings

The properties of photodiodes are determined by the following characteristics:

Volt-ampere. Determines the change in the magnitude of the light current in accordance with the changing voltage with a stable light flow and dark current
Spectral. Characterizes the effect of light wavelength on photocurrent
The time constant is the period during which the current responds to an increase in darkness or illumination of 63% of the set value
Sensitivity threshold - the minimum luminous flux to which the diode reacts
Dark resistance is an indicator characteristic of a semiconductor in the absence of light
Inertia

What does a photodiode consist of?

Types 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 resistance value. When light hits this area, pairs of holes and electrons appear. The electric field in this region is constant, there is no space charge. Such an 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 light flux into an electrical signal, amplified using the avalanche multiplication effect. Can be used in conditions of low luminous flux. Avalanche photodiodes use superlattices to reduce interference during signal transmission.

With 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 primary rather than additional charge carriers.

With heterostructure

Formed from two semiconductors having different band gaps. The layer located between them is called heterogeneous. By selecting such semiconductors, it is possible to create a device that operates in the full range of wavelengths. Its disadvantage is the high complexity of manufacturing.

Applications of photodiodes

Optoelectronic integrated circuits. Semiconductors provide optical communication, which ensures efficient galvanic isolation of power and control circuits while maintaining functional communication.
Multi-element photodetectors - scanistors, photosensitive devices, photodiode matrices. The optoelectric element is capable of perceiving not only the brightness characteristics of an object and its change over time, but also creating a complete visual image.

Other areas of use: fiber optic lines, laser range finders, positron emission tomography installations.

9. LEDs. Purpose, device, operating principle, main parameters and characteristics.

Purpose: LED is a semiconductor device that emits light when current is passed through it in the forward direction.

Operating principle: The work is based on the physical phenomenon of the occurrence of light radiation when an electric current passes through a p-n junction. The color of the glow (the maximum wavelength of the emission spectrum) is determined by the type of semiconductor materials used that form the p-n junction.

An LED is a semiconductor emitting device with one or more n-p junctions that converts electrical energy into the energy of incoherent light radiation. Emission occurs as a result of recombination of injected carriers in one of the regions adjacent to the n-p junction. Recombination occurs when carriers move from upper to lower levels.

Characteristics and parameters: the main parameter of LEDs is the internal quantum efficiency (the ratio of the number of photons to the number of carriers injected into the base) and external efficiency (the ratio of the flux of photons from the LED to the flux of charge carriers in it). External efficiency is largely determined by technology and can be significantly increased as its level increases.

The main characteristics of LEDs are current-voltage, brightness and spectral. The main parameters of light-emitting diodes are wavelength, half-width of the radiation spectrum, radiation power, operating frequency and radiation pattern.

LEDs are widely used in digital indicators, light displays, and optoelectronics devices. It is fundamentally possible to form a color television screen on their basis.

The photodiode is actively used in modern electronic devices; from the name it becomes clear that the device is a structure using a semiconductor, so let's look at what a photodiode is. A photodiode is a semiconductor diode that has the property of one-way 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 terminals and mounted in a plastic or metal case.

There are two modes of operation of the photodiode.

1) photodiode - when the external circuit of the photodiode contains a direct current source, which creates a reverse bias at the junction and a valve bias when such a source is absent. In photodiode mode, a photodiode, like a photoresistor, is used to control the current. The photocurrent of the 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 a photodiode are the sensitivity threshold, noise level, the spectral sensitivity range ranges from 0.3 to 15 μm (micrometers), inertia is the recovery time of the photocurrent. There are also photodiodes with a direct structure. The photodiode is an integral element in many optoelectronic devices . Photodiodes and photodetectors are widely used in opron pairs and radiation receivers for video and audio signals. Widely used for receiving signals from laser diodes in CD and DVD drives.

The signal from the laser diode, which contains encoded information, first hits the photodiode, which in these devices has a complex design, then, after decoding, the information goes to the central processor, where after processing it turns into an audio or video signal. All modern disk drives operate on this principle. Photodiodes are also used in various security devices, in infrared motion and presence sensors. Another review for a beginner radio amateur has come to an end, good luck in the world of radio electronics - AKA.

Theory for beginners

Discuss the article PHOTODIODES

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description of the operating principle, diagram, characteristics, methods of application

The principle of operation of photodiodes

When light hits a pn junction, light quanta are absorbed to form photocarriers
Photocarriers located in the n region approach the boundary at which they are separated under the influence of the electric field
Holes move to the p zone, and electrons collect in the n zone or near the boundary
Holes charge the p-region positively, and electrons charge the n-zone negatively. A potential difference is formed
The higher the illumination, the greater the reverse current

Photodiode circuit

Operating modes

Photodiodes are divided according to their operating mode.

Photo generator mode

Photoconversion mode

The power source is connected to the circuit with reverse polarity, the photodiode in this case serves as a light sensor.

Main settings

The properties of photodiodes are determined by the following characteristics:

Volt-ampere. Determines the change in the magnitude of the light current in accordance with the changing voltage with a stable light flow and dark current
Spectral. Characterizes the effect of light wavelength on photocurrent
The time constant is the period during which the current responds to an increase in darkness or illumination of 63% of the set value
Sensitivity threshold - the minimum luminous flux to which the diode reacts
Dark resistance is an indicator characteristic of a semiconductor in the absence of light
Inertia

What does a photodiode consist of?

Types of photodiodes

P-i-n

Avalanche

With Schottky barrier

With heterostructure

Applications of photodiodes

Optoelectronic integrated circuits. Semiconductors provide optical communication, which ensures efficient galvanic isolation of power and control circuits while maintaining functional communication.
Multi-element photodetectors - scanistors, photosensitive devices, photodiode matrices. The optoelectric element is capable of perceiving not only the brightness characteristics of an object and its change over time, but also creating a complete visual image.

Other areas of use: fiber optic lines, laser range finders, positron emission tomography installations.

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Photodiodes

A photodiode is usually called a semiconductor device with one p-n junction, the current-voltage characteristic of which depends on the light acting on it.

The symbolic graphic designation, structure and appearance of the photodiode are presented in Fig. 17.6.

Rice. 17.6. Photodiode:

a - conventional graphic designation; b – structure; c – appearance

The simplest photodiode is a conventional semiconductor diode, which allows the influence of optical radiation on the p-n junction. In an equilibrium state, when the radiation flux is completely absent, the carrier concentration, potential distribution and energy band diagram of the photodiode are completely consistent with a conventional 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. When photocarriers diffuse deep into the n region, the majority of electrons and holes do not have time to recombine and reach the boundary of the pn junction. Here, photocarriers are separated by the electric field of the p-n junction, with holes moving into the p region, and electrons cannot overcome the transition field, and accumulate at the boundary of the p-n junction and the n region. However, the current through the p-n junction is caused by the drift of non-base carriers - holes. The drift current of photocarriers is usually called photocurrent.

Photocarriers - holes charge the p region positively relative to the n region, and photocarriers - electrons - charge the n region negatively relative to the p region. The resulting potential difference is usually called photo emf Ef. The generated current in the photodiode is reverse, it is directed from the cathode to the anode, and its value is 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 operates in converter mode, a reverse voltage is applied to it (Fig. 17.7, a). The reverse branches of the current-voltage characteristics of the photodiode are used at different illumination levels F, F1, F2 (Fig. 17.7, b).

Taking into account the dependence of the illumination level, the reverse current of the photodiode changes, and the voltage across the load resistor changes. In railway automation systems, according to this scheme, a germanium photosensor is included in devices for detecting a heated axlebox (germanium is sensitive to IR rays, and silicon is sensitive to visible light).


A)	b)

Rice. 17.7. Operation of a photodiode in photoconverter mode:

a – connection diagram; b – current-voltage characteristics

Photodiodes operating in generator mode are used as power sources that convert solar radiation energy into electrical energy. These are called solar cells and are part of solar panels. The output voltage of a solar cell is highly dependent on the light level. To obtain a stable voltage at the load, a solar battery is used in conjunction with a battery. The diagram of the solar battery is shown in Fig. 17.8.

Rice. 17.8. Schematic diagram of a solar battery

At maximum illumination, the solar battery powers the load and charges the battery. Posted on ref.rf In the dark, the load is powered only by the battery, and to prevent the battery from being discharged by the solar battery, a 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 of 200 W/kg and 1 kW/m2, respectively.

More detailed information about photodiodes is given in the literature.

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Photodiodes | Techniques and Programs

Operating principle of a photodiode

A semiconductor photodiode is a semiconductor diode whose reverse current depends on illumination.

Typically, semiconductor diodes with a pn junction are used as a photodiode, which is reverse biased by an external power source. When light quanta are absorbed in a pn junction or in areas adjacent to it, new charge carriers are formed. Minority charge carriers that arise in 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 influence of an electric field. That is, the reverse current increases when illuminated. Absorption of quanta directly in the pn junction leads to similar results. The amount by which the reverse current increases is called photocurrent.

Characteristics of photodiodes

The properties of a photodiode can be characterized by the following characteristics:

The current-voltage characteristic of a photodiode is the dependence of the light current at a constant light flux and the dark current 1t on voltage.

The light characteristic of a photodiode is determined by the dependence of the photocurrent on illumination. As illumination increases, 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 rate and an increase in the influence of surface recombination of charge carriers with a decrease in the wavelength of light quanta. That is, the short-wavelength limit of sensitivity depends on the thickness of the base and the speed of surface recombination. 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 photocurrent of the photodiode changes after illumination or after darkening of the photodiode by e times (63%) relative to the steady-state value.

Dark resistance is the resistance of the photodiode in the absence of illumination.

The integral sensitivity is determined by the formula:

where 1ph is photocurrent, Ф is illumination.

Inertia

There are three physical factors that influence inertia:

1. Time of diffusion or drift of nonequilibrium carriers through the base t;

2. Time of flight through the p-n junction t;

3. Recharging time of the barrier capacitance of the pn junction, characterized by the time constant RC6ap.

The thickness of the pn junction, depending on the reverse voltage and the concentration of impurities in the base, is usually less than 5 μm, which means t - 0.1 ns. RC6ap is determined by the barrier capacitance of the pn junction, which depends on the voltage and resistance of the photodiode base at low load resistance in the external circuit. The magnitude of RC6ap is usually several nanoseconds.

Calculation of photodiode efficiency and power

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 photodiode power is illustrated in Fig. 2.12 and table 2.1.

Rice. 2.12. Dependence of photodiode power on voltage and current

The maximum power of the photodiode corresponds to the maximum area of a given rectangle.

Table 2.1. Dependence of power on efficiency

Illumination power, mW	Current strength, mA	Voltage, V

Application of photodiode in oltoelectronics

The photodiode is an integral element in many complex optoelectronic devices:

Optoelectronic integrated circuits.

A photodiode may be faster, but its photocurrent gain does not exceed unity. Thanks to the presence of optical communication, optoelectronic integrated circuits have a number of significant advantages, namely: almost ideal galvanic isolation of control circuits from power circuits while maintaining a strong functional connection between them.

Multi-element photodetectors.

These devices (scanistor, photodiode matrix controlled by a MOS transistor, photosensitive charge-coupled 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, perceiving its full visual image.

The number of photosensitive cells in the device is quite large, so in addition to all the problems of a discrete photodetector (sensitivity, speed, spectral region), the problem of reading information also has to be solved. All multi-element photodetectors are scanning systems, that is, devices that make it possible to analyze the space under study by sequentially viewing it (element-by-element decomposition).

How does image perception occur?

The brightness distribution of the observed object is converted into an optical image and focused onto a 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 brightness pattern is converted into electrical relief. The scanning circuit periodically sequentially polls each element and reads the information contained in it. Then at the output of the device we receive a sequence of video pulses in which the perceived image is encoded.

When creating multi-element photodetectors, they strive to ensure the best performance of conversion and scanning functions. Optocouplers.

An optocoupler is an optoelectronic device in which there is a source and a radiation receiver 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, on the order of several mA), where the emitter is connected, and the executive circuit, in which the photodetector operates, and control information is transmitted through light radiation.

This property of an optoelectronic pair (and in some types of optocouplers there are even several optical optocouplers not connected to each other) turned out to be indispensable in those electronic units where it is necessary to eliminate as much as possible the influence of output electrical circuits on input ones. For all discrete elements (transistors, thyristors, microcircuits that are switching assemblies, or microcircuits with an output that allows switching high-power loads), the control and executive circuits are electrically connected to each other. This is often unacceptable when switching high voltage loads. In addition, the resulting feedback inevitably leads to additional interference.

Structurally, the photodetector is usually mounted on the bottom of the housing, and the emitter is mounted on the top. The gap between the emitter and the photodetector is filled with immersion material - most often this role is performed by polymer optical glue. This material acts as a lens that focuses radiation onto the sensitive layer of the photodetector. The immersion material is coated on the outside with a special film that reflects light rays inward to prevent radiation from scattering outside the working area of the photodetector.

The role of emitters in optocouplers is usually performed by LEDs based on gallium arsenide. Photosensitive elements in optocouplers can be photodiodes (optocouplers of the AOD... series), phototransistors, phototrinistors (optocouplers of the AOU... series) and highly integrated photorelay circuits. In a diode optocoupler, for example, a silicon-based photodiode is used as a photoreceiving element, and an infrared emitting diode serves as the emitter. The maximum spectral characteristics of the diode radiation occur at a wavelength of about 1 micron. 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 influencing the LED) and electrically via 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). For a field-effect transistor, control is carried out through the gate circuit.

In addition, the phototransistor can operate in switching and amplification modes, and the photodiode can only operate in switching mode. Optocouplers with composite transistors (for example, AOT1YUB) have the highest gain (like a conventional unit on a composite transistor), can switch voltage and current of sufficiently large values and in these parameters are second only to thyristor optocouplers and optoelectronic relays of the KR293KP2 - KR293KP4 type, which suitable for switching high-voltage and high-current circuits. Today, new optoelectronic relays of the K449 and K294 series have appeared in retail sales. 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 replace low-power electromagnetic relays and have a lot of advantages over relays (quiet operation, reliability, durability, absence of mechanical contacts, wide operating voltage range). In addition, their affordable price is explained by the fact that there is no need to use precious metals (in relays they cover the switching contacts).

In resistor optocouplers (for example, OEP-1), the emitters are electric mini-incandescent lamps, also placed in one housing.

The graphic designations of optocouplers according to GOST are assigned a conventional code - the Latin letter U, followed by the serial number of the device in the circuit.

Chapter 3 of the book describes instruments 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.

Compared to other, more complex photodetectors, they have the greatest stability of temperature characteristics and better performance properties.

The main drawback that is usually pointed out is the lack of amplification. But it is quite conventional. In almost every optoelectronic device, the photodetector operates on one or another matching electronic circuit. And introducing an amplification stage into it is much simpler and more expedient than giving the photodetector unusual amplification functions.

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 range. The small wavelength of light vibrations ensures a high achievable density of information recording in optical storage devices (up to 108 bits/cm2).

Sharp directionality (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 concentrated and low-loss transmission of electrical energy to any area of space.

Possibility of double - temporal and spatial - modulation of the light beam. Since the source and receiver in optoelectronics are not electrically connected to each other, and the connection between them is carried out only through a light beam (electrically neutral photons), they do not influence each other. And therefore, in an optoelectronic device, the flow of information is transmitted in only 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 performance. They can operate at frequencies up to several MHz. usually made from germanium or silicon.

The photodiode is a potential broadband receiver. This determines its widespread use and popularity.

IR spectrum

An infrared emitting diode (IR diode) is a semiconductor diode that, when direct current flows through it, emits electromagnetic energy in the infrared region of the spectrum.

Unlike the radiation spectrum visible to the human eye (such as, for example, produced by a conventional light-emitting diode based on gallium phosphide), IR 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 in 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 range of 0.87...0.96 microns. The radiation efficiency and efficiency of these devices is higher than that of light-emitting diodes.

Based on IR diodes (which occupy an important place in electronic designs as pulse transmitters in the IR spectrum), fiber-optic lines (advantageously distinguished by their speed and noise immunity), multifaceted electronic household units and, of course, electronic security units are constructed. This has its advantage, because... The IR beam is invisible to the human eye and in some cases (if several multidirectional IR beams are used), it is impossible to visually determine the presence of the security device itself until it goes into “alarm” mode). Experience in the field of production and maintenance of security systems based on IR emitters allows us to give some recommendations for determining the operating condition of IR emitters.

If you look closely at the emitting surface of an 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 peering into a device emitting IR light energy for a long time is undesirable from a medical point of view.

In addition to security systems, IR emitting diodes are currently used in car alarm key fobs and various types of wireless signal transmitters over a distance. For example, by connecting a modulated low-frequency signal from an amplifier to the transmitter, using an IR receiver at a certain distance (depending on the radiation power and terrain) you can listen to audio information; telephone conversations can also be broadcast over a distance. This method is less effective today, but is still an alternative to a home radiotelephone. The most popular (in everyday life) application of IR emitting diodes is remote controls for various household appliances.

As any radio amateur can easily verify 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 operating modes of IR diodes (forward and reverse maximum permissible 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. The 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 direct current flowing through the diode. The radiation pattern of a diode shows a decrease in radiation power depending on the angle between the direction of radiation and the optical axis of the device. Modern IR diodes differ between having highly directional radiation and scattered radiation.

When designing electronic components, it should be taken into account that the transmission range of the IR signal directly depends on the angle of inclination (the combination of the transmitting and receiving parts of the device) and the power of the IR diode. When interchanging IR diodes, it is necessary to take into account this radiation power parameter. Some reference data on domestic IR diodes are given in table. 2.2.

Data on interchanges of foreign and domestic devices are given in the appendix. Today, the most popular types of IR diodes among radio amateurs are considered to be devices of the AL 156 and AL147 model series. They are optimal in terms of versatility of use and cost.

Pulse radiation power - Rizl im - the amplitude of the radiation flux, measured for a given direct current pulse through the diode.

The width of the radiation spectrum is the wavelength interval in which the spectral radiation power density is half the maximum.

The maximum permissible forward pulse current is 1 direct (IR diodes are mainly used in pulsed operating mode).

Table 2.2. Infrared emitting diodes

Radiation power, mW	Wavelength, µm	Spectral width, µm	Device voltage, V	Radiation angle, degrees






		no data		no data

The rise time of the radiation pulse tHapizl is the time interval during which the diode radiation power increases from 10 to 100% of the maximum value.

The pulse decay time parameter tcnM3J1 is similar to the previous one.

Duty factor - Q - the ratio of the period of pulse 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 this is not the only way to use the operating principle of an IR diode. Such opto-relays can also operate in the mode of responding to the reflection of rays (the photodetector is placed next to the emitter). This principle is embodied in electronic components that respond to the approach of an object or person to the combined receiving-transmitting node, 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.

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A photodiode is... What is a Photodiode?

Photodiode FD-10-100 active area - 10x10 mm² FD1604 (active cell area 1.2x4mm2 - 16 pcs) Designation on diagrams

A photodiode is an optical radiation receiver that converts light incident on its photosensitive region into an electric charge due to processes in the p-n junction.

A photodiode, whose operation is based on the photovoltaic effect (the separation of electrons and holes in the p- and n-regions, due to which a charge and emf is formed), is called a solar cell. In addition to p-n photodiodes, there are also 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 - direct current source; RH - load.

Principle of operation:

When exposed to radiation quanta in the base, free carriers are generated, which rush to the boundary of the p-n junction. The width of the base (n-region) is made such that holes do not have time to recombine before moving to the p-region. The photodiode current is determined by the minority carrier current - 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 capacitance of the p-n junction Cp-n

The photodiode can operate in two modes:

photovoltaic - no external voltage
photodiode - with external reverse voltage

Peculiarities:

simplicity of manufacturing technology and structure
combination of high photosensitivity and speed
low base resistance
low inertia

Parameters and characteristics of photodiodes

Options:

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, sensitivity is measured by the ratio of the change in the electrical characteristic recorded at the output of the photodetector to the luminous flux or radiation flux that caused it. ; - current sensitivity according to light flux; - voltaic sensitivity to energy flow
noise, in addition to the useful signal, a chaotic signal with a random amplitude and spectrum appears at the output of the photodiode - photodiode noise. It does not allow recording arbitrarily small useful signals. Photodiode noise is a combination of semiconductor material noise and photon noise.

Characteristics:

current-voltage characteristic (volt-ampere characteristic) is the 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 band gap, at short wavelengths by a large absorption rate and an increase in the influence of surface recombination of charge carriers with a decrease in the wavelength of light quanta. That is, the short-wavelength limit of sensitivity depends on the thickness of the base and the speed of surface recombination. 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 photocurrent on illumination corresponds to direct proportionality of photocurrent on illumination. This is due to the fact that the thickness of the photodiode base is significantly 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 photocurrent.
The time constant is the time during which the photocurrent of the photodiode changes after illumination or after darkening of the photodiode by e times (63%) relative to the steady-state value.
dark resistance is the resistance of a photodiode in the absence of illumination.
inertia

Classification

In a p-i-n structure, the middle i-region is located between two regions of opposite conductivity. At a sufficiently high voltage, it penetrates the i-region, and free carriers, which appear due to photons during irradiation, are accelerated by the electric field of the p-n junctions. This gives a gain in speed and sensitivity. The increase in performance in a 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 Uarb≈0.1V the p-i-n photodiode has an advantage in performance.

Advantages: 1) it is possible to provide sensitivity in the long-wavelength part of the spectrum by changing the width of the i-region. 2) high sensitivity and speed 3) low operating voltage Urab Disadvantages: difficulty in obtaining high purity of the i-region

Schottky photodiode (Schottky barrier photodiode) Metal-semiconductor structure. When the structure is formed, some electrons will transfer from the metal to the p-type semiconductor.

Avalanche photodiode

The structure uses avalanche breakdown. It occurs 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 so that, over the mean free path, the electron gains energy greater than the bandgap width: 2) The width of the space charge region must be significantly greater than the free path: The value of the internal gain factors is M=10-100 depending on the type of photodiodes.

Photodiode with a heterostructure A heterojunction is a layer that appears at the interface of two semiconductors with different band gaps. One p+ layer plays the role of a “receiving window”. Charges are generated in the central region. By selecting semiconductors with different band gaps, it is possible to cover the entire wavelength range. The disadvantage is the complexity of manufacturing.