Analysis of the working principle of semiconductor optoelectronic devices

Semiconductor optoelectronic devices are new semiconductor devices that connect the two physical quantities of light and electricity and convert them into each other. Optoelectronic devices mainly include photoconductive devices that work using the photosensitivity of semiconductors, photocells that work using the photovoltaic effect of semiconductors, and semiconductor light-emitting devices. This section briefly introduces the working principles of these optoelectronic devices.

1. Photoconductive devices

The first section of this chapter introduced the photosensitive properties of semiconductor materials, that is, when semiconductor materials are irradiated with light of a certain wavelength, their resistivity decreases significantly, or their conductivity increases. This phenomenon is also called the photoconductivity of semiconductors. Semiconductor devices made using this property are called photoconductive devices.

The conductivity of semiconductor materials is determined by the carrier concentration. Carriers are electrons that escape from semiconductor atoms and the vacancies they leave behind, namely holes. For electricity to escape from atoms, it must break free from the atoms and do work. Light is a form of energy that provides energy to electrons, enabling them to escape. Therefore, light can change the concentration of carriers, thereby changing the conductivity of semiconductors.

Photoconductive devices mainly include photoresistors, photodiodes, phototransistors, etc.

1. Photoresistor.

This is a semiconductor resistor. When there is no light, the resistance is very large; under light within a certain wavelength range, the resistance value becomes significantly smaller. The main materials for making photoresistors are silicon, germanium, cadmium sulfide, indium antimonide, lead sulfide, cadmium selenide, lead selenide, etc. Cadmium sulfide photoresistors are sensitive to visible light, and photoresistors made of cadmium sulfide single crystals are also sensitive to X-rays and gamma rays; lead sulfide and indium antimonide are sensitive to infrared light. Various light detectors can be made using these photoresistors.

Photoresistors with large photosensitive areas can obtain a larger difference between light and dark resistances. For example, the domestic 625-A cadmium sulfide photoresistor has a light resistance of less than 50 kilo-ohms and a dark resistance of more than 50 megohms.

2. Photodiode

The core of the photodiode is also a PN junction, but the junction area is larger than that of an ordinary diode, which is convenient for receiving light. However, unlike ordinary diodes, photodiodes work under reverse voltage. Its dark current is very small, only about 0.1 microamperes. The electron-hole pairs generated under light irradiation are called photogenerated carriers, and their participation in conduction will increase the reverse saturation current. The number of photogenerated carriers is related to the light intensity. Therefore, the reverse saturation current will change with the change of light intensity, so that the change of light signal can be converted into the change of current and voltage.

Photodiodes are mainly used in near-infrared detectors and automatic control instruments for photoelectric conversion, and can also be used as receiving devices for optical fiber communications.

3. Phototransistor:

The structure of the phototransistor is the same as that of an ordinary transistor, but the base area is larger, which is convenient for receiving more incident light. When the incident light excites electrons-holes in the base area, a base current is formed, and the collector current is B times the base current, so light can effectively control the collector current. Phototransistors have higher sensitivity than photodiodes. 2. Photovoltaic devices--silicon photovoltaic cells

The effect that a semiconductor PN junction can generate an electromotive force when exposed to light is called the photovoltaic effect. Silicon photovoltaic cells are semiconductor devices that use the photovoltaic effect to directly convert light energy into electrical energy.

A silicon photovoltaic cell is a large-area PN junction. Illumination can cause a thin P-type region to generate a large number of photogenerated carriers. These photogenerated electrons and holes will diffuse toward the PN junction. During the diffusion process, some electrons and holes recombine and disappear, and most diffuse to the edge of the PN junction. Under the action of the junction electric field, most of the photogenerated holes are pushed back to the P-type region by the electric field and cannot cross the PN junction; most of the photogenerated resistors are accelerated by the junction electric field to cross the PN junction and reach the N-type region. With the accumulation of photogenerated electrons in the N-type region and the accumulation of photogenerated holes in the P-type region, a stable potential difference will be generated on both sides of the PN pair, which is the photogenerated electromotive force. When a load is connected to both ends of the photovoltaic cell, a current will flow through the load, acting as a battery.

Silicon photovoltaic cells are widely used. They are mainly used in the following aspects:

Energy----Silicon photovoltaic cells are connected in series or in parallel to form battery packs and can be used as power sources for artificial satellites, spacecraft, navigation lights, unmanned weather stations and other equipment; they can also be used as power sources for electronic watches, electronic calculators, small cars, yachts, etc.

Photoelectric detection devices----photoreceptors used as near-infrared detectors, photoelectric readouts, photoelectric coupling, laser collimation, movie sound reproduction and other equipment.

Photoelectric control devices----used as conversion devices for photoelectric control devices such as photoelectric switches.

3. Semiconductor light-emitting devices

Semiconductor light-emitting devices are devices that convert electrical energy into light energy. They include light-emitting diodes, infrared light sources, semiconductor light-emitting digital tubes, etc.

1. Light Emitting Diode

The tube core of the light-emitting diode is also a PN junction, and has unidirectional conductivity. When a forward voltage is applied to the PN junction, electrons cross (diffuse) from the N region to the space charge region and recombine with holes to release energy. Most of this energy appears in the form of light, so electrical energy can be directly converted into light energy. The light color (wavelength) of the light-emitting diode varies depending on the semiconductor material and doping composition. Commonly used light-emitting diodes are yellow, green, red and other colors.

The working voltage of light emitting diode is very low (15-3V), the working current is very small (10-30 mA), and the power consumption is very low. It can be used as a light signal display, a fast light source, and it can also play the role of rectification and light emission at the same time.

2. Luminous digital tube

The gallium phosphide light-emitting tube or the tube core of the gallium phosphide light-emitting tube is made into a strip shape, and seven light-emitting tubes are used to form a seven-segment digital display tube, which can display ten numbers from 0 to 9. The advantages of this semiconductor digital display tube are small size, low power consumption, long life, and fast response speed. It can be used as a digital display for various small calculators and digital display instruments.

3. Photocoupler

The semiconductor light-emitting device and the photosensitive device are combined and sealed together to form a photoelectric coupler with the function of electricity-light-electricity conversion. Obviously, when an electrical signal is input to the coupler, the light-emitting device emits light, and after the light is received by the light receiving device, it is converted into an electrical signal output. Because the input and output are coupled with light. Therefore, there is no feedback from the output end to the input end, and it has excellent isolation and anti-interference performance. The photoelectric coupler is also a photoelectric switch. This type of photoelectric switch does not have the problem of easy fatigue of mechanical points in relays, and has high reliability.