A sensor consists of two parts: a sensitive element (sensing element) and a conversion device. Some semiconductor sensitive elements can directly output electrical signals, thus constituting a sensor themselves. Sensitive elements are diverse, and based on their principles of sensing external information, they can be divided into: ① Physical types, based on physical effects such as force, heat, light, electricity, magnetism, and sound; ② Chemical types, based on the principles of chemical reactions; ③ Biological types, based on molecular recognition functions such as enzymes, antibodies, and hormones. Generally, based on their basic sensing functions, sensors can be classified into ten major categories: thermal sensors, photosensitive sensors, gas sensors, force sensors, magnetic sensors, humidity sensors, acoustic sensors, radiation sensors, color sensors, and taste sensors (some have even classified sensors into 46 categories). The following is an introduction to commonly used thermal, photosensitive, gas-sensitive, force-sensitive, and magnetic sensors and their sensitive elements.
I. Temperature Sensors and Thermistors
Temperature sensors are mainly composed of thermistors. There are many types of thermistors available on the market, including bimetallic strips, copper resistance thermometers, platinum resistance thermometers, thermocouples, and semiconductor thermistors. Temperature sensors using semiconductor thermistors as the sensing element are widely used because, within the allowable operating conditions, semiconductor thermistors are characterized by small size, high sensitivity, and high accuracy, and are also simple to manufacture and inexpensive.
1. Working principle of semiconductor thermistors
Thermistors can be divided into two categories according to their temperature characteristics: those whose resistance increases with temperature are positive temperature coefficient thermistors, and those whose resistance decreases with temperature are negative temperature coefficient thermistors.
(1) Working principle of positive temperature coefficient thermistor
This type of thermistor uses barium titanate (BaTiO3) as the base material, with appropriate amounts of rare earth elements added, and is sintered at high temperatures using ceramic technology. Pure barium titanate is an insulating material, but after adding appropriate amounts of rare earth elements such as lanthanum (La) and niobium (Nb), it becomes a semiconductor material, known as semiconducting barium titanate. It is a polycrystalline material with grain interfaces between the grains. For conductive electrons, the grain interfaces act as a potential barrier. At low temperatures, due to the internal electric field of semiconducting barium titanate, conductive electrons can easily cross the potential barrier, resulting in a low resistance value. When the temperature rises to the Curie point (i.e., the critical temperature; the 'temperature control point' of this element is generally 120°C for barium titanate), the internal electric field is disrupted, preventing conductive electrons from crossing the potential barrier, thus resulting in a sharp increase in resistance. Because this type of element has a very slow resistance change with temperature before reaching the Curie point, it has constant temperature, temperature adjustment and automatic temperature control functions, it only heats up but does not turn red, has no open flame, is not easily flammable, can operate on AC or DC voltages of 3~440V, has a long service life, and is very suitable for overheat detection of electrical devices such as motors.
(2) Working principle of negative temperature coefficient thermistors
Negative temperature coefficient (NTC) thermistors are manufactured using ceramic processes with metal oxides such as manganese oxide, cobalt oxide, nickel oxide, copper oxide, and aluminum oxide as the main raw materials. These metal oxide materials all possess semiconductor properties, completely similar to germanium and silicon crystal materials. They have a low number of charge carriers (electrons and holes) and high resistance; as temperature increases, the number of charge carriers increases, naturally decreasing the resistance. There are many types of NTC thermistors, categorized by low temperature (-60~300℃), medium temperature (300~600℃), and high temperature (>600℃). They offer advantages such as high sensitivity, good stability, fast response, long lifespan, and low price, and are widely used in automatic temperature control circuits requiring fixed-point temperature measurement, such as temperature control systems in refrigerators, air conditioners, and greenhouses.
A thermistor, combined with a simple amplifier circuit, can detect temperature changes of one-thousandth of a degree Celsius. Therefore, when combined with electronic instruments to form a thermometer, it can perform high-precision temperature measurements. General-purpose thermistors operate at temperatures from -55℃ to +315℃, while special low-temperature thermistors operate at temperatures below -55℃, reaching as low as -273℃.
2. Thermistor model
China's domestically produced thermistors are modeled according to the ministerial standard SJ1155-82 and consist of four parts.
Part 1: Subject, indicated by the letter 'M' for sensitive element.
Part Two: Category, indicated by the letter 'Z' for positive temperature coefficient thermistors or the letter 'F' for negative temperature coefficient thermistors.
Part Three: Application or Characteristics, represented by a single digit (0-9). Generally, '1' indicates general application, '2' indicates voltage regulation (negative temperature coefficient thermistor), '3' indicates microwave measurement (negative temperature coefficient thermistor), '4' indicates externally heated (negative temperature coefficient thermistor), '5' indicates temperature measurement, '6' indicates temperature control, '7' indicates demagnetization (positive temperature coefficient thermistor), '8' indicates linear type (negative temperature coefficient thermistor), '9' indicates isothermal type (positive temperature coefficient thermistor), and '0' indicates special type (negative temperature coefficient thermistor).
Part Four: Serial Number, also represented by numbers, indicates specifications and performance.
Manufacturers often add a 'derivative serial number' after the serial number to distinguish products in this series. This serial number is composed of letters, numbers, and '-' signs.
3. Main parameters of thermistors
The operating conditions of all thermistors must be within the limits allowed by their factory specifications. The main parameters of a thermistor include more than ten items: nominal resistance value, operating ambient temperature (maximum operating temperature), measuring power, rated power, nominal voltage (maximum operating voltage), operating current, temperature coefficient, material constant, and time constant. The nominal resistance value is the resistance value at zero power in 25℃; in reality, there is always a certain error, which should be within ±10%. Ordinary thermistors have a wide operating temperature range, from -55℃ to +315℃, depending on the needs. It is worth noting that the maximum operating temperature varies greatly between different models of thermistors. For example, the MF11 chip negative temperature coefficient thermistor has a maximum operating temperature of +125℃, while the MF53-1 only has +70℃. Students should pay attention to this during experiments (generally, the temperature should not exceed 50℃).
4. Selection of thermistors for experiments
General-purpose negative temperature coefficient thermistors are preferred because their temperature changes are generally easier to observe than those of positive temperature coefficient thermistors, with a noticeable and continuous decrease in resistance. If a positive temperature coefficient thermistor is chosen, the experimental temperature should be near the Curie point of the element.
For rough measurement of a thermistor's value, a multimeter with a moderate range and a small current draw when measuring the thermistor is preferred. If the thermistor is around 10kΩ, an MF10 multimeter can be used. Set its range switch to the ohms range (R×100), and use alligator clips instead of test leads to clamp the two leads of the thermistor. When the ambient temperature is significantly lower than body temperature, the reading will be 10.2kΩ. Squeeze the thermistor with your hand; you will see the resistance reading gradually decrease. After releasing your hand, the resistance will increase and gradually return to its original value. Such a thermistor is suitable (maximum operating temperature around 100℃).
II. Optical Sensors and Photosensitive Elements
Optical sensors are mainly composed of photosensitive elements. Currently, photosensitive elements are developing rapidly, with a wide variety of types and applications. Commercially available products include photoresistors, photodiodes, phototransistors, optocouplers, and photovoltaic cells.
1. Photoresistor
Photoresistors are composed of light-transmitting semiconductor photoelectric crystals. Due to the different compositions of semiconductor photoelectric crystals, they are divided into visible light photoresistors (cadmium sulfide crystals), infrared light photoresistors (gallium arsenide crystals), and ultraviolet light photoresistors (zinc sulfide crystals). When light of a sensitive wavelength shines on the surface of a semiconductor photoelectric crystal, the number of charge carriers in the crystal increases, thereby increasing its conductivity (i.e., decreasing its resistance).
It is worth noting that the light characteristics (characteristics that change with light intensity), temperature coefficient (characteristics that change with temperature), and current-voltage characteristics are not linear. For example, the photoresistance of a CdS (cadmium sulfide) photoresistor sometimes increases with increasing temperature and sometimes decreases with increasing temperature.
2. Photodiode
Compared to ordinary diodes, photodiodes differ significantly except that their die is also a PN junction with unidirectional conductivity. Firstly, the PN junction depth within the die is shallower (less than 1 micrometer) to improve photoelectric conversion efficiency. Secondly, the PN junction area is larger, while the electrode area is smaller, allowing the photosensitive surface to collect more light. Thirdly, photodiodes have a sealed "window" made of plexiglass that focuses light onto the photosensitive surface. Therefore, the sensitivity and response time of photodiodes are far superior to those of photoresistors.
A phototransistor can be considered a combination of a photodiode and a transistor. Due to its amplification function, its dark current, photocurrent, and photosensitive sensitivity are much higher than those of a photodiode. However, its structure increases the junction capacitance, resulting in a deterioration in its response characteristics. It is widely used in low-frequency optoelectronic control circuits.
The common shapes and symbols of phototransistors are as follows:
Semiconductor optoelectronic devices also include MOS structures. For example, the CCD (charge-coupled device) commonly used in scanners and cameras is an integrated photodiode or MOS structure array.
III. Gas Sensors and Gas-Sensing Elements
Semiconductor gas sensors are classified into N-type and P-type. In N-type sensors, the resistance decreases with increasing gas concentration; in P-type sensors, the resistance increases with increasing gas concentration. SnO2 metal oxide semiconductor gas sensors, for example, are N-type semiconductors. At temperatures of 200-300°C, they adsorb oxygen from the air, forming oxygen ions. This reduces the electron density in the semiconductor, thus increasing its resistance. When exposed to a combustible gas (such as CO), the adsorbed oxygen desorbs, and the combustible gas is adsorbed as positive ions on the surface of the metal oxide semiconductor. The oxygen desorption releases electrons, and the adsorbed combustible gas also releases electrons, increasing the electron density in the conduction band of the oxide semiconductor and decreasing its resistance. Once the combustible gas is gone, the metal oxide semiconductor automatically resumes the adsorption of oxygen ions, restoring the resistance to its initial state. This is the basic principle behind semiconductor gas sensors for detecting combustible gases.
Currently, there are two types of domestically produced gas-sensitive elements. One type is directly heated, where the heating wire and measuring electrode are sintered together inside a metal oxide semiconductor die; the other type uses a ceramic tube as a substrate, with a heating wire running through the tube and two measuring electrodes on the outside of the tube. The space between the measuring electrodes is filled with a metal oxide gas-sensitive material, which is then sintered at high temperature.
IV. Force Sensors and Force-Sensing Elements
There are many types of force sensors. Traditional measurement methods use the deformation and displacement of elastic materials to represent force. With the development of microelectronics technology, force sensors with small size, light weight, and high sensitivity have been developed by utilizing the piezoresistive effect of semiconductor materials (that is, the resistivity changes when pressure is applied in a certain direction) and their good elasticity. These sensors are widely used for measuring physical and mechanical quantities such as pressure and acceleration.
V. Magnetic Sensors and Magnetic Elements
Currently, magnetic sensitive elements include Hall devices (based on the Hall effect), magnetoresistive devices (based on the magnetoresistive effect: an applied magnetic field causes the resistance of a semiconductor to increase with the increase of the magnetic field), magnetic diodes, and transistors. Magnetic sensors based on magnetic sensitive elements are widely used in the measurement of some electrical, magnetic, and mechanical quantities.
