Background:
The gas sensor is the heart of the gas detection system. Usually installed in the probe head. Essentially, a gas sensor is a transducer that converts a certain gas volume fraction into a corresponding electrical signal. The probe conditioned the gas sample through the gas sensor. Usually includes filtration of impurities and interfering gases, drying or refrigeration, and sample aspiration. Samples are even chemically treated for faster measurements by chemical sensors.
There are many types of gases with different properties, so there are many types of gas sensors. According to the nature of the gas to be detected, it can be divided into: sensors for detecting flammable and explosive gases, such as hydrogen, carbon monoxide, gas, gasoline volatile gas, etc. Sensors for the detection of toxic gases such as chlorine, hydrogen sulfide, arsine, etc. Sensors for the detection of industrial process gases, such as oxygen in steelmaking furnaces, carbon dioxide in heat treatment furnaces. Sensors used to detect air pollution, such as NOx, CH4, O3 that form acid rain, household pollution such as formaldehyde, etc.
Semiconductor gas sensor：
Semiconductor gas sensors can be divided into resistive type and non-resistive type (junction type, MOSFET type, capacitive type). The principle of resistive gas sensor is that gas molecules cause changes in the resistance of sensitive materials. Non-resistive gas sensors mainly include M()s diodes, junction diodes and field effect transistors (M()SFETs). It exploits the principle that sensitive gas will change the turn-on voltage of a MOSFET. Its principle structure is the same as that of the ISFET ion-sensitive sensor.
The semiconductor gas sensor is made by using the redox reaction of gas on the surface of the semiconductor to change the tissue of the sensitive element. when the semiconductor device is heated to a steady state. When the gas contacts the surface of the semiconductor and is adsorbed, the adsorbed molecules first diffuse freely on the surface of the object. Loss of motion energy, part of the molecules are evaporated, and the other part of the residual molecules are thermally decomposed and adsorbed on the surface of the object. When the work function of the semiconductor is smaller than the affinity of the adsorbed molecule, the adsorbed molecule will take away electrons from the device and become negative ion adsorption, and the surface of the semiconductor presents a charge layer.
Junction gas sensor：
The junction gas sensing device is also called a gas sensing diode. This type of gas sensing device uses gas to change the rectification characteristics of the diode to work. Its principle is: the noble metal Pd is selective to hydrogen, and it contacts with the semiconductor to form a contact barrier. When the diode is forward biased, the flow of electrons from the semiconductor to the metal will increase. So the forward direction is conducting. When a negative bias is applied, the carriers are essentially unchanged. This is the rectifying characteristic of a Schottky diode. In the detection atmosphere, due to the adsorption of hydrogen. The work function of the precious metal changes and the contact barrier is weakened. As a result, the carrier increases, the forward current increases, and the rectification characteristic curve of the diode will shift to the left.
MOSFET type gas sensor：
The left shift of the characteristic curve of the gas-sensing diode can be regarded as a change in the conduction voltage of the diode. If this characteristic occurs at the gate of the FET, it will change the threshold voltage UT of the FET. Using this principle, a MOSFET gas sensor can be made. Hydrogen-sensitive MOSFET is one of the most typical gas-sensing devices, which uses metal palladium (Pd) to make a palladium gate. in an atmosphere containing hydrogen. Due to the catalysis of palladium, hydrogen molecules are decomposed into hydrogen atoms that diffuse to the interface of palladium and silica. Eventually, the threshold voltage UT of the MOSFET changes. The gate-drain is often short-circuited to ensure that the MOSFET works in the saturation region. At this time, the drain current ID = β (UGS-UT) 2, and the hydrogen concentration can be measured using this circuit.
In practical applications, there is a characteristic that UT drifts with time. To this end, a layer of SiO2 insulating layer is grown in HCl atmosphere, which can significantly improve the drift of UT. Gases other than hydrogen cannot pass through the palladium grid. Certain measures should be taken to make Pd-MOSFET gas sensors of other gases. For example, when making a CO-sensitive MOSFET, a small hole of about 20 nm should be made on the palladium gate to allow CO gas to pass through. In addition, due to the high sensitivity of Pd-MOSFET to hydrogen, the sensitivity to CO is low. To this end, a layer of aluminum with a thickness of about 20 nm can be evaporated on the palladium gate as a protective layer to prevent the passage of hydrogen.
Solid Electrolyte Gas Sensor：
A solid electrolyte is a solid substance that has the same ionic conductive properties as an aqueous electrolyte. When used as a gas sensor, it is a battery. It does not need to dissolve the gas in the electrolyte through a breathable membrane, which can avoid evaporation and electrode consumption and other problems. This sensor has high conductivity, sensitivity and selectivity. Almost in the petrochemical, environmental protection, mining, food and other fields have been widely used. Its importance is only secondary to metal - oxide - semiconductor gas sensors.
Portable sensor with smooth precision：
The multi gas detector supports the detection of combustible gases, toxic and harmful gases. The multi gas detector can monitor 4 toxic combustible gases at the same time, and supports free customization of gas detection parameters 4 and 1, 2 and 1, 3 and 1.Multi gas detector, waterproof, dustproof, accurate detection data, sound and light alarm, long service life. The Multi gas detector can be used in gas leakage, pharmaceutical factories and other scenes.
