# IoT-Enabled Gas Sensors: Technologies, Applications, and Opportunities
## 5 Sensing Technologies
### Metal Oxide Semiconductors  
Metal oxide semiconductor (MOS) gas sensors are broadly used in hospitals to grant the correct mixture of O\({}_{2}\) and nitrogen (N\({}_{2}\)) to patients as well as at the industry to avoid toxic gas leakages. These sensors are simple to fabricate, easy to reproduce, and are of low cost, compared to other technologies. The sensors can be fabricated using commercial complementary metal oxide semiconductor (CMOS) processes, which means the easy fabrication of numerous components, with low cost, and identical characteristics [120]. These sensors operate at high temperatures, the reaction with the target gas generates a variation on its internal resistance and the sensor responds with a proportional output tension, according to the concentration of the gas it is sensing; the sensors are normally designed within a heater. The transducers are sensitive to more than one gas and the temperature of operation is a key to determine the gas that will be sensed [28, 29, 38, 120, 121]. Figure 2: Schematics of an amperometric electrochemical gas sensor, where the reduction of the cathode to hydroxyl ions generates a variation of the electrical current, proportional to the concentration of the target gas. MOS sensors are usually constructed with a ceramic tube where the heater is placed in contact with the electrode; the sensing layer is made with the metal oxide semiconductor, placed in between the electrode and a heater; the sensor could also be printed in a ceramic wafer [22; 30; 118]. Figure 3 shows both MOS sensors, where (a) represents the tubular gas sensor and (b) the wafer gas sensor. These sensors can be easily miniaturized, including the use of nanomaterials to develop the sensing elements, which have demonstrated an increased performance on these transducers; the use of an external stimulus, as ultraviolet (UV) light, have shown better efficiency on measurements at room temperature [118; 119]. The most common materials used on these transducers are tin oxide (SnO\({}_{2}\)), zinc oxide (ZnO), tungsten trioxide (WO\({}_{3}\)) and titanium dioxide (TiO\({}_{2}\)). The transducers need to be heated to achieve the sensing point, and are consumed with the reaction with the gas it is sensing, making necessary a periodic calibration and limiting the lifetime of these devices. The heating process must be adequate to the target gas, as the sensor will respond to different gases if the temperature is not suitable for the gas of interest. A physical gas filter, to eliminate interference from other gases, can be added to the sensing element, upgrading the performance of the measurements [28; 38; 119; 120]. These sensors, in contact with the target gas and elevated temperature, have their internal resistance altered, which also vary the output voltage; the variation of the internal resistance is determined by the concentration of the target gas and the semiconductor that composes the transducer. The concentration of the target gas (R) is calculated as a relation between the internal resistance on the presence of the target gas (Rs) and the output resistance at the presence of a reference gas (Ro), as shown in Equation (1) [70]. \[\mathrm{R}=\mathrm{R}_{\mathrm{s}}/\mathrm{R}_{\mathrm{o}}, \tag{1}\]  
There are two types of MOS sensors, the n-type, where there are more electrons than protons, and the p-type, where the number of electrons is inferior, and the electricity flows through the positive holes. The n-type sensor have the internal resistance decreased by reducing gases, and increased by the oxidizing gases, whereas the p-type presents the opposite effect [28; 122; 123]. The use of UV light within the sensing element allows a lower sensing temperature, produces lower internal resistance, as electrons are inducted to pair, and the resistance of the sensing material will increase, proportionally to the increase of the target gas [29].