# A TSV-Structured Room Temperature p-Type TiO\({}_{2}\) Nitric Oxide Gas Sensor
## 3 Results
The minority carriers in p-type MOS are electrons (e\({}^{-}\)) and the majority carriers are holes (h\({}^{+}\)), thus when O\({}_{2}\) captures an electron, the number of hole carriers increases since the electron-hole (e\({}^{-}\)-h\({}^{+}\)) pair recombination is inhibited and the resistance of the sensor is decreased. When a reducing gas (e.g., SO\({}_{2}\), H\({}_{2}\)S, etc.) is injected, it reacts with oxygen ions, thus the trapped electrons return to the material and there is an increase in e\({}^{-}\)-h\({}^{+}\) pair recombination and the resistance of the p-type MOS increases. With this sensing mechanism, ((\(\rm{R_{air}-R_{gas}}\))/\(\rm{R_{air}}\)) \(\times\) 100 [34] and ((\(\rm{R_{gas}-R_{air}}\))/\(\rm{R_{air}}\)) \(\times\) 100 [35], respectively define the response of the sensor to oxidizing gas and reducing gas, where R\({}_{air}\) is the sensor's resistance and R\({}_{gas}\) is the sensor's resistance in air containing an oxidizing or reducing gas. Using this definition, Figure 7 displays the sensor response of the TSV-structured TiO\({}_{2}\) sensor. During the measurement, the NO concentration is increased from 0.5 to 8 ppm and the sensor operates at room temperature (RT). The respective sensor response for the TSV-structured TiO\({}_{2}\) gas sensor is 6.4%, 8.2%, 10.9%, 16.7%, and 21.3% for NO concentrations of 0.5, 1, 2, 4, and 8 ppm. In other words, the response of the sensor is increased with the increasing NO concentration. The stability and reproducibility of the TSV-structured TiO\({}_{2}\) NO gas sensor was measured over five cycles. Each cycle involves 5 min of NO gas injection and 5 min of pumping, as shown in Figure 8. During the measurement, the NO concentration is 4 ppm and the operating temperature is RT. The sensor's resistance rapidly decreases when NO gas is injected. The sensor's resistance increases when the test chamber is pumped. Of note, at this time, the ambient gas is also introduced. This dynamic behavior is consistent with the sensing mechanism for a p-type MOS that is previously described. Using the same definition for sensor response, the average sensor response is 16.5%, with an inaccuracy of \(<\)\(\pm\)0.5%. In other words, the sensor is stable and reversible at RT. Moreover, it was found that the average response and recovery time of the TSV-structured TiO\({}_{2}\) NO gas sensor are  
Figure 7: The sensor response for various concentrations. about 109 and 144 s. The response and recovery time are defined as the time required for a 90% change in the signal. Furthermore, this result indicated that the TSV-structured TiO\({}_{2}\) NO gas sensor had a better response and recovery time than the previous studies [36; 37; 38; 39; 40], as shown in Table 1.  
A MOS gas sensor must sense a specific gas selectively. Figure 9 shows the measurements for a TSV-structured RT TiO\({}_{2}\) gas sensor. During this experiment, CO\({}_{2}\), SO\({}_{2}\), H\({}_{2}\)S, CO, and NO were injected at a flow rate of about 10 ppm. The sensor's response to NO is 24.4%. The sensor is only very slightly responsive to gases, such as CO, H\({}_{2}\)S, SO\({}_{2}\), and CO\({}_{2}\). The TiO\({}_{2}\) gas sensor with TSV structure exhibits good selectivity to NO gas. This should be attributed to the lower activation energy of TiO\({}_{2}\) film for NO gas. Furthermore, since the sensor operates at room temperature, the TiO\({}_{2}\) surface is difficult to adsorb ionized oxygen species. As a result, the response of the sensor is poor when reducing gases, such as CO\({}_{2}\), CO, H\({}_{2}\)S, and SO\({}_{2}\), are introduced [41].