# A Review of Hydrogen Sensors for ECLSS: Fundamentals, Recent Advances, and Challenges
## 4 Semiconductor Hydrogen Sensors
### Metal Oxide Semiconductor Hydrogen-Sensitive Materials and Sensors  
The metal oxide-type hydrogen sensor includes a metal oxide layer with semiconductor characteristics that is deposited on a heater, thereby raising the temperature of the layer to an operating temperature (500 \({}^{\circ}\)C). The principle of operation is that when oxygen from the environment is adsorbed in the metal oxide layer, which has a high resistivity when hydrogen diffuses into the sensing layer and reacts with oxygen, it is adsorbed on the surface of the semiconductor metal oxide and the resistivity of the adsorbed layer decreases, while the value of the decrease increases with the increase in the concentration of hydrogen (see Figure 5). The semiconductor metal oxide-type hydrogen sensor has the advantages of simple structure, cheap price, high sensitivity, fast response, and easy compounding; thus, it is conducive to mass production [47].  
Most metal oxide semiconductor-type hydrogen sensors are highly sensitive, with average response times being between 4 and 20 s. However, the poor response speed of a single metal oxide makes it difficult to meet practical needs. Moreover, the single metal oxide has poor selectivity to hydrogen, lacks sensitivity to hydrogen, and is highly susceptible to interference from other reducing gases, such as CO, CH4, alcohol, etc. To improve their selectivity, this problem can be solved via doping with noble metals with good selectivity for hydrogen, such as Pt, Pd, Au, etc. [PERSON] et al. [48] presented a method for hydrogen detection at room temperature using ZnO nanorods doped with a Pd catalyst. Pd nanoparticles with a size of about 10 nm were used as catalysts and deposited into the ZnO nanorod array network via RF magnetron sputtering to detect hydrogen concentration in the range of 0.2 to 1000 ppm at room temperature with fast response and stable performance. [PERSON] et al. [49] reviewed the research work on improving the performance of resistive hydrogen sensors using Pd. However, precious metals are costly and sensitive to chemicals such as sulfur-containing substances and CO. To overcome these drawbacks, it was shown that transition metal dopants, such as zinc, manganese, cobalt, and copper, have significant effects in improving metal oxide sensors. The selectivity, optimum operating temperature, and response/recovery time of the sensors were improved [50]. In addition, doping with rare earth elements can also significantly enhance the sensor response [51]. In addition to metal oxides, semiconductor materials include sulfides, nitrides, etc. [PERSON] et al. [52] presented a highly sensitive hydrogen sensor that used  
Figure 5: Schematic diagram of metal oxide semiconductor-type hydrogen sensor structure.  
MoS\({}_{2}\)-Pt nanoparticle films as the active layer. For 100 ppm H\({}_{2}\), the sensor achieved ultrafast response and recovery rates of 4 s and 19 s, respectively. When exposed to 100 ppm H\({}_{2}\), the MoS\({}_{2}\)-Pt composite film exhibited a high sensor response, which was superior to existing metal sulfide-based sensors. [PERSON] et al. [53] studied a GaN hydrogen sensor. The GaN hydrogen sensor exhibited high stability and sensitivity under a high-temperature environment. [PERSON] et al. [54] studied a wide band gap aluminum nitride (AlN) sensor material, which can withstand high-temperature environments. Three unique AlN morphologies (rod, nested, and hexagonal plate) were synthesized via direct nitridation at 1400 \({}^{\circ}\)C using \(\gamma\)-AlOOH as a precursor. The gas-sensing performance showed that the hexagonal plate form exhibited good reproducibility and the highest response to 750 ppm H\({}_{2}\) leakage at high temperature (500 \({}^{\circ}\)C) compared to the rod and nested forms.