High-Sensitivity Metal–Semiconductor–Metal Hydrogen Sensors With a Mixture of Pd and $\hbox{SiO}_{2}$ Forming Three-Dimensional Dipoles

New metal-semiconductor-metal hydrogen sensors are fabricated to take advantages of symmetrically bidirectional detection. Unlike commonly used single catalytic metal layers, a mixture of Pd and SiO
₂
inserted between Pd and GaN was employed as sensing media. There are three sensing regions (i.e., 2-D dipole, transient, and 3-D dipole regions) observed in static response. Room-temperature sensitivity larger than 10
⁷
was obtained in 1080-ppm H
₂
/N
₂
ambient. The barrier-height variation is as high as 422 mV. To our best knowledge, these are the highest values ever reported. According to transient response, a short response time of 70 s is obtained at room temperature. Thus, a newly developed concept of forming 3-D dipoles is introduced to possibly explain experimental results.