| Summary: | 碩士 === 國立臺灣海洋大學 === 電機工程學系 === 102 === The purpose of this thesis, a new metal-semiconductor-metal (MSM) hydrogen sensor was proposed to avoid (or to reduce) false alarms due to temperature drift when it is used in differential-pair hydrogen-sensing systems. A GaN semiconductor layer together with catalytic metal of Pt as sensing metal and non-catalytic metal of gold (Au) as Schottky metal was employed to structure an Au-GaN-Pt MSM sensor. In particular, the structured Au-GaN-Pt MSM sensor can function as an active sensor and a reference sensor depending on the polarity of applied voltage. Possible sensing mechanisms associated with the Au-GaN-Pt MSM sensor were described first to include band diagrams and graphical analysis. Experimental results reveal that an active sensor by forward-biasing the Au-GaN-Pt MSM sensor responses well to hydrogen-containing gases (50, 500, and 5000 ppm H2/N2) and produces high sensing current gains over 104 at various temperatures (25C, 50C, 70C, and 90C). Further, the Au-GaN-Pt MSM sensor can also be reverse-biased to act as a reference sensor which shows a bit responsive to hydrogen-containing gases. The use of differential-pair sensing circuit with the proposed Au-GaN-Pt MSM sensor reduces false alarms due to ambient temperature variation while it provides a short detection time.
Then, the zigzag-shaped pure-Pd thin film and Pd-SiO2 thin-film mixture as resistive-type hydrogen sensors were deposited on cover-glass substrates through a multiple-boat thermal evaporator. The resistance of the pure-Pd resistive-type sensor showed a relative sensitivity of 6.1% in the hydrogen gases concentrations 1% H2/N2. Most importantly, Above all, we measure a variety of thin-film thickness and relative sensitivity, and a variety of thin-film thickness and response time subjected to a variety hydrogen concentration at room temperature. We find that the thin-film thickness and relative sensitivity (SH) is the independent, because the hydrogen atoms are to fill all thin-film thickness no matter that is thicker or thinner. The response time (ta) is different in many thin-film thicknesses. Thin-film thickness and response time (ta) are in direct inverse proportion, because the hydrogen atoms must be to fill all thickness no matter that is thicker or thinner. Furthermore, phase transition from the phase of Pd to phase hydride not occurs at a high hydrogen concentration of 3%, because it is an irreversible process.
Sensing properties of the Pd-SiO2 resistive-type sensor responding to the represent of 1% H2/N2 are much better than those of the pure-Pd one, including a higher relative sensitivity (8.7% to 6.1%), a faster response time (20 s to 33 s), and a lower detection concentration limit (50 ppm to 100 ppm). A higher dissociation rate and a faster diffusion rate due to porous-like properties and more hydrogen atoms caught due to oxygen associated with the Pd-SiO2 thin-film mixture explain why the Pd-SiO2 resistive-type sensor has a higher relative sensitivity with a shorter response time. But SiO2 is doping too much. When the measurements hydrogen-containing gases back to air, atmospheric oxygen is very difficult to catch the hydrogen atoms in Pd-SiO2. It takes more time to come back baseline current.
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