Investigation of High-Performance Compound Semiconductor Based Hydrogen Sensors

• Main Authors: Ching-Wen Hung, 洪慶文
• Other Authors: Wen-Chau Liu
• Format: Others
• Language: en_US
• Online Access: http://ndltd.ncl.edu.tw/handle/78588104375859508057

博士 === 國立成功大學 === 微電子工程研究所碩博士班 === 96 === In this dissertation, a series of high-performance compound semiconductor based hydrogen sensors, including Schottky diodes, field-effect resistors, and heterostructure field-effect transistors, are fabricated and studied. Pd and Pt are used as sensing metals due to their excellent catalytic activity towards hydrogen gas. GaAs, AlGaAs, and InAlAs materials are served as sensing platforms because of their larger band gap than that of Si-based materials. We present the related electric characteristics, detection performance, and dynamic behaviors of these sensors measured under different hydrogen concentrations at different temperatures. First, the temperature-dependent hydrogen-detection characteristics of a Pt/InAlAs Schottky diode-type sensor are studied and demonstrated. The studied device exhibits significant sensing performance, including high relative sensitivity ratio, large current variation, widespread reverse voltage regime, and fast transient response time. A comparative study between forward and reverse biases is presented. A simple detection model is proposed to elucidate the hydrogen sensing behavior under forward and reverse biases. Thermionic emission (TE) and field emission (FE) exhibit considerable influences on the hydrogen sensing properties. Second, a three-terminal-controlled field-effect resistive hydrogen sensor, based on the current-voltage characteristics in the linear region of a Pd/oxide/AlGaAs pseudomorphic high electron mobility transistor, is fabricated and studied. The dissociation of H2, diffusion of H atoms and formation of a dipolar layer cause a significant decrease in channel resistance. In comparison with other resistor-type hydrogen sensors, the studied device demonstrates the considerable advantages of lower detection limit and higher sensitivity at room temperature. In addition, the influences of gate-source bias (VGS) on the hydrogen sensing properties are presented. Third, field-effect transistors based on GaAs/AlGaAs heterostructures are fabricated with catalytically active palladium (Pd) gate electrodes to induce the sensitivity of hydrogen gas. For the studied device, a 5 nm-thick undoped GaAs cap layer is grown to suppress the oxidation of the underneath Al0.24Ga0.76As layer. Comprehensive analysis on the electrical properties including equilibrium adsorption and kinetic adsorption is presented. The negative reaction enthalpy and entropy indicate that hydrogen adsorption process is exothermic and hydrogen atoms are more ordered when they are adsorbed at the Pd/GaAs interface. Hydrogen sensing characteristics in air and nitrogen environments are comparatively studied to investigate the influence of oxygen effect on the studied device. Finally, hydrogen-induced switching behaviors of a Pd/GaAs-based field-effect transistor are studied. A drastic change of hydrogen detection sensitivity is observed in the cut-off region. To further understand the stability and reliability of the Pd/GaAs transistor-type hydrogen sensor, the accelerated lifetime-test sensing behaviors toward high hydrogen concentration at high temperature, including two- and three-terminal electrical characteristics and dynamic responses, are presented. Based on the experimental results, these studied devices provide the promise for smart hydrogen sensors and micro-electro-mechanical system (MEMS) applications.