2532 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 50, NO. 12, DECEMBER 2003 Investigation of Hydrogen-Sensing Properties of Pd/AlGaAs-Based Schottky Diodes Yan-Ying Tsai, Kun-Wei Lin, Chun-Tsen Lu, Huey-Ing Chen, Hung-Ming Chuang, Chun-Yuan Chen, Chin-Chuan Cheng, and Wen-Chau Liu, Senior Member, IEEE Abstract—The hydrogen response characteristics and sensing properties of catalytic Pd/Al Ga As metal–oxide–semicon- ductor (MOS) and metal–semiconductor (MS) Schottky diodes are systematically studied. The effects of hydrogen adsorption on device performances such as the current-voltage characteristics, sensitivity, barrier height variation, heat of adsorption, and transient response are investigated. The studied devices can be operated under very wide hydrogen concentration regimes with remarkable hydrogen-sensing properties. Particularly, at an extremely low hydrogen concentration of 15 ppm H /air, both steady-state and transient responses at room temperature can be detected. In addition, under the presence of oxide layer in the studied MOS device, a larger change of barrier height and higher hydrogen response are observed. In addition, according to the van’t Hoff equation, the initial values of heat adsorption for Pd/semiconductor and Pd/oxide interface are calculated as 7.29 and 49.6 KJ/mole, respectively. Index Terms—Barrier height, Fermi-level pinning, hydrogen sensors, Schottky diode. I. INTRODUCTION T HE CATALYTIC metal–oxide–semiconductor (MOS) structure was first proposed by Lundström et al. in 1975 [1]. Based on the MOS structure, a variety of semiconductor devices have been reported to sense different gases, such as carbon monoxide, ammonia and hydrogen, etc., [2]–[4]. Due to the significant potentiality for scaling down of device size, semiconductor-type sensors have attracted considerable atten- tion in many applications, e.g., industrial fabrication processes, medical installations, laboratories, and fueled motor vehicles [5], [6]. For the operation of gas sensor, the temperature-depen- dent characteristic is an important factor. For the operation in harsh environments, such as aircraft, automotive process, space technology, and other applications [7]–[13], large energy gap semiconductor materials are considered. For example, the Pd, Pt/oxide/SiC Schottky diodes were reported to present good hydrogen detection sensitivity at high-temperature regime [14], [15]. III–V compound semiconductor materials with higher Manuscript received May 9, 2003; revised August 29, 2003. This work was supported in part by the National Science Council of Taiwan, R.O.C., under Grant NSC 92-2218-E006-032. The review of this paper was arranged by Editor K. Najafi. Y.-Y. Tsai, C.-T. Lu, H.-M. Chuang, C.-Y. Chen, C.-C Cheng, and W.-C. Liu are with the Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, Tainan, Taiwan 70101, R.O.C. (e-mail: wcliu@mail.ncku.edu.tw). K.-W. Lin is with the Department of Electrical Engineering, Chien Kuo In- stitute of Technology, Changhua, Taiwan, R.O.C. H.-I. Chen is with the Department of Chemical Engineer, National Cheng-Kung University, Tainan, Taiwan 70101, R.O.C. Digital Object Identifier 10.1109/TED.2003.819656 Fig. 1. AES depth profiles of the studied Pd/AlGaAs MS Schottky diode. energy bandgaps are also good candidates for high-temperature applications [16]–[18]. In addition, for MOS Schottky diode structures, the introduction of an adsorptive oxide layer between the catalytic metal, e.g., Pd, and semiconductor could increase the hydrogen detection sensitivity [19]. In this work, we report the fabrication and characterization of Pd/oxide/Al Ga As MOS Schottky diode for hydrogen detection. For comparison, a Pd/ Al Ga As MS Schottky diode is also fabricated and investigated in this work. There are many advantages by using AlGaAs material, such as large bandgap, high reactivity with oxygen, and lattice matched to GaAs [20]. Hence, high hydrogen detection sensitivity over wide temperature operation regime can be expected. Because the AlGaAs/GaAs material system has been widely employed to fabricate high-speed microwave and opto-electronic devices [21], therefore, the studied device has a good potential to be integrated with the AlGaAs-based devices to form a high-speed and high hydrogen detection system. II. EXPERIMENTAL The studied Pd/oxide/Al Ga As Schottky diodes were grown by metal–organic chemical vapor deposi- tion (MOCVD) on a (100) oriented semi-insulated (S.I.) GaAs substrate. The epitaxial structure consisted of a 5000- -thick GaAs undoped buffer layer, a 3000- -thick 0018-9383/03$17.00 © 2003 IEEE