Wireless hydrogen sensor network using AlGaN/GaN high electron mobility transistor differential diode sensors

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Abstract

We have demonstrated a wireless hydrogen sensing system using commercially available wireless components and AlGaN/GaN high electron mobility transistor (HEMTs) differential sensing diodes as the sensing devices. The active device in the differential pair is coated with 10 nm of Pt to enhance catalytic dissociation of molecular hydrogen, while the reference diode is coated with Ti/Au. Our sensors have a wide range of detection from ppm levels to ∼30%, with the added advantages of a very rapid response time within a couple of seconds, and rapid recovery. The sensors have shown good stability for more than 18 months in an outdoor field test. Currently, the wireless sensing system consists of six wireless sensor nodes and a base station. The wireless sensor node consists of a sensor, a power management system with back-up batteries in case of power outages and a wireless transceiver. The base station consists of a high sensitivity receiver and an in-house developed intelligent monitoring software that does basic data logging and tracking of each individual sensor. The software defines and implements the monitoring states, transitions, and actions of the hydrogen sensor network. Also, the software is able to warn the user of potential sensor failure, power outages and network failures through cell phone network and Internet. Real-time responses of the sensors are displayed through a web site on the Internet. (http://ren.che.ufl.edu/app/default.aspx).

Introduction

There is great interest in detection of hydrogen sensors for use in hydrogen-fuelled automobiles and with proton-exchange membrane (PEM) and solid oxide fuel cells for space craft and other long-term sensing applications. These sensors are required to detect hydrogen near room temperature with minimal power consumption and weight and with a low rate of false alarms. Due to their low intrinsic carrier concentrations, GaN- and SiC-based wide band gap semiconductor sensors can be operated at lower current levels than conventional Si-based devices and offer the capability of detection to ∼600 °C [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. The ability of electronic devices fabricated in these materials to function in high temperature, high power and high flux/energy radiation conditions enable performance enhancements in a wide variety of spacecraft, satellite, homeland defense, mining, automobile, nuclear power, and radar applications.

AlGaN/GaN high electron mobility transistors (HEMTs) show promising performance for use in broadband power amplifiers in base station applications due to the high sheet carrier concentration, electron mobility in the two-dimensional electron gas (2DEG) channel and high saturation velocity. The high electron sheet carrier concentration of nitride HEMTs is induced by piezoelectric polarization of the strained AlGaN layer and spontaneous polarization is very large in wurtzite III-nitrides. This provides an increased sensitivity relative to simple Schottky diodes fabricated on GaN layers [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. An additional attractive attribute of AlGaN/GaN diodes is the fact that gas sensors based on this material could be integrated with high-temperature electronic devices on the same chip. The advantages of GaN over SiC for sensing include the presence of the polarization-induced charge, the availability of a heterostructure and more rapid pace of device technology development for GaN which borrows from the commercialized light-emitting diode and laser diode businesses.

In this paper, we report on a demonstration of hydrogen sensing system using differential pair of AlGaN/GaN HEMT diodes for hydrogen sensing near room temperature. This configuration provides a built-in control diode to reduce false alarms due to temperature swings or voltage transients. The design and optimization of the detection circuitry, digital signal processing, wireless network, and monitoring states to maintain an accurate and reliable system were investigated.

Section snippets

Experiments

AlGaN/GaN HEMT layer structures were grown on C-plane Al2O3 substrates by a molecular beam epitaxy (MBE) system. The layer structure included an initial 2 μm thick undoped GaN buffer followed by a 35 nm thick unintentionally doped Al0.28Ga0.72N layer. The sheet carrier concentration was ∼1 × 1013 cm−2 with a mobility of 980 cm2/(V s) at room temperature. We designed a mask that employed a differential diode configuration, with a Pt-contact device as the active member of the pair and a Ti/Au contact

System overview

The wireless sensing system consists of six wireless sensor nodes and a base station including a wireless receiver and a computer equipped with monitoring software. Each sensor node consists of a differential sensor pair, detection circuits, microcontroller, wireless transceiver, and power management circuits. The main part of the detection circuits is an instrumentation amplifier used to sense the change of current in the device. The current variation, embodied as a change in the output

Field test

Field tests have been conducted both at University of Florida and at Greenway Ford in Orlando, FL. The outdoor tests at University of Florida have been conducted several times for a period of 2 weeks, to test a range of possible real world conditions in a more controlled setting. Hydrogen leakage was successfully detected for hydrogen concentrations in a range from 1% to 100% at the point of the leak and heights ranging from 1 to 10 ft in an outdoor environment. The setup at Greenway Ford was

Conclusion

In conclusion, a wireless sensor network which uses the IEEE 802.15.4 standards has been constructed to transmit data from a number of hydrogen sensors to a base station. A user-friendly program has been developed to share the data collected by base station to Internet, so that the data can be analyzed and monitored from anywhere with an Internet connection. A cell phone alarm has been implemented to report any potential hydrogen leakage to responsible personnel. The entire system has been

Acknowledgements

The work at UF is partially supported by ONR (N00014-98-1-02-04, H. B. Dietrich), NSF (CTS-0301178, monitored by Dr. M. Burka and Dr. D. Senich), by NASA Glenn Research Center under grant NAG 3-2930 monitored by Timothy Smith and also by Florida Department of Environmental Protection, U.S. Dept. of Energy (DE-FG26-05R410962 by Jill Stoyshich). The authors would like to thank the management and technical team in the Greenway Ford, Orlando, Florida for their technical support.

Xiaogang Yu received the B.S. degree in Physics from Nanjing University, Nanjing, China, in 2004, the M.S. degree in electrical and computer engineering from the University of Florida, Gainesville, in 2007, and is currently working toward the Ph.D. degree in electrical and computer engineering at the University of Florida. His research interests include wireless sensor, biomedical applications of microwave/RF systems, and microwave/millimeter-wave circuits.

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