Travis Anderson1, Hung-Ta Wang1, Byoung Sam Kang1, Changzhi Li1, Zhen-Ning Low1, Jenshan Lin1, Steve Pearton1, John Painter2, Canan Balaban1, A. Osinsky2, Amir Dabiran2, P. Chow2, and Fan Ren1. (1) University of Florida, (2) J. Painter Consulting, LLC
We have demonstrated a wireless hydrogen sensing system using commercially available wireless components and Schottky diodes fabricated on an AlGaN/GaN High Electron Mobility Transistor (HEMT) structure with platinum as the sensing metal. The wide bandgap nature of GaN means the devices can be operated in high temperature, high power, high radiation or harsh environments. The platinum metal creates a catalytic surface for cracking molecular hydrogen. The atomic hydrogen then diffuses to the metal-semiconductor interface and alters the device characteristics through its effect on surface charge, changing the measured current level of the device. The HEMT structure in particular shows increased sensitivity since it operates with an amplification effect. This is fundamentally different from conventional sensing by detecting a conductivity change in the materials. Our sensors have demonstrated a large range of detection, from 100% to 10 ppm hydrogen concentrations, with the advantages of a response time within a couple of seconds and rapid recovery. The magnitude of the current change can be related to the hydrogen concentration, which is a requirement for a marketable sensor. A field test of the sensors is underway at Greenway Ford in Orlando, FL. This is the host service station for a hydrogen powered bus fleet operating around Orlando International Airport. The field test has been underway for 15 months and has allowed for extensive reliability testing. We initially noticed periodic current changes in the sensor that were attributed to temperature effects. We have redesigned the diodes to include an encapsulated reference diode. The sensor pair detects a change in current between the pair of diodes rather than measuring absolute current and setting an alarm threshold. With this design the effect of ambient temperature can be eliminated. We also found that the sensors degraded during the first few months of the field test, and have altered our design to employ TiB2-based ohmic contacts. These have established significant improvements in stability of the metal contacts. These two improvements have produced a very stable and reliable sensor package.
Our wireless network sensing system enables monitoring of multiple independent, battery powered, sensor nodes. This is especially useful in manufacturing plants and hydrogen-fuelled automobile dealerships, where a number of sensors, possibly with each detecting different chemicals, would be required. The particular transmission protocol is designed to handle a city-wide deployment level (up to 100,000 nodes). The sensor data is transmitted wirelessly to a central computer. The computer processes the sensor data and performs pre-programmed emergency actions, such as triggering alarms, shutting off valves, and notifying relevant personnel via email, text messages, or phone calls. The base station also acts as a web server, so the data can be monitored remotely through the internet. The entire package can be built for less than $50 making it very competitive in the emerging market.
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