AlGaN/GaN High Electron Mobility Transistors(HEMTs) Based Sensors
 
HEMT pH Sensors
The use of Sc2O3 gate dielectric produced superior results to either a native oxide or UV ozone-induced oxide in the gate region. The ungated HEMTs with Sc2O3 in the gate region exhibited a linear change in current between pH 3-10 of 37µA/pH .The HEMT pH sensors show stable operation with a resolution of < 0.1 pH over the entire pH range. The results indicate that the HEMTs may have application in monitoring pH solution changes between 7 and 8, the range of interest for testing human blood.
Figure Caption: (Left) A cross sectional view of the AlGaN/GaN HEMT Sensor. (right) Drain current as a function of the pH value and measuring time.
 
Hg Detection
The mercury(II) (Hg2+) ion is one of the environmentally most important cations whose toxicity has long been recognized as a chronic environmental problem. Mercury is released into the environment through a variety of courses including the combustion of fossil fuels, mining, volcanic emissions and solid waste incineration. Certain bacteria convert inorganic mercury Hg+2 into neuro-toxic organic-mercury compounds, which bio-accumulate through plants, animals and the human food chain and can affect the entire eco-system.
Figure Caption: A cross sectional view of the AlGaN/GaN HEMT Hg Sensor.
It is desirable to develop sensitive, selective analytical methods for the quantitative detection of Hg2+, applicable in a wide range of different environments. We have functionalized Au-gated AlGaN/GaN high electron mobility transistors (HEMTs) surface with specific chemicals , which showed detection time of less than 5 seconds. This is the shortest response time ever reported for mercury detection. The sensors were able to detect mercury (II) ion concentrations as low as 10-7 M. The sensors showed an excellent sensing selectivity of more than 100 for detecting mercury ions over sodium or magnesium ions.
Figure Caption: Drain IV of AlGaN/GaN HEMT as a function of concenration of heavy ion solution.
 
Hydrogen Sensing
We have demonstrated a wireless hydrogen sensing system using commercially available wireless components and GaN Schottky diodes or AlGaN/GaN High Electron Mobility Transistors (HEMTs) as the sensing devices. Our sensors have achieved ppm level detection, 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 10 months in an outdoor field test. Currently, the wireless sensing system consists of 6 wireless sensor nodes and a base station. Using a Zigbee compliant wireless network, we can easily scale up to city wide coverage as well as 0.75 million independent sensor nodes within a single zone. The wireless sensor node consists of the sensor, power management system to switch to back up batteries in case of power outages and a Zigbee compliant 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, it is able to warn the user of potential sensor failure, power outages and network failures. This is especially useful in facilities for hydrogen storage, hydrogen-fuelled automobile dealerships and future home garages with hydrogen vehicles, and manufacturing plants, where a number of sensors, possibly with each detecting different chemicals, would be required. The system can be implemented to act as a real time warning system to the 911 centers so that the emergency services are able to act immediately to contain any potential threats. We have also developed an energy-efficient transmission protocol to reduce the power consumption and enable very long lifetime operation using batteries. Experimental results showed that a 150 meter transmission distance can be achieved with 10 mW total power consumption. The entire sensor package can be built for less than $30 at a 1 thousand quantity, making it extremely competitive in today's market.
Figure Caption: (Left) AlGaN/GaN diode based Hydrogen sensorintegrated with a wireless transmitter. (center) Hydrogen sening system including a transmitter and a receiver. (right) A wireless receiver connected with a laptop.

 
DNA Sensing
Au-gated AlGaN/GaN HEMTs functionalized in the gate region with label free 3'-thiol modified oligonucleotides, which serves as a binding layer to the AlGaN surface can detect the hybridization of matched target DNAs. XPS shows that immobilization of thiol modified DNA covalently bonded with gold on the gated region. Hybridization between probe DNA and matched or mismatched target DNA on the Au-gated HEMT was detected. The HEMT drain-source current showed a clear decrease of 115 µA as this matched target DNA was introduced to the probe DNA on the surface, showing the promise of the DNA sequence detection for biological sensing.
Figure Caption: High resolution Ga 3s and S 2p3 XPS peaks before and after thiol-modification of Au coated GaN samples
 
Protein Sensing
Ungated AlGaN/GaN High Electron Mobility Transistor structures were functionalized in the gate region with aminopropyl silane, which served as a binding layer to the AlGaN surface for attachment of biotin. Biotin has very high affinity to streptavidin proteins. Each time the chemicals attached to the AlGaN/GaN HEMT the charges on the attached chemicals affected the HEMT drain-source current. The HEMT showed a clear decrease of 4 µA as this protein was introduced to the surface, showing the promise of this all-electronic detection approach for biological sensing.
Figure Caption: A cross sectional view of the AlGaN/GaN HEMT protein Sensor.