PORTLAND, Ore. New sensors based on gallium nitride could improve the detection of bioterror agents like anthrax.
Researchers at the Georgia Institute of Technology in Savannah said gallium nitride avalanche photodectors could provide real-time detection of bioterrorism agents like anthrax by inducing fluorescence in the biological molecules floating in the air at ultraviolet wavelengths. Since gallium nitride photodiodes do not respond to visible light, they enable noise-free avalanche photodetectors--unlike silicon photodiodes which require strong filters to reduce noise at visible wavelengths.
"With careful modeling of gallium nitride, we believe that it is possible to fabricate arrays [with] identical photodiodes on the same chip, which is what you will need for future bioterrorism-agent detectors," said Georgia Tech professor Douglas Yoder. Yoder specializes in Monte Carlo random-sampling simulations of gallium nitride photodiodes to insure design uniformity for avalanche-mode detectors.
Avalanche photodiode detectors work by applying a high reverse-bias voltage--so high that when photons create electron-hole pairs (by knocking electrons from the valence to the conduction band) their bias-induced high velocity causes an avalanche effect that dislodges up to 1 million more electron-hole pairs. As a result, the researchers were able to demonstrate gallium nitride photodetectors with gains as high as 100,000 at the ultraviolet wavelengths of from 280 to 360 nanometers.
Gallium nitride is a direct-bandgap semiconductor with a wurtzite crystal structure and a very wide bandgap of 3.4 electron volts. Gallium nitride is widely used for enabling high-power, high-temperature transistors with up to 2.5 times the power density of high voltage gallium arsenide devices. When fabricated in photodiodes, gallium nitride is insensitive to visible light, thus eliminating the noise that confounds ultraviolet wavelength detectors using silicon photodetectors.
The photodetectors were fabricated using metal-organic chemical vapor deposition of many layers with precisely controlled thickness and composition, resulting in photodiodes with UV optical gains of greater than 10,000.
The research was funded by the Defense Advanced Research Project Agency's Deep Ultraviolet Avalanche Photodetectors program.