To improve the sensitivity at ultraviolet wavelengths, the researchers designed the gallium nitride photodiodes to operate in a mode that employs avalanche multiplication. The avalanche multiplication phenomenon is used to multiply normally tiny currents by factors of up to one million, thus dramatically increasing the device gain.
Avalanche photodiodes can create much larger currents for each photon compared to normal photodiodes. Once the necessary electric field strength has been achieved inside the device, the avalanche effect starts with just one free electron. Since the illuminated photodiode will contain many free electrons, an avalanche will always occur if the electric field is large enough.
"One electron-hole pair that is produced by a photon absorption event creates a million other electron-hole pairs and the current becomes a pulse of current that you can detect with special electronics," added Dupuis.
The researchers fabricated high-performance gallium nitride ultraviolet avalanche photodiodes on bulk gallium nitride substrates that demonstrate optical gains of 100,000 at ultraviolet wavelengths from 280 to 360 nanometers.
The gallium nitride device structures were grown by metalorganic chemical vapor deposition, a technique for depositing thin layers of atoms onto a semiconductor wafer. Many layers can be built up, each of a precisely controlled thickness and composition, to create a material which has specific optical and electrical properties. This is the first time gallium nitride was successfully used in the fabrication of photodiodes having ultraviolet optical gains greater than 10,000.
Since demonstrating the feasibility of the photodiodes to exhibit the avalanche effect, the research team has been developin
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Georgia Institute of Technology Research News