Researchers at the University of Sheffield designed and developed an avalanche photodiode (APD) with significant potential for low photon detection. The highly sensitive gallium arsenide antimonide/aluminum gallium arsenide antimonide (GaAsSb/AlGaAsSb) separate absorption and multiplication avalanche photodiode (SAM-APD), which the researchers believe signifies a milestone in the development of infrared (IR) APDs, demonstrates very little added noise to interfere with signal recognition. The novel APD features a GaAsSb absorption region and an AlGaAsSb avalanche region. It features low tunneling current in addition to high usable avalanche gain and extremely low excess noise factors. APDs are widely used in optical receivers for high-speed, optical fiber-based communication and lidar applications, which require extremely sensitive photodiodes that are capable of detecting very low levels of light intensity — in some cases, detection down to a few photons or single-photon level. When operated in the Geiger mode, APDs can be used in single-photon detection, such as quantum key distribution and quantum imaging. The signal-to-noise ratio (SNR) in an APD-preamplifier module can be enhanced by the internal avalanche gain of APDs if the dominant noise source is the preamplifier. A significant increase in SNR, relative to pin photodiodes, can be achieved if the randomness in the impact ionization process is small. APDs typically have a higher SNR than pin photodiodes because APDs apply reverse voltage, which causes them to experience internal gain. To build the APD, the researchers combined a semiconductor alloy with a wider bandgap semiconductor. The semiconductor alloy is based on a GaAsSb absorption region that has excellent detection efficiency at IR wavelengths up to 1700 nm. Researcher Tarick Blain and team members designed an extremely low excess noise avalanche photodiode with a GaAsSb absorption region and an AlGaAsSb avalanche region. Core applications for the APD include lidar and optical communications. Courtesy of the University of Sheffield. The low-noise APD incorporates an appropriate doping profile to suppress tunneling current from the absorption region. It achieves an avalanche gain of about 130 at −49.6 V at room temperature, and it exhibits low excess noise factors of 1.52 and 2.48 at the gain of 10 and 20, respectively. At the gain of 20, the measured excess noise factor of 2.48 is more than 3× lower than a commercial indium gallium arsenide/indium phosphide (InGaAs/InP) SAM-APD. “One of the long-standing limitations of infrared APDs is a relatively high added noise from the multiplication process that limits the maximum multiplication factor,” said professor Chee Hing Tan. “This, in turn, prevented infrared APDs from reaching the performance limit predicted by established models. “Our breakthrough result, with an excess noise factor of 2.48, is approaching the theoretical lower limit of 2. This provides the pathway to realize extremely low-noise APD that I believe can generate step changes in optical communication and long-range lidar.” The research was published in Applied Physics Letters (www.doi.org/10.1063/5.0139495).