Draper reports that its Single Photon Detector (SPD) patent has been awarded by by the Boston Patent Law Association at its 10th Annual Invented Here! event.
Draper’s SPD is intended to be configured in an array of multiple SPDs capable of detecting a single photon with high timing resolution, speed and efficiency over an unparalleled wavelength range, from visible to infrared.
Draper’s SPD uses a silicon-germanium photodiode, which enables detection of the longer wavelengths of light and performance similar to a conventional silicon SPD. It can also function at room temperature. The performance of the new detector is intended to enable ultralow dark-counting rate and timing resolution of better than one nanosecond.
Steven Spector, principal member of Draper’s technical staff, developed the technology with a team at Draper. He said, “Draper’s SPD is so fast and efficient that it can absorb and detect a single particle of light and reset itself for the next one within a nanosecond.”
The patent US10636918 "Single electron transistor triggered by photovoltaic diode" presents the idea using a single electron bipolar avalanche transistor (SEBAT) circuit:
"Embodiments of the present invention are directed to a single photon detection circuit that includes a germanium photodiode configured with zero voltage bias to avoid dark current output when no photon input is present and also configured to respond to a single photon input by generating a photovoltaic output voltage and current. A single electron bipolar avalanche transistor (SEBAT) has a base emitter junction connected in parallel with the germanium photodiode and is configured so that the photovoltaic output voltage triggers an avalanche collector current output.
The SEBAT may be formed of silicon material. The germanium photodiode may be formed using germanium material used as deposited plus any dopings and anneals needed to turn it into a photodiode.
Unlike SPADs, which are triggered by single photons, SEBATs are triggered by single electrons. For single electron detection operation, the collector voltage Vcc is set to a high voltage above the breakdown of the collector-base junction. If a negative voltage is applied to the emitter E, the base-emitter junction is forward biased and an emitter current IE appears."
Very short patent with no evidence of reduction to practice. Electrons collected by the photodiode create a miniscule current that increases the voltage bias of the EB junction. This increase in voltage bias causes a concomitant increase in the forward bias current of the transistor, potentially triggering avalanche. Unless this is a deep nanoscale version of the transistor, the injected electron is lost in the sea of majority electrons in the emitter, and does not itself wander to the EB junction to be collected. Furthermore, there are significant time delays, including the dime delay for photon collection in the zero biased diode and charging time of the EB junction. I see no evidence that this could achieve timing performance "better than one nanosecond".
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