MDPI paper "Toward the Super Temporal Resolution Image Sensor with a Germanium Photodiode for Visible Light" by Nguyen Hoai Ngo, Anh Quang Nguyen, Fabian M. Bufler, Yoshinari Kamakura, Hideki Mutoh, Takayoshi Shimura, Takuji Hosoi, Heiji Watanabe, Philippe Matagne, Kazuhiro Shimonomura, Kohsei Takehara, Edoardo Charbon, and Takeharu Goji Etoh from Ritsumeikan University, Hanoi University of Science and Technology, IMEC, Osaka Institute of Technology, Osaka University, Link Research Corporation, Kindai University, and EPFL presents Ge PD to overcome the Si speed limit:
"The theoretical temporal resolution limit tT of a silicon photodiode (Si PD) is 11.1 ps. We call “super temporal resolution” the temporal resolution that is shorter than that limit. To achieve this resolution, Germanium is selected as a candidate material for the photodiode (Ge PD) for visible light since the absorption coefficient of Ge for the wavelength is several tens of times higher than that of Si, allowing a very thin PD. On the other hand, the saturation drift velocity of electrons in Ge is about 2/3 of that in Si. The ratio suggests an ultra-short propagation time of electrons in the Ge PD. However, the diffusion coefficient of electrons in Ge is four times higher than that of Si. Therefore, Monte Carlo simulations were applied to analyze the temporal resolution of the Ge PD. The estimated theoretical temporal resolution limit is 0.26 ps, while the practical limit is 1.41 ps. To achieve a super temporal resolution better than 11.1 ps, the driver circuit must operate at least 100 GHz. It is thus proposed to develop, at first, a short-wavelength infrared (SWIR) ultra-high-speed image sensor with a thicker and wider Ge PD, and then gradually decrease the size along with the progress of the driver circuits."
These would potentially be useful not only in communications and instrumentation but also in more practical developments of the single-pixel camera using linear arrays of pixels.
ReplyDeleteThere's even a pretty proven path to making optically efficient hyperspectral (~1k line spectrum/ sub-nm spectral resolution) cameras with high temporal resolution using variations on this compressed-sensing approach, though they'll be tens of k$ and only useful to the sorts of applications that are willing to pay that. (still patentable, I think, if anyone's interested)
The processing power needed seems to have only recently become available for more normal speed PD arrays, but 1.5 ps resolution PDs are going to require some incredible preprocessing electronics to get the data from arrays of those into processors.