Toward the Super Temporal Resolution Image Sensor with a Germanium Photodiode for Visible Light
School of Science and Engineering, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan
School of Electronics and Telecommunications, Hanoi University of Science and Technology, 1 Dai Co Viet, Bach Khoa, Hai Ba Trung, Hanoi 100803, Vietnam
IMEC, 3001 Leuven, Belgium
Faculty of Information Science and Technology, Osaka Institute of Technology, Hirakata Campus, 1-79-1 Kitayama, Hirakata City, Osaka 573-0196, Japan
Link Research Corporation, 291-4, Kuno, Odawara-shi, Kanagawa 250-0055, Japan
Graduate School of Engineering, Osaka University, 1-1 Yamada-oka, Suita, Osaka 565-0871, Japan
Department of Civil and Environmental Engineering, School of Science and Engineering, Kindai University, 3-4-1 Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
Advanced Quantum Architecture Laboratory, EPFL, Rue de la Maladière 71b, Case Postale 526, CH-2002 Neuchâtel, Switzerland
Author to whom correspondence should be addressed.
Sensors 2020, 20(23), 6895; https://doi.org/10.3390/s20236895
Received: 30 October 2020 / Revised: 19 November 2020 / Accepted: 24 November 2020 / Published: 2 December 2020
(This article belongs to the Special Issue Advances in High-Speed CMOS Image Sensor and Related Technologies)
The theoretical temporal resolution limit
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.