The development of a correlation-assisted direct time-of-flight (CA-dToF) pixel provides a novel solution for time-of-flight applications that combines low power consumption, robust ambient shot noise suppression, and a compact design. However, the pixel’s implementation introduces systematic errors, affecting its performance. We investigate the pixel’s robustness against various noise sources, including timing jitter, kTC noise, switching noise, and photon shot noise. Additionally, we address limitations such as the SPAD deadtime, and source follower gain mismatch and offset, identifying their impact on performance. The paper also proposes solutions to enhance the pixel’s overall reliability and to improve the pixel’s implementation.

A video rate fluorescence lifetime (FLT) imaging system is presented featuring in-pixel lifetime calculation based on the center-of-mass of the times-of-arrival (ToA) of photons received by the single-photon avalanche diode (SPAD) of the pixel. The ToAs of photons received during a gating window placed in the decay of the returned fluorescent emission are averaged by a switched-capacitor circuit, implementing the center-of-mass method (CMM) in a compact pixel circuit. Binning of ToAs or complex time-to-digital circuits are avoided, and instead, optimal use of all received photons is achieved on a small pixel area with low communication and computational overhead. The system is validated through analytical modeling, Monte Carlo simulations, and real-world experiments using fluorophores with known lifetimes, demonstrating agreement with the analytical model and achieving accurate FLT estimations across a large range of photon counts. A dedicated QVGA (320 × 240pixel) image sensor that includes all necessary circuitry for the CMM currently reaches 10 frames per second and demonstrates the real-time FLT imaging capability of the system.

A 128×128-pixel current-assisted photonic sampler (CAPS) image sensor is designed for macroscale fluorescence lifetime imaging (MFLI). Current assistance is used for swift detection and time gating of the photogenerated carriers within the whole substrate. The sensor is fabricated in a commercial 350-nm CMOS process on a 15- $\mu$ m epilayer, with 19- $\mu$ m pixels and a 62% FF. The pixel features gating widths below 1 ns subject to a 42-ps standard deviation on the gate width uniformity and a 27-ps standard deviation on its temporal position. The pixel has a quantum efficiency (QE) above 40% for near-infrared (NIR) wavelengths, whereas the peak QE is 60% at 675 nm. The gate’s intrinsic decay is observed to be 280 ps, which allows for the MFLI of subnanosecond lifetime dyes. A multiexposure method has been implemented, where the pixel is nondestructively read-out multiple times—with different exposure times—after each reset. This allows for imaging with an extended dynamic range (DR) and is validated on commercial fluorescence phantoms. The MFLI capabilities are further demonstrated in a preclinical experiment.

Current-Assistance (CA) has emerged as a popular pathway for high-performance time-resolved imaging in the near-infrared spectrum. It demodulates photogenerated charges classically by injecting Ohmic majority currents in a low-resistivity epilayer, obtaining a uniform drift field deep into the photodetection volume. To reduce its power consumption, recent implementations utilize high-resistivity epilayers, which introduce strong nonlinearities in the resulting drift fields. This work provides a physical description of the distribution of drift fields in highresistivity silicon substrates by considering that the injected majority currents are in fact Space-Charge-Limited (SCL). A two-dimensional analytical model for the potential and electric field in the epilayer is derived and validated using TCAD drift-diffusion simulations, introducing the concept of the Virtual Cathode (VC) for CA. The SCL nature of the injected majority current is verified through I-V measurements. These insights establish a physical understanding for the design of future current-assisted photodetectors in high-resistivity epilayers.