This review paper presents an assortment of research on a family of photodetectors which use the same base mechanism, current assistance, for the operation. Current assistance is used to create a drift field in the semiconductor, more specifically silicon, in order to improve the bandwidth and the quantum efficiency. Based on the detector and application, the drift field can be static or modulated. Applications include 3D imaging (both direct and indirect time-of-flight), optical receivers and fluorescence lifetime imaging. This work discusses the current-assistance principle, the various photodetectors using this principle and a comparison is made with other state-of-the-art photodetectors used for the same application.

Correlation-Assisted Direct Time-of-Flight (CA-dToF) is demonstrated for the first time on a large 320 × 240-pixel SPAD array sensor that includes on-chip high-speed timing support circuitry. SPAD events are processed in-pixel, avoiding data communication over the array and/or storage bottlenecks. This is accomplished by sampling two orthogonal triangle waves that are synchronized with short light pulses illuminating the scene. Using small switched-capacitor circuits, exponential moving averaging (EMA) is applied to the sampled voltages, delivering two analog voltages (VQ2, VI2). These contain the phase delay, or the time of flight between the light pulse and photon’s time of arrival (ToA). Uncorrelated ambient photons and dark counts are averaged out, leaving only their associated shot noise impacting the phase precision. The QVGA camera allows for capturing depth-sense images with sub-cm precision over a 6 m range of detection, even with a small PDE of 0.7% at an 850 nm wavelength.

Correlation-Assisted Direct Time-of-Flight (CA-dToF) is demonstrated for the first time with a 320×240-pixel SPAD array. SPAD triggers are processed in-pixel, avoiding data communication over the array and/or storage bottlenecks. This is accomplished by sampling two orthogonal triangle waves that are synchronized with short light pulses illuminating the scene. Using small switched-capacitor circuits, exponential-moving-averaging (EMA) is applied to the sampled voltages, delivering two analog voltages (VQ2, VI2). These contain the phase delay, or the time of flight between the light pulse and photon’s time of arrival (ToA). Uncorrelated ambient photons and dark counts are averaged out, leaving only their associated shot noise impacting the phase precision. The QVGA camera allows capturing depth-sense images with sub-cm precision over 6-m range of detection, even with a small PDE of 0.7% at 850-nm wavelength.

This letter proposes a novel SPAD-based pixel, designed for direct time-of-flight (dToF) imaging with in-pixel averaging, which provides a promising advancement in low-power and potentially high image resolution for outdoor applications. By utilizing a laser pulse and two orthogonal sinusoidal signals, the pixel averages out the detected ambient light while accumulating the laser pulse round-trip time. A prototype pixel array was fabricated using a 180 nm CMOS process, featuring a commercial SPAD cell. By characterizing one pixel and employing a 100 klux solar emulator as an ambient light source with a fixed 40 ambient-to-signal ratio (ASR) over a 360 $^{\circ }$ phase shift, equivalent to 6 m detection range, the maximum detected accuracy error was 3.3 %, with a 5 cm precision.

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.