SPAD pixels

SPAD pixel and arrays which I designed during my DPhil studies

As early part of my doctorate studies at Oxford, I developed an analogue SPAD pixel which utilised the small gap between circular SPADs. The readout was compatible with conventional CMOS pixels. Digital SPAD pixels used a counter to store the number of detected photons. However, this counter occupied a large space relative to the diode. Instead the analogue pixel, stored the number of detected photons as a charge on the integrated metal capacitor which was placed above the quenching circuit. The stored charge was readout as an analogue voltage similar to CMOS pixel arrays.


(1/4) My first chip back in 2009 UMC 0.18 µm MMRF included three geometrical SPAD structures: circle, octagonal and square. The circle was repeated twice, since a better performance was expected.

(2/4) The 2x2 analogue SPAD array in UMC 0.18 µm, MMRF was a proof-of-concept SPAD pixel structure which was compatible with conventional CMOS image sensor pixel.

(3/4) Multiple SPADs some including passive quenching circuits in AMS 0.35 µm. The bottom structure is a digital counter

(4/4) The fourth and final chip during my DPhil was a 64 actively quenched array of SPADs with a high-speed current-mode differential output optimised for optical communications.


Image credit: © Danial Chitnis

Analouge SPAD pixel in UMC 0.18µm MMRF

Danial Chitnis, Steve Collins, "A flexible compact readout circuit for SPAD arrays", Proc. SPIE 7780, Detectors and Imaging Devices: Infrared, Focal Plane, Single Photon, 77801E (18 August 2010)

Single SPADs in AMS 0.35µm

M. A. Al-Rawhani, D. Chitnis, J. Beeley, S. Collins and D. R. S. Cumming, "Design and Implementation of a Wireless Capsule Suitable for Autofluorescence Intensity Detection in Biological Tissues", in IEEE Transactions on Biomedical Engineering, vol. 60, no. 1, pp. 55-62, Jan. 2013.


SPAD array with active quenching in UMC 0.18µm MMRF

Danial Chitnis and Steve Collins, "A SPAD-Based Photon Detecting System for Optical Communications," J. Lightwave Technology. 32, 2028-2034 (2014)