Current-Assisted SPAD with Improved p-n Junction and Enhanced NIR Performance

Abstract

Single-photon avalanche diodes (SPADs) fabricated in conventional CMOS processes typically have limited near infra-red (NIR) sensitivity. This is the consequence of isolating the SPADs in a lowly-doped deep N-type well. In this work, we present a second improved version of the “current-assisted” single-photon avalanche diode, fabricated in a conventional 350 nm CMOS process, having good NIR sensitivity owing to 14 μm thick epilayer for photon absorption. The presented device has a photon absorption area of 30 × 30 µm2, with a much smaller central active area for avalanche multiplication. The photo-electrons generated in the absorption area are guided swiftly towards the central area with a drift field created by the “current-assistance” principle. The central active avalanche area has a cylindrical p-n junction as opposed to the square geometry from the previous iteration. The presented device shows improved performance in all aspects, most notably in photon detection probability. The p-n junction capacitance is estimated to be ~1 fF and on-chip passive quenching with source followers is employed to conserve the small capacitance for bringing monitoring signals off-chip. Device physics simulations are presented along with measured dark count rate (DCR), timing jitter, after-pulsing probability (APP) and photon detection probability (PDP). The presented device has a peak PDP of 22.2% at a wavelength of 600 nm and a timing jitter of 220 ps at a wavelength of 750 nm.

An Overview of CMOS Photodetectors Utilizing Current-Assistance for Swift and Efficient Photo-Carrier Detection

“…This could be due to the small junction capacitance of the CA-SPAD-2 and re-triggering of the SPAD by carriers, generated by an avalanche event, due to fast recharge. This effect is further explained in Jegannathan et al [25]. This "re-triggering" leads to extended deadtimes which could saturate the detector at high counting rates.…”

“…Although the device did not perform up to the existing FSI SPADs, it was vital in demonstrating that the principle works as expected and the main issues were identified. The second iteration (we will refer to it as CA-SPAD-2 [25]) was also fabricated in X-Fab's XO035 technology. A few changes were made in the design of the pixel: (1) The junction was made with a cylindrical symmetry to avoid sharp corners (shown in Figure 21), (2) The "ring" terminal is chosen to be a P-well (with a p + contact) to realize a more ohmic contact, (3) The pixel pitch was reduced to 30 × 30 µm.…”

“…Over the years, the prefix “current-assistance” has been found in an increasing number of different photonic sensors: current-assisted photonic demodulators (CAPD) for time-of-flight imaging [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ], fast time-gated current-assisted photonic samplers (CAPS) for high-precision fluorescence lifetime imaging [ 19 , 20 , 21 , 22 ], current-assisted photodetector (CAP) for optical receivers [ 23 ] and current-assisted single-photon avalanche diodes (CA-SPAD) for fast and efficient single-photon detection [ 24 , 25 ]. This review paper focusses on research performed at the Department of electronics and informatics (ETRO), Vrije Universiteit Brussel (VUB) using this “current-assistance” principle.…”

A Near-Infrared Enhanced Silicon Single-Photon Avalanche Diode With a Spherically Uniform Electric Field Peak

“…As a result, the SPAD contains a depleted absorption volume of nearly 15 µm × 15 µm × 18 µm in which an electric field is present. Electrons generated in the absorption region move efficiently towards the cathode by drift, without requiring additional assistance [25]. The significant thickness of the absorption volume and the drift-based transport enhance the NIR sensitivity and timing performance of the detector, respectively.…”

A Backside-Illuminated Charge-Focusing Silicon SPAD with Enhanced Near-Infrared Sensitivity

“…However, the larger band gap also leads to a relatively low absorption coefficient for NIR photons [8]. Due to the inefficient absorption of infrared light, silicon SPADs may require an absorption volume with a depth exceeding 5 µm to achieve a PDE above 10% at 905nm [9][10][11][12][13][14][15][16].…”