Characterization of Single-Photon Avalanche Diodes in a 0.5 $\mu$m Standard CMOS Process—Part 1: Perimeter Breakdown Suppression

Abstract

We report on the breakdown characteristics of a single-photon avalanche diode structure fabricated in a 0 5 m single-well CMOS process. This paper features two mechanisms for reducing perimeter breakdown. The first mechanism consists of using the lateral diffusion of adjacent n-wells to reduce the electric field at the diode's periphery, and the second makes use of a poly-silicon gate over the high field regions to modulate the electric field. We studied each technique independently as well as their combined effect on the devices' avalanche profiles. In addition to marked alterations in the current-voltage curves near and above breakdown, the diodes' breakdown voltages were increased by more than 4 V, indicating that perimeter breakdown was curtailed.We verified this assertion through a self-consistently solved 2-D numerical model based on Poisson's equation and the hole and electron current continuity equations coupled with rate equations for carrier generation due to impact ionization. The model revealed spatial maxima of the charge generation rates, thereby indicating regions susceptible to breakdown. Our investigation revealed that in native diodes, the generation rate peaked at the perimeter and near the junction's surface, suggesting perimeter breakdown. Conversely, in devices where suppression techniques were used, the region of maximum generation spread laterally and away from the surface, indicating full volumetric breakdown was achieved.

Modeling Emerging Semiconductor Devices for Circuit Simulation

“…Single photon avalanche diodes (SPADs) based optical detectors have gained interest for use in a wide range of applications such as biochemical analysis, imaging and light ranging applications [26][27][28][29][30][31][32]. The main causes of the increased popularity are the exceptional level of miniaturization and portability, overall high performance and low costs due to the integration of SPADs with mixed-signal readout circuits in standard complementary metal-oxide-semiconductor (CMOS) technology [26][27][28][29][30][31][32]. The capability to detect single photon and provide subnanosecond time resolution along with the low-power and high-speed CMOS readout circuits have made SPADs superior to other optical detectors in high performance weak optical signal detection applications.…”

“…This rapid multiplication process results in a sudden large avalanche current. The reverse voltage above which this multiplication process occurs is called breakdown voltage [26][27][28][29][30][31][32]. In order to respond to an incident photon, impact ionization is necessary for a SPAD.…”

“…The generation of an electron-hole pairs causes the device to be in an "ON" or "OFF" state. Since the generation of an avalanche gives a current or voltage spike, the device exhibits an inherently digital operation [26][27][28][29][30][31][32]. Figure 5 shows the Geiger mode operation of SPAD and quenching of the avalanche current.…”

A perimeter gated single photon avalanche diode based silicon photomultiplier as optical detector

“…In this mode of operation the SPAD is said to be operating in Geiger mode. Standard SPADs suffer a reduction in detection efficiency due to premature breakdown at the junction edges of the device [5][6][7]. The electric field distributions show maxima at the periphery due to the planar nature of the junction [7].…”

“…The detection efficiency is reduced due to the reduction in active area by this premature edge breakdown. To overcome this various approaches have been proposed including field limiting guard rings and gated placed on top of the gap [5][6][7][8][9]. A polysilicon gate can be placed on top of the junction of the SPAD to prevent premature breakdown around the junction edges, creating a perimeter gated SPAD or PGSPAD ( Figure 1) [5][6][7][8].…”