IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 24, NO. 2, MARCH/APRIL 2018 3800510 Large-Format Geiger-Mode Avalanche Photodiode Arrays and Readout Circuits Brian F. Aull, Senior Member, IEEE, Erik K. Duerr, Jonathan P. Frechette, K. Alexander McIntosh, Daniel R. Schuette, and Richard D. Younger (Invited Paper) Abstract—Over the past 20 years, we have developed arrays of A Geiger-mode avalanche photodiode (GmAPD) can detect a custom-fabricated silicon and InP Geiger-mode avalanche photo- single photon and produce an electrical pulse of sufficient ampli- diode arrays, CMOS readout circuits to digitally count or time tude to directly trigger a logic element. This “photon-to-digital stamp single-photon detection events, and techniques to integrate these two components to make back-illuminated solid-state image conversion” makes for simple interfacing of the GmAPD to an sensors for lidar, optical communications, and passive imaging. all-digital CMOS pixel circuit [5]. There are mature fabrication Starting with 4 × 4 arrays, we have recently demonstrated 256 × techniques to make dense large-format arrays of GmAPDs. Ar- 256 arrays, and are working to scale to megapixel-class imagers. rays and image sensors based on this technology have many In this paper, we review this progress and discuss key technical scientific applications such as biomedical imaging [6], spec- challenges to scaling to large format. troscopy [7], time-resolved fluorescence [8], and astronomy [9]. Index Terms—Image sensors, Geiger-mode avalanche photodi- It is now even finding its way into commercial applications, such odes, photon counting, lidar. as lidar for self-driving cars [10]. There are, however, some challenges to scaling to large for- I. INTRODUCTION mat and high pixel density. First, the avalanche discharge in OLID-STATE image sensors have a multi-billion-dollar a GmAPD produces a small amount of hot-carrier light emis- S global market. The microelectronics revolution has enabled sion that triggers spurious detection events in neighboring pixels rapid progress in CMOS image sensors, which are now mass [11]. Mitigation of this optical crosstalk is therefore necessary produced for cell phones, consumer electronics, and medical, for successful scaling. Second, while the design of an all-digital automotive, and industrial applications. Most of these image CMOS pixel circuit may seem to be a straightforward engi- sensors are based on the integration of photocurrent by a capac- neering task, scaling to large arrays requires solutions to issues itor in the pixel circuit. The photocharge is sensed by an analog such as data readout bandwidth and power distribution. Third, amplifier and digitized at the periphery of the array, a process hybrid integration of custom GmAPD arrays to CMOS read- that adds readout noise [1], [2]. outs is often favored over monolithic approaches, either for If photons are scarce (or, equivalently, if integration time is performance optimization or for the use of non-silicon detector short), it is desirable for the pixel to be able to digitally count or arrays. This presents non-trivial fabrication issues for large- time stamp individual photon arrivals. This capability eliminates format back-illuminated devices. The next three sections of this the need for analog sensing circuitry in the detection and readout paper respectively discuss the detector physics and technology, path, and thus eliminates the readout noise. CMOS readout architectures, and hybridization techniques. The Airborne flash lidar systems, for example, can achieve high final section discusses the technical progress and future research area coverage rates by using large arrays of detectors, each directions in the path to high-performance large-format Geiger- of which precisely time stamps individual photons [3], [4]. mode image sensors. However, specialized photodetectors with fast, single-photon response were until recently available only in the form of dis- II. GEIGER-MODE APD TECHNOLOGY crete devices with single detectors or small-scale arrays. A. Background and Pixel Architecture Manuscript received May 27, 2017; revised July 31, 2017; accepted July 31, An avalanche photodiode (APD) is a pn junction designed to 2017. Date of publication August 11, 2017; date of current version August 25, 2017. This work was supported by the U.S. Air Force under Contract FA8721- support high electric fields that cause the primary photogener- 05-C-0002 and/or Contract FA8702-15-D-0001. (Corresponding author: Brian ated carrier to initiate an avalanche, that is, a chain of impact F. Aull.) ionization events. The electron-hole pairs generated in this chain The authors are with the Lincoln Laboratory, Massachusetts Institute of Tech- nology, Lexington, MA 02141 USA (e-mail: [email protected]; [email protected]. mediate internal gain. At biases below the breakdown voltage the edu; [email protected]; [email protected]; [email protected]; younger@ chain is self-terminating; the average value and the variance of ll.mit.edu). the gain are determined by the randomness of the avalanche pro- Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. cess. This way of using the APD is referred to as linear-mode Digital Object Identifier 10.1109/JSTQE.2017.2736440 operation, because one obtains a continuous-time photocurrent 1077-260X © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information. 3800510 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 24, NO. 2, MARCH/APRIL 2018 Fig. 2. Separate-absorber-multiplier structure. The GmAPD doping profile forms an n+-π-p-π-p+ structure, where π denotes very light p-type background doping. The middle p-layer separates the multiplier region, where avalanche Fig. 1. Direct digital sensing scheme. The Geiger-mode APD is biased on the multiplication occurs, from the absorber, where the incident photon is absorbed. p side at a negative voltage whose magnitude VA is slightly below the avalanche The electric field profile for a Si APD is shown for the armed (solid line) and breakdown voltage. The n-side is armed to a logic-high voltage VDD by briefly disarmed (dashed line) state. turning on a tri-state buffer. When the APD detects a photon, it discharges the parasitic capacitance on the n-side node until its reverse bias reaches avalanche breakdown. At this point, the APD shuts itself off and the n-side node is at a voltage close to a logic zero. small transistor count. Second, the APD itself functions as a CMOS-compatible logic element. While a voltage VA of several tens of volts is required for APD biasing, the voltage swing on proportional to the intensity of the optical signal. Gains in the the CMOS side needed for efficient photon detection is in the range 10–103 are typically used. range of 3–5 V. Thus the APD is CMOS-logic-compatible in It is also possible to operate the APD above the breakdown many older CMOS processes. In advanced process (180 nm voltage and obtain discrete electrical pulses in response to indi- and below) that have smaller voltage swings, circuit techniques vidual photons. This is known as Geiger-mode operation. When are used to augment the voltage swing, such as cascoding the an avalanche is first initiated, the average rate at which carriers transistors used for arming and disarming. Like a CMOS logic are generated exceeds the rate at which they are collected at element, the APD draws negligible static bias current. the device terminals. Barring an early random fluctuation that terminates the avalanche, this imbalance causes the current to B. GmAPD Structure and Operation grow until the internal fields are reduced by series resistance. A structure typically used in our GmAPDs is the separate- At this point, the current reaches a self-sustaining steady state. absorber-multiplier (SAM) structure [14]. Fig. 2 shows an ide- The total number of electron-hole pairs generated can in prin- alized one-dimensional doping profile of our silicon GmAPDs ciple be infinite. To be useful, however, the APD is connected and electric field distributions in the armed and disarmed states. to a circuit that detects the current flow and reduces the bias to In this device, a p-type separator layer partitions the diode into below breakdown long enough to terminate the avalanche and separate regions for photon absorption and avalanche multipli- shut the device off. This bias reduction is commonly referred cation. Specific layer thicknesses, doping polarities, and doping to as quenching and the subsequent bias restoration as either levels depend on the APD material system (Si or InGaAsP) and arming or reset. optimization criteria. A number of circuit architectures have been demonstrated A photon enters the absorber layer and is absorbed, generat- for arming, quenching, and event sensing [12], [13]. Geiger- ing an electron-hole pair. In the silicon SAM structure shown in mode operation, however, lends itself to a simple architectural Fig. 2, the hole is collected at the p+ contact. When the APD concept: direct connection of the APD to a logic circuit. Fig. 1 is in the armed state, a modest electric field exists in the ab- illustrates the concept. The n side of the APD is initially armed sorber that sweeps the electron into the multiplier region, where V to a voltage level DD corresponding to a logic high. The p the electric field is high enough to induce an avalanche. After V side is tied to a negative voltage supply whose amplitude A the discharge, the field in the multiplier is reduced to below the is slightly less than the avalanche breakdown voltage. After breakdown value. The doping polarities favor electron-initiated V +V arming, the reverse bias on the APD is DD A , or roughly avalanches over hole-initiated avalanches.