Since the photomultiplier tube (PMT) was invented in the mid-1930s, it has remained the principal detector for experiments involving a small number of photons. However, its low photon detection efficiency and high sensitivity to magnetic fields forced researchers and systems designers worldwide to find alternatives for single-photon detection. The foundation for the development of the Geiger-mode silicon avalanche photodiode (G-SAPD) was laid in 1961, when Dr. Robert J. McIntyre of RCA Electro-Optics Canada presented his theory of microplasma instability in silicon. McIntyre’s team then researched operating the avalanche photodiode (APD) above the breakdown voltage – the so-called Geiger mode – and enabling single-photon detection with silicon APDs. The first G-SAPD single-photon-counting detector, generally referred to as the single-photon avalanche diode, was developed in 1986 using the SLIK structure (superlow K-factor, or superlow excess noise factor structure) by Excelitas Technologies Canada, the former Electro-Optics division of RCA Canada. In the wake of innovations by Rockwell International, G-SAPD photon-counting detector structures were introduced by Radiation Monitoring Devices Inc. and Hamamatsu Photonics. In the past 50 years, tremendous progress has been made in the area of single-photon counting based on G-SAPD. Applications such as adaptive optics, single-molecule analysis, positron emission tomography and time-domain fluorescence spectroscopy, as well as market demand for cost-effectiveness, have been instrumental in driving its development. These advances range from a single-active-area device to multicell silicon photomultipliers (SiPMs); from the simple analog-mode avalanche diode to a digital photon-counting module; and from multiphoton detection to the analysis of their spatial resolution. To facilitate the use of G-SAPDs, the first commercialized single-photon-counting module was introduced in 1987 (Figure 1). It was a self-contained, user-friendly device with built-in temperature control, a stabilized high-voltage supply and a Geiger-mode avalanche photodiode passive quenching circuit. With its low dark count, low timing jitter, low afterpulse and a photon detection efficiency (PDE) of more than 50 percent, this first-generation module enabled single-photon studies to move deeper into the red and near-infrared regions of the electromagnetic spectrum. Figure 1. The first commercial Geiger-mode silicon avalanche photodiode single-photon-counting module by RCA, the SPCM-100, was introduced in 1987. Courtesy of Excelitas Technologies Corp. In the 1990s, this single-photon-counting module enabled the start and development of fields such as single-molecule detection, fluorescence sensing and quantum cryptography, which by now are critical in research and commercial fields such as biology and astronomy. The ability to detect single photons made it possible to work with nanoliter or picoliter samples without amplification. Biomedical studies of vaccines, bioreagents and pharmaceuticals were some of the first beneficiaries of this compact solid-state photon-counting module. Applications such as near-field scanning microscopy and laser-induced fluorescence in single-molecule detection became easier and more effective with the high PDE. More choices opened up as well within the laser-induced fluorescence sector because it was now possible to see and analyze weak fluorescence in the green and red. But a passive quenched solid-state photon-counting module is quite slow; the device is inadequate for time-correlated fluorescence spectroscopy and Förster resonance energy transfer, which need faster rise times and higher count rates. The next-generation module evolved to integrate active quenching circuitry, which today is the norm for all such devices. Photon-counting detectors with good timing resolution or time jitter can measure the molecular dynamics of nanometer-scale structures at picosecond-long fluorescence lifetime changes. New applications in time-resolved fluorescence studies have led to the development of low-timing-resolution G-SAPD single-photon-counting modules by various companies within the past decade. These optimized modules have superior low timing jitter performance of less than 100 ps, although most have active detection areas of 50 μm or smaller and lower PDE in the green and red regions. In the early to mid-1990s, high-throughput fluorescence detection for drug discovery and genome DNA-sequencing projects led the trend toward larger-number screening applications in biotechnology using small amounts of reagents and samples. At the same time, single-photon counting in high-resolution imaging was being explored for time-resolved fluorescence microscopy, x-ray imaging and PET with CT imaging. These movements fueled an increase in demand for multichannel G-SAPD photon-counting arrays for increasing throughput in screening experiments and various types of ultrahigh-resolution imaging. Figure 2. (a) Four- and (b) 16-channel single-photon-counting modules. Courtesy of Excelitas Technologies Corp. Such an array would provide researchers the means to study several aspects of photons simultaneously over many channels. It would mean that original equipment manufacturers such as pharmaceutical companies could increase the speed of multiple drug analysis. Time-resolved acquisition of fluorescence lifetimes in DNA microarray imaging could be achieved with high resolution in the red. A PET/CT combination to provide whole-body anatomical (CT) and functional (PET) diagnostics, including localization of cancer, could significantly reduce the time and cost associated with disease investigation and treatment. Various advances were made in building multichannel G-SAPD single-photon-counting arrays in the 1990s through early 2000s. Formats included monolithic G-SAPD chip arrays and individual single-photon-counting modules with output signals from each detector while sharing the high-voltage power supply and quenching electronics (Figure 2). In the late 1990s came one of the most promising single-photon-counting arrays to evolve. It was the complementary metal oxide semiconductor (CMOS) process-based multipixel Geiger-mode avalanche photodiode, often referred to as the SiPM, the solid-state photomultiplier or the multipixel photon counter. This silicon device is a matrix of small Geiger-mode-operated APD cells connected in parallel to a quenching resistor in series. The basis for this design originated in Russia, and by and large it owes its existence to the metal resistor semiconductor APD, a p-n structured silicon APD with a thin layer of titanium and a layer of high-resistivity SiC or SixOy to limit its Geiger-mode breakdown (Figure 3). Figure 3. (a, b) A silicon photomultiplier by Moscow State Engineering Physics Institute. The SiPM has some of the most desirable attributes of a solid-state single-photon-counting device: high gain (105 to 107), high PDE in the blue, low operating voltage (40 to 100 V), low single-photon timing resolution (average 100 ps), magnetic field insensitivity, compact size but larger active areas (1 to 10 mm2), and the ability to differentiate between single and multiple photons. The most attractive feature of these microcell G-SAPDs is that they can be manufactured using the conventional CMOS process. They could be easily producible in high volume and at fairly low cost; at the wafer level, low-power CMOS electronics, such as analog-to-digital conversion circuitry, could be integrated into the SiPM. For applications such as x-ray and medical imaging where many detectors are needed, the SiPM could significantly reduce the overall system cost and footprint. For point-of-care diagnostics, where small, rugged, portable and inexpensive instruments are sought after, the SiPM would make that possible. Currently, single SiPMs and SiPM arrays in either diode or module form are available commercially from several companies. However, the weaknesses of the SiPM – such as high dark count, high optical crosstalk, high afterpulse and low PDE in the green through red – have hindered its adaptation in many popular biomedical single-photon-counting domains. Through ongoing research, many investigators have already found promising ways to improve the design and performance of the SiPM. Recently announced results from Excelitas Technologies show dark counts of their 1-mm2 SiPM to be orders of magnitude lower than those of previous models, while the PDE is higher than that of currently available commercial devices (Figure 4). Nevertheless, more work is needed on the overall performance of this product for it to be widely established commercially in applications such as cell sorting and sizing, protein analysis and biomolecular diagnostics. Figure 4. (a, b) These graphs demonstrate recently published results from Excelitas Technologies on its 1-mm2 silicon photomultipliers. Presented in July 2011 in Lyon, France, at the International Conference on New Developments in Photodetection. Other frontiers of the G-SAPD single-photon detector are being explored. Among these are pushes for a fully integrated digital SiPM variant that can detect single and multiple photon hits as well as their timing and intensity, and single and compact monolithic G-SAPD arrays with much higher PDE for photon counting in the red and some even in the near-infrared. Ultimately, the G-SAPD single-photon-counting detector of the future must be a combination of all the sought-after features of the PMT and solid-state single-photon-counting technologies and must meet the growing needs of biomedical and life sciences applications. Meet the author Bernicy Fong is an applications specialist at Excelitas Technologies Corp. in Vaudreuil, Québec; e-mail: bernicy.fong@excelitas.com.