SPAD stands for single-photon avalanche diode, and some people may call it Geiger-mode avalanche diode. A SPAD is a semiconductor device that can detect low-intensity light, which means it can count single-photon events. SPADs were first introduced in the 1960s and ‘70s, but only recently have they gained more popularity especially in applications such as LiDAR, quantum computing, etc. See SPAD products on our main website.
SPADs are related to SiPMs. A SPAD connected to a quenching resistor is the microcell in an SiPM. Imagine arranging many of these SPADs with quenching resistors in an array, and it will form an SiPM. The operating regime in an I-V curve for SPADs is the same as SiPMs, where the reverse bias voltage is above the breakdown voltage. To learn more about SPADs and SiPMs, check out our collection of technical notes.
To operate a SPAD, you will need to bias the voltage above the breakdown voltage. When a SPAD is biased above the breakdown voltage, this state is called a meta-stable state; in other words, it means that the current does not flow. If a photon strikes the SPAD, it triggers an avalanche, and the current increases rapidly and reaches a state called steady state. Once the current reaches the steady state, the SPAD is no longer sensitive to light. To bring the SPAD back to the sensitive state, either an active or a passive quenching resistor is required.
There are three types of photodetectors that are excellent in detecting a single photon, and they have very high internal gain and serve a diverse range of applications. Hamamatsu offers all three:
The table below can help you differentiate among SPAD, SiPM, and PMT to enable you to choose which photodetector type best fits your needs. To learn more about photon counting detectors, check out our technical notes and webinars:
|Detector||MPPC (SiPM)||SPPC (SPAD)||PMT|
|Spectral range||120 to 1000 nm||320 to 1000 nm||185 to 900 nm|
|Max count rate||Low||High||Medium|
|Magnetic field||Immune||Immune||Needs protection|
|Light intensity detection||Easy||Hard||Easy|
Table 1. General comparison of photon counting detectors
MPPC stands for Multi-Pixel Photon Counter, the product family that includes our SiPMs. Since we believe the future of Geiger-mode gain detectors includes more than just silicon-based devices, we’ve adopted the term MPPC to cover all the various materials (like InGaAs) that can be used to catch those shifty little photons!
If your detector is or has the potential to be exposed to strong light, try to keep the detector’s average current below 100 mA. The detector will be non-linear well before this current range, and exceeding it may cause damage—especially in devices with a wirebond connection. While the detector won’t explode or disintegrate (well, maybe disintegrate if it’s reaaaaalllyyy strong laser light), the wirebond will disconnect and create an open circuit. This destroys a detector’s ability to send current outside of its package.
The microcell size affects a few parameters of the MPPC. As you probably know, each microcell requires a quenching resistor which takes up a portion of each microcell’s area, reducing the fill factor. The quenching resistor takes up a smaller percentage of the microcell area as the microcell sizes become larger and larger, so larger microcells have higher fill factor and subsequently higher photon detection efficiency (PDE).
However, larger microcells also mean there are less microcells per unit area. Since the upper limit of an MPPC’s linearity is determined by the number of available microcells, then MPPCs with larger microcells will have a lower upper limit or linearity than an identical-sized MPPC with smaller microcells.
There are other factors to consider as well; for example, larger microcells have larger gain but slower time response due to higher capacitance. Dark counts are pretty much identical, but crosstalk may be higher with larger microcells.
Our largest off-the-shelf array is 4x4ch 6mm sq. MPPC array or 8x8ch 3mm sq. MPPC array. We can also offer custom-size arrays or the individual tile-able package MPPC, so the customer can create an array of their own size or shape.
The photon counting method is the better choice when you’re measuring extremely low light levels, so low that each individual photon arrival can be measured separately and the number of photons can be counted.
When you use photon counting, the following noise factors decrease:
The noise reduction will help improve the signal-to-noise ratio (SNR) in your measurements. It also gives you more accurate results compared to ordinary light measurement techniques that measure the output current as analog signals.
The MPPC/SiPM and PMT are both suitable for photon counting due to their excellent time resolution, high gain, and low noise.
Visit our main website to see our MPPC products
You should choose analog mode when you’re measuring higher light levels, where the output pulse intervals are narrow or overlap each other when you read the output on an oscilloscope. In analog mode, the output signal is the mean value of signals. By choosing analog mode, you will preserve the amplitude information, in contrast to photon counting where multiple photons detected simultaneously are counted as one photon.
Dino Butron is an Applications Engineer, specializing in all of our point detectors and signal-to-noise simulations. He and his photonic sidekick (his cat Bash) tackle technical issues daily.
Shwu Fei Tan is a Marketing Engineer, in Hamamatsu Corporation’s San Jose, CA, office. As part of the MPPC (Multi-Pixel Photon Counter) project team, she likes learning about new and interesting applications of MPPCs, and also creating marketing collateral that explains highly technical concepts in simpler terms. In her spare time, Shwu Fei enjoys challenging yoga poses, which help her focus and connect her body and mind.
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