Selecting an Infrared Photodiode Array for Your Next Optical System

When most people think of detectors for optical systems, they probably default to CCD arrays or CMOS sensors. This is understandable; these components have a long, successful history and are a familiar camera component. However, when it comes to gathering sensitive measurements outside of the visible spectrum, a CCD or CMOS sensor is not the best choice. Instead, you should use a photodiode or photodiode array to gather these sensitive measurements. Here’s what you need to know about photodiode arrays and how to select the best component for your next optical system.

Measurement vs. Imaging

On the surface, CCDs, CMOS sensors, and photodiode arrays might sound like the perfect replacements for each other in different applications, but this is not the case. Each of these components has their place in different applications, and it’s not the best idea to mix and match components and applications.

CMOS sensors and CCDs are pixel array sensors that are primarily used for imaging. Light exposure in each pixel is used to build an intensity map for light from the surrounding environment. When you add color filtration to different pixels, you can then build color images. Contrast this with a photodiode array; a photodiode array is formed from a number of elements (usually less than 1024) with a common cathode. Each element resembles a pixel and acts as its own p-n photodiode. In some applications, such as military and aerospace, a photodiode array might be assembled from several individual photodiode array detectors.

A photodiode array is typically optimized to provide high-sensitivity response within a narrow band of wavelengths, whereas CMOS sensors and CCDs are typically built to operate throughout the visible range. Because the number of elements is so small, and the desired response is confined to a smaller range of wavelengths, a photodiode array is typically used for very niche imaging applications or for gathering sensitive measurements at specific wavelengths. If you need to gather extremely sensitive measurements other than what can be gathered with a photodiode array, a photomultiplier should be used instead.

(Alt text: Small photodiode array) https://www.shutterstock.com/image-photo/digital-electronic-colour-sensor-chip-solder-1077154646

Infrared Photodiode Array Selection Criteria

The following points should be considered when selecting a photodiode for use in any infrared imaging system:

• Sensitivity spectrum: Datasheets will normally quote the sensitivity at a single wavelength (normally the wavelength with maximum sensitivity). However, like most components, the sensitivity depends upon frequency. Sensitivity is normally quoted in units of A/W or as a percentage of the maximum sensitivity.
• Wavelength range: Photodiode arrays are designed to operate in different wavelength ranges. Photodiode arrays that span portions of the visible, ultraviolet, and infrared spectra are available from different manufacturers.
• Maximum output current: This is the maximum current that an element in a photodiode array can produce.
• Breakdown voltage: When operated in photoconductive mode, the component is driven in reverse bias. This is the maximum reverse bias voltage that will cause breakdown. Note that a photodiode array can be operated in photovoltaic mode (i.e., unbiased).

Optek OPR2100T

The OPR2100T photodiode array is a 6 element, 12-pin SMD package that is ideal for industrial motor encoder applications. This photodiode array has an opaque package that blocks stray light from the surrounding environment. The package is rather rugged and has 3.45 sq. mm active area. Maximum responsivity is seen at 850 nm.

Photograph of the OPR2100T photodiode array and its responsivity. From the OPR2100T datasheet.

Advanced Photonix PDB-C216

The PDB-C216 is a linear photodiode array with maximum response at 950 nm. The sensitivity spectrum spans into the visible range, although this can be eliminated with the proper filter if detection at infrared wavelengths is desired. This photodiode array finds it home in baggage scanners and other applications requiring line scan measurements.

The PDB-C216 linear photodiode array and its responsivity. From the PDB-C216 datasheet.

Broadcom SPD2025-12X

In the telecommunications arena, the SPD2025-12X linear photodiode array from Broadcom is ideal for high performance optical receiver designs. This 12-element photodiode array provides high responsivity, low dark current, and low capacitance for high-bandwidth products. Some telecommunications applications include use in receivers that use multimode 12×25-Gb/s and 12×28-Gb/s NRZ or PAM-4 signalling.

Layout of the SPD2025-12X linear photodiode array. From the SPD2025-12X product brief. (Alt text: Layout showing the SPD2025-12X linear photodiode array) https://drive.google.com/open?id=14d5QeekNnjB2DXV8t2mBGLZl3as3_AKc

With new applications in computer vision, greater use of optical systems, and sensitivity requirements in applications like lidar, you’ll need to choose the right photodiode array for your next system. Octopart^® gives you access to a range of photodiode arrays and other detectors for your next imaging or measurement system. Try using our Part Selector guide to determine the best option for your next product.

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