Introduction to Photoconductive Detector Technology

The electrical conductivity of photoconductive detectors is changed by the free-charge carriers generated by absorbed incident photons. A current starts flowing due to an applied voltage or bias. The current flow varies in proportion to the photon irradiance. Lead selenide and lead sulfide detectors fall into this category of detectors. This concept is shown in Figure 1.

Schematic of a photoconductive detector.

Figure 1. Schematic of a photoconductive detector.

PbS and PbSe Detectors

PbS and PbSe detectors are constructed through chemical deposition of polycrystalline film on a quartz substrate. Gold electrodes are plated to the film edges to provide electrical contact, and the entire assembly is sealed within a package with a suitable window, quartz or sapphire. A typical lead salt detector is depicted in Figure 2.

Typical lead salt detector.

Figure 2. Typical lead salt detector.

The cooled lead salt detectors also incorporate a thermistor sensor, and the TE cooling stages within the housing. This reduces the cooling requirements due to the reduction in heat transfer between the environment and the detector element. The heat produced by the TE coolers also provides protection to the window from condensation.

TE Cooled HgCdZnTe Family

Liquid Nitrogen cooled MCT detectors employed in the Oriel MIR 8000™ FT-IR instruments provide ultimate detectivity in the range of 2-17µm. The TE cooled HgCdZnTe family delivers somewhat lower performance, and eliminates the requirement for liquid nitrogen refilling.

The epitaxially grown HgCdZnTe family of detectors typically has low impedance, but provides linear performance when appropriately biased and interfaced. This versatile room, and near room, temperature operation detector line does not exactly come under the description of either the junction or bulk photoconductor. Nevertheless, it acts as either the junction or bulk photoconductor in its different modes of operation. In the photoconductive mode of operation, these detectors have notably faster risetimes when compared to the lead salt detectors, particularly at longer wavelengths.

In smaller detectors bias prerequisites, and the related heating and noise generation, are smaller. Optical immersion makes a small detector look like a big detector optically (Figure 3). This method makes the surface of the detector look n² times larger for the hyperspherical lens utilized in the Oriel optically immersed detectors. Here, n is the refractive index of the lens. This a highly effective method for HgCdZnTe detectors, thanks to their monolithic structure. Typical detectivities of some of the Oriel cooled IR detectors are delineated in Figure 4.

Principle of optical immersion.

Figure 3. Principle of optical immersion.

Typical detectivities of some of our cooled IR detectors.

Figure 4. Typical detectivities of some of our cooled IR detectors.

About Oriel Instruments

Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range, from UV to IR, pulsed or continuous, and low to high power.

Oriel also offers monochromators and spectrographs, as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators, that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.

This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.

For more information on this source, please visit Oriel Instruments.

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