Generally speaking, sensor manufacturers are driven to improve performance (i.e. offering greater sensitivity, smaller packages, more features) and lower costs. The prioritization of these sometimes conflicting demands varies with each target sensor application. For detector manufacturers, this translates into an assortment of requirements including high sensitivity, expanded range of operation, more integrated products, and low cost solutions. Final detector selection will depend on which attributes are most important for the target application.
To identify the technologies best suited for a particular application, it is helpful to begin with a general overview of the advantages and disadvantages of different types of detectors and the tradeoffs that each provide.
Some complex detector technologies — including InSb, InAs and HgCdTe — typically operate at very cold temperatures to reduce thermally generated free carriers and achieve adequate performance. These detectors normally provide higher performance with broadband wavelength coverage. Unfortunately they often require extensive cooling and a complex design. The resulting solution is bulkier and more costly than alternative technologies. These detector materials should be considered when high sensitivity is required in conjunction with broadband coverage into the far-IR and cost is not a factor.
If low cost is the highest priority, thermal detectors applying pyroelectric, bolometric or thermopile materials are a viable option. These detectors typically have simple designs, broad wavelength response, and are very cost-effective. However they have low sensitivity and slow response speed compared to photon detectors, so they should be used for applications where these attributes are not an issue.
The detector materials that remain for consideration are InGaAs photodiodes (both standard and extended) and lead salt photoconductors (PbS and PbSe). All of these detectors have reasonable cooling requirements and good response times. Standard InGaAs photodiodes have excellent sensitivity and speed and are generally low-cost as well, making them a strong option for applications that only require a spectral response up to wavelengths of 1.7 μm. Extended InGaAs photodiodes have reasonable sensitivity up to 2.2μm although there is some degradation in performance and increased costs. Some extended InGaAs photodiodes provide spectral coverage up to 2.6μm, however, these often come with significant performance degradation and notably higher cost. InGasAs is not a viable alternative for applications requiring a spectral response beyond 2.6μm.
The remaining alternatives, lead selenide (PbS) and lead sulfide (PbSe) photoconductors, have characteristics that make them excellent candidates for meeting demanding industry requirements including high performance, fast response, smaller packages, and costeffective solutions. PbS and PbSe detectors stand out for their industry-leading sensitivity-versus-cost ratio across the entire 1 to 5.5 micron infrared spectrum. PbS has detectivity values that are at least as good as Extended InGaAs with coverage up to 3μm. PbSe is approximately an order of magnitude less sensitive than PbS, but fully covers the mid-IR spectrum with a 1 to 5μm operating range.
The Evolution of Infrared Lead Salt Technologies
Lead salt detector technology has emerged in roughly three stages. PbS detectors were developed during World War II by the German military for use as heat-seeking sensors for weapons. During the ensuing 50 years, the major volume users of the detectors continued to be primarily military, for applications in missile guidance and surveillance systems. The second stage began around 1985 and marked an era of emerging new commercial applications including spectrometry, protein analysis, fire detection systems, combustion control, and moisture detection and control. The most recent stage, which emerged during the last five years, involves an expansion of commercial applications taking advantage of the extended wavelength coverage provided by PbSe technology. Much of this growth is centered around environmental applications, such as pollution detection and medical applications, such as blood protein and medical gas analysis.