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Infrared image sensors are essential for automotive vehicle night vision, rescue robot-eye vision, thermal imaging in biology and medicine, remote sensing in security surveillance systems, and terahertz electronics applications. Two different kinds of approaches are used to detect infrared radiation, particularly for far-infrared radiation. These include very narrow-band photon detectors and broad-band thermal detectors. Photon detectors are used because of their high detectivity and high-speed performance. However, they need cooling with liquid nitrogen and/or liquid helium and thus make the system expensive and complex to handle. Based on the AlGaAs/GaAs material system, quantum-well infrared photodetectors (QWIPs) at a wavelength of 8.3 μm operating at 77 K, and multicolor QWIPs at wavelengths of 8-9 μm and 10-15 μm operating at 40-70 K have been realized. Conventional HgCdTe-based photodiodes can be used for the far-infrared range up to 50 μm. On the other hand, uncooled broad-band thermal detectors for detection of far-infrared radiation of around 10-μm wavelength, either of pyroelectric, bolometric, or thermopile types, still require high-speed operation as well as high-responsivity performances with low cost. The thermoelectric approach offers a low-cost potential due to its simple operation. This is because temperature stabilization takes place by the Seebeck effect, and biasing with self-heating limitations such as those in the bolometric type is not required. In addition, this approach has good compatibility with integrated technology, unlike the pyroelectric discrete approach. The polysilicon thermopile, which is compatible with complementary metal-oxide semiconductor (CMOS) technology, has served in low-cost approaches up until now and led to the demonstration of an infrared image sensor. However, it showed inferior performance compared to commercial bolometric sensors. Heterostructure-based InGaAs/InP thermopiles and modulation-doped heterostructure thermopiles (H-PILEs) were also reported. However, as yet, their performance advantages have not been demonstrated in detail. In this chapter, the performance advantages of the modulation-doped AlGaAs/InGaAs H-PILEs are presented based on demonstrated data. These devices can achieve high performance due to the advantages of both superior Seebeck coefficient and the exceedingly high mobility of 2D electron gas (2DEG) and 2D hole gas (2DHG) at the heterojunction interface. This chapter first describes the features of the Seebeck effect consideration and device design considerations in Sections 21.2 and 21.3, respectively. The fabrication technology for realizing infrared sensors and the measured performances are presented in Sections 21.4 and 21.5, respectively. Finally, the future prospects for uncooled infrared FPA image sensor applications is discussed.
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