Introduction
Infrared (IR) radiation itself was unknown until 220 years ago, when Herschel’s experiment with the thermometer was first reported. The first detector consisted of a liquid in a glass thermometer with a specially blackened bulb, to absorb radiation. Herschel built a crude monochromator that used a thermometer as a detector, so that he could measure the distribution of energy in sunlight [1].
The early history of IR was reviewed about 60 years ago in two well- known monographs [2,3]. Much historical information can be also found in more recently published papers [4,5]. The initial infrared detectors were based on the class of thermal detectors: thermometers, thermocouples, and bolometers [6]. In 1821, T.J. Seebeck discovered the thermoelectric effect, and soon afterward, in 1829, L. Nobili created the first thermocouple. In 1833, M. Melloni modified the thermocouple and used bismuth and antimony for its design [7]. Then, in 1835, Nobili, together with Melloni, constructed a thermopile capable of sensing a person 10 m away. The third type of thermal detector, the bolometer/thermistor, was invented by S.R Langley in 1878. By 1900, his bolometer was 400 times more sensitive than his first efforts, and his latest bolometer could detect the heat from a cow at a distance of 14 mile [8].
The photoconductive effect was discovered by W. Smith in 1873, when he experimented with selenium as an insulator for submarine cables [9]. This discovery provided a fertile field of investigation for several decades, though most of the effort was of doubtful quality. By 1927, over 1500 articles and 100 patents had been published on photosensitive selenium [10]. Work on the IR photovoltaic effect in naturally occurring lead sulfide, or galena, was first published by Bose in 1904 [11]; however, the IR photovoltaic effect was not exploited in a radiation detector for several more decades.
The photon detectors were developed in the twentieth century. The first IR photoconductor was developed by T.W. Case in 1917 [12]. He discovered that a substance composed of thallium and sulfur exhibited photoconductivity. Later, he found that the addition of oxygen greatly enhanced the response [13]. However, the instability of the resistance in the presence of light or polarizing voltage, the loss of responsivity due to overexposure to light or high noise, its sluggish response, and the lack of reproducibility seemed to be inherent weaknesses.
Since about 1930, the development of IR technology has been dominated by photon detectors. In about 1930, the appearance of the Cs-O-Ag phototube, with more stable characteristics, discouraged further development of photoconductive cells to a great extent until about 1940. At that time, interest in improved detectors had begun [14,15]. In 1933, Kutzscher, at the University of Berlin, discovered that lead sulfide (from natural galena found in Sardinia) was photoconductive and had a response to about 3 pm. This work was, of course, carried out under great secrecy and the results were not generally known until after 1945. Lead sulfide was the first practical IR detector deployed in a variety of applications during the war. In 1941, Cashman improved the technology of thallous sulfide detectors, which led to successful production [16]. After success with thallous sulfide detectors, Cashman concentrated his efforts on lead sulfide and, after World War II, found that other semiconductors of the lead salt family (PbSe and PbTe) showed promise as IR detectors [17]. Lead sulfide photoconductors were brought to the manufacturing stage of development in Germany in about 1943. They were first produced in the United States at Northwestern University, Evanston, Illinois in 1944 and, in 1945, at the Admiralty Research Laboratory in England [17].
Many materials have been investigated in the IR field. Observing a history of the development of the IR detector technology, a simple theorem, after Norton [18], can be stated: “All physical phenomena in the range of about 0.1-1 eV can be proposed for IR detectors.” Among these effects are: thermoelectric power (thermocouples), change in electrical conductivity (bolometers), gas expansion (the Golay cell), pyroelectricity (pyroelectric detectors), photon drag, the fosephson effect (Josephson junctions, SQUIDs), internal emission (PtSi Schottky barriers), fundamental absorption (intrinsic photodetectors), impurity absorption (extrinsic photodetectors), low-dimensional solids [superlattice (SL), quantum well (QW), and quantum dot (QD) detectors], different types of phase transitions, and so on.
Figure 1.1 gives approximate dates for significant developments for the materials mentioned. The years during World War II saw the origins of modern IR detector technology, supported by the discovery of the transistor in 1947 by W. Shockley, J. Bardeen, and W. Brattain [19]. Recent success in applying IR technology to remote sensing problems has been made possible by the successful development of high-performance IR detectors over the past seven decades. Photon IR technology, combined with semiconductor material science, photolithography technology developed for integrated circuits, and the impetus of Cold War military preparedness, propelled extraordinary advances in IR capabilities within a short period of time during the past century [20].
FIGURE 1.1 The history of the development of IR detectors and systems. For principal military and civilian applications, four generation systems can be considered: first-generation (scanning systems), second-generation (staring systems, electronically scanned), third-generation (staring systems, with large number of pixels and two-color functionality), and fourth-generation (staring systems with very large number of pixels, multi-color functionality, 3D ROIC, and other on-chip functions) systems, offering other functions, e.g. better radiation/pixel coupling, avalanche multiplication in pixels, and polarization/phase sensitivity.
