Gen.Scan to image PA-ROIC FPA-ROIC 194 195 196M 1980 1990 2000 201 ent of infrared de nd systems.Three generation systems can b canning systems).2 cally scanned)an Conduction The spectral current responsivity of photon detectors is (1) (c) wavelength the Planck's the contacts of the device is noisy due to the statistica sorption he noise current Photon detector 月=2g2g2(Gp+Gh+M4. Fig.8.Relative spectral response for a photon and thermal detector D=A)次 NEP Opto-Electron.Rev.20.no.3.2012 A.Rogalski 285 generation of charge carriers. The thermal transitions com− pete with the optical ones, making non−cooled devices very noisy. The spectral current responsivity of photon detectors is equal to R hc qg i , (1) where is the wavelength, h is the Planck’s constant, c is the velocity of light, q is the electron charge, and g is the photoelectric current gain. The current that flows through the contacts of the device is noisy due to the statistical nature of the generation and recombination processes – fluc− tuation of optical generation, thermal generation, and radia− tive and nonradiative recombination rates. Assuming that the current gain for the photocurrent and the noise current are the same, the noise current is I qg G G R f n op th 2 22 2 ( ) , (2) where Gop is the optical generation rate, Gth is the thermal generation rate, R is the resulting recombination rate, and f is the frequency band. It was found by Jones [42], that for many detectors the noise equivalent power (NEP) is proportional to the square root of the detector signal that is proportional to the detector area, Ad. The normalized detectivity D* (or D−star) sug− gested by Jones is defined as D A NEP  d ( )1 2 . (3) Opto−Electron. Rev., 20, no. 3, 2012 A. Rogalski 285 Fig. 6. History of the development of infrared detectors and systems. Three generation systems can be considered for principal military and civilian applications: 1st Gen (scanning systems), 2nd Gen (staring systems – electronically scanned) and 3rd Gen (multicolour functionality and other on−chip functions). Fig. 7. Fundamental optical excitation processes in semiconductors: (a) intrinsic absorption, (b) extrinsic absorption, (c) free carrier ab− sorption. Fig. 8. Relative spectral response for a photon and thermal detector.