Chapter 2 - Comparison of photon and thermal detector performance

Abstract

The chapter investigates the performance of both photon and thermal detectors, with emphasis on the material properties, device structure, and their impact on focal plane arrays (FPA) performance, especially in the long wavelength infrared (LWIR) and very long wavelength infrared (VLWIR) spectral regions. In comparison with photon detectors, thermal detectors have been considerably less exploited in commercial and military systems. The reason for this disparity is that thermal detectors are popularly believed to be rather slow and insensitive in comparison with photon detectors. The effort to develop thermal detectors has been extremely small relative to that of photon detectors. The speed of thermal detectors is quite adequate for non-scanned images with two-dimensional detectors. In comparative studies, more attention is paid to a wide family of photon detectors. Progress in infrared (IR) detector technology is connected with semiconductor IR detectors that are included in the class of photon detectors. The photon detectors show a selective wavelength dependence of response per unit incident radiation power. They exhibit both perfect signal-to-noise performance and a very fast response. Thermal detectors operate on a simple principle, that is, when these detectors are heated by incoming IR radiation their temperature increases and the temperature changes are measured by any temperature-dependent mechanism, such as thermoelectric voltage, resistance, pyroelectric voltage. The extrinsic photon detectors require more cooling than intrinsic photon detectors having the same long wavelength limit. The theoretical detectivity value for the thermal detectors is much less temperature dependent than for the photon detectors. The temperature requirements to attain background fluctuation noise performance, in general favor thermal detectors at the higher cryogenic temperatures and photon detectors at the lower cryogenic temperatures.