The purpose of this chapter is to provide an understanding of infrared (IR) detector focal plane arrays (FPAs) and to describe the important parameters that contribute to overall system performance. The emphasis here is on building an intuitive view of how IR FPAs work and less on a detailed treatment of the semiconductor physics of the detector materials. There are many excellent books that treat detector physics and materials in much greater depth. A partial list used in preparing this chapter includes Singh, Henini and Razeghi, and Kinch. Section 12.1 is an overview of the basic principles of photon detector/FPA operation. Section 12.2 describes the development of the background-limited photoconductor (BLIP) and flux-based expressions for signal-to-noise ratio. Section 12.3 summarizes the capabilities, limitations, and typical performance parameters for cooled FPAs. Section 12.4 discusses uncooled (bolometer) FPAs.12.1 Photon Detector Infrared Focal Plane Arrays
12.1.1 Photon detector basic principles
Figure 12.1 shows an abstracted cross-sectional view of a few pixels of a photon detector IR FPA. This particular detector structure is a double-layer heterojunction (DLHJ) photovoltaic (PV) detector with an n-type absorber region. There are many other detector structures possible, but their general properties are similar. The DLHJ is a widely used structure for HgCdTe FPAs; it is of considerable practical importance and is easy to understand in cross section. The end-to-end signal chain involves a number of steps:
(1) IR radiation (i.e. light) impinges from the top.
(2) The first layer is an antireflection coating that reduces first surface reflection from 20-30% to 2-3%.
(3) The light passes through a transparent but electrically inactive substrate. This detector is "backside" illuminated. Some detectors have no substrate or have the substrate removed.
(4) Photons of sufficient energy to overcome the detector's bandgap are absorbed in the n-type absorber region generating electron-hole pairs.