<title>R<formula><inf><roman>D</roman></inf></formula>A Requirements For Optimum Hybrid Focal Plane Performance</title>

Abstract

AbstractIn analyzing the performance of direct-injected hybrid focal plane arrays, many factors must ne considered in determining the minimum detector resistance-area product R^A neces­ sary to ootain Dackground-limited performance (BLIP) and good array uniformity. In photo- dioae arrays, a necessary but not sufficient condition is that noise due to the diode generation-recombination and diffusion currents are less than the background photon shot noise. Tnis places a minimum requirement on the magnitude of R0A, the zero bias resist­ ance area product. In addition, there are generally much more stringent requirements on RjjA aue to input MOSFET 1/f noise and threshold variations which exceed the single detec­ tor R0A requirement for BLIP operation at a given background. In general, the input thres- nola variations require that the photodiodes be somewhat back-biased. This produces a sub­ stantially higner average R^A at the expense of higher detector 1/f noise due to surface leakage. In tnis study we have investigated the detector impedance requirements in terms ol tne injection efficiency, threshold nonuniformities, the input MOSFET excess (1/f) noise, and tne detector excess noise. For state-of-the-art parameters, it was determined tnat tne input MOSFET 1/f noise always dominates the other elements in determining the re­ quired detector impedance.*IntroductionHiyh-performance electro-optical infrared (IR) sensors which operate under low back­ ground conditions are essential to many military, earth resources, and astronomical sensors now being developed. Many of these sensor development programs have been emphasizing mono­ lithic and hybrid mosaic focal plane detector arrays with charge-coupled device (CCD) read­ outs. There is a growing trend toward hybrid focal planes consisting of intrinsic semicon­ ductor detector materials and on-focal plane silicon multiplexers. These hybrids take advantage of the higher quantum efficiency of intrinsic detector materials and their higher temperature operation.In focal planes based on discrete photodiode detector/preamp configurations, the most stringent requirement on the detector impedance-area ratio R^A often proves to be the re­ quirement tnat the background shot noise dominate the thermal dark current shot noise, re­ sulting in background-limited performance (BLIP). Other publications have considered dio­ des in nybrid IRCCDs that do not meet the BLIP requirement at zero bias.2~7 In this paper,1 we will consider diode arrays that do meet the minimum BLIP requirement at zero Dias (tne R0A requirement) and consider what additional requirements there may be on the diode operating characteristics. These additional requirements may require a diode opera­ ting impedance area product (RpA) several orders of magnitude greater than the zero-bias (koA) requirement. These higher operating impedances are often obtained by reverse biasing tne diodes wnicn translates the impedance requirements into requirements on the leakage or tunneling currents to allow a sufficient reverse bias range for satisfactory diode operation.Tne performance of a hybrid mosaic focal plane depends critically on the characteristics of tne electrical interface between the detectors and their CCD readout. From circuit analysis, the relationships between the various interface parameters such as injection efficiency, CCD input MOSFET channel excess noise, and MOSFET threshold voltage variations can be developed. During this study, we included the full impact of the MOSFET threshold variations and the effects of the frequency-dependent injection efficiency.The study unambiguously establishes the input MOSFET channel excess noise as the primary determinant of the photodiode impedance required for optimum device performance. Given tnis condition, the diode operating point is then modified by the magnitude of the MOSFET threshold variations. Once these two conditions are satisfied, the requirements for high*Supported by the LMSC Independent Research Program