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We present a study demonstrating the dependence of apparent activation energies on signal charge in digital image sensors. The data presented in this paper are for a charge-coupled device imager, but the analysis can be applied to CMOS sensors or generally to any system that shows nonlinearity with respect to time. Activation energies for some pixels are observed to vary between values at about half the bandgap of silicon, when calculated at low signal-charge levels, to values approaching the bandgap when calculated at high signal-charge levels. As such, the traditional method of calculating activation energies using a single exposure time at varying temperatures will result in different activation energies dependent on the exposure time chosen, making it difficult to draw physically meaningful conclusions from its value. Therefore, a method of calculating activation energies using a single signal-charge level is proposed, making it easier to correlate activation energies with impurities. Further, we demonstrate how a model of a pixel with a fixed location impurity in conjunction with a moving depletion edge, due to a changing depletion region size, can lead to this behavior. [ABSTRACT FROM AUTHOR]

A model explaining nonlinear time dependence of pixels in charge-coupled device (CCD) pixels is presented. The model describes the movement of the pixel's depletion edge based on varying quantities of signal charge. Dynamic depletion edges will affect dark current collected by pixels with impurities located in the region of movement of the edge. The model attempts to address nonlinear behavior of dark current with respect to exposure time well below the saturation level seen in some CCD imagers. Modeling an imager by giving pixels varying number of impurities and depths of those impurities, assuming a uniform distribution, leads to characteristic behavior observed in the imagers. [ABSTRACT FROM AUTHOR]

The intrinsic resolution of a solid-state detector such as a CCD is often dominated by diffusion of photogenerated charges. The resolution for imaging detectors has been successfully predicted using the Blouke-Robinson model, which calculates modulation-transfer-function curve in the spatial-frequency domain. For applications that image point-source objects, the shape of the point spread function (PSF) is a more appropriate tool for modeling the detector performance. A direct calculation of charge-carrier diffusion developed by Widenhorn et al. provides the needed mathematical model, but it suffers from some practical difficulties in its calculation involving singularities and slow convergence of the series. We present an equivalent model and implementation that converges rapidly in all cases of interest. Predictions of this PSF model are compared against Monte Carlo simulations of carrier transport. [ABSTRACT FROM AUTHOR]

A program is under way to develop charge-coupled device (CCD) sensors for space-based x-ray astronomy imaging spectrometers. To date, laboratory line emission spectra have been acquired throughout the range of 277 to 8000 eV (carbon through copper Kα emission) and CCD sensitivity has been demonstrated throughout the range of 1.1 through 8000 eV. Image resolution is excellent, limited almost entirely by the 15-µm pixel size. These results are presented and specialized techniques are described which permit such low energy response, high spectral resolution, and efficient charge collection. Finality, analysis is presented of one particular CCD characteristic which currently limits UV and x-ray performance: charge diffusion. [ABSTRACT FROM AUTHOR]