CCD scanning in photodiode arrays

Resendiz-Rodriguez, Francisco (1980) CCD scanning in photodiode arrays. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

This thesis deals with the charge coupling between light sensors and charge-coupled devices in linear imagers. The main aim of this work is the study of the charge transfer mechanism between photodiode and CCD scanner which occurs whilst the sensor is being read out. The physical model is based on MOS theory applied to the optical input stage formed by the photodiode, the MOS switch and the CCD potential well. The analysis of the charge transfer mechanism depends on the previous photodiode history. Therefore the physical phenomena associated with the charge collection due to optical and/or thermal generation of the photosite had to be treated independently. A generalized model has been derived to predict the discharge of p-n step junctions and MOS induced junction as light sensors under any conditions of illumination. The charge transport produced whilst the photodiode injects charge into the CCD is analysed in terms of strong inversion i. e. linear mode, and weak inversion, i. e. subthreshold regimes. From the analysis it has been deduced that the amount of charge stored at the photodiode eventually defines the operating regime. For large charge packets as those-generated at normal light level or at low light levels using long integration times the operation can be considered as purely strong inversion condition. For snail charge packets generated at normal illumination during short integration periods 1'q sets) or at low light levels in periods of time of a few milliseconds, the transfer needs to be treated as a combination of both regimes. At the lower end of the low light level where the amounts of charge collected are comparable to the ones produced by thermal generation, the charge transfer is mostly connected with the subthreshold mode. Experimental tests on P-channel and N-channel S CCD running backwards i. e. from output to input, enabled the sensor diode of the output stage to be characterized as a photodiode. Time measurements for the photodiode recharge during the CCD readout process show reasonable agreement with predictions derived from the theoretical model. The thesis includes the processing of a 10-bit linear imager, its optical and electrical characterization and a novel technique to determine the fast surface state density.

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