Charge-transfer efficiency in a buried-channel charge-coupled device at very low signal levels

A measurement of charge-transfer efficiency (CTE) is described for a 256-element charge-coupled linear imaging device operated below room temperature and at a very low total charge level per charge packet, that is, at a level of approximately 1/20 000th of saturation. This measurement was carried out at a register frequency near 15.75 kHz, the standard television-line frequency. CTE was measured by noting the dependence of the size of the principal charge packet upon the number of transfers. It was observed that at a dark current level of 4 electrons per packet and a photosignal level of 15 electrons the signal loss through approximately 250 transfers was 1 ± 5 electrons at the 50 percent confidence level. A signal-averaging technique was used to obtain this small probable error in the measurement. An analysis has been performed which assumes bulk trapping as the limiting mechanism and from which it may be concluded that to the same probable error the bulk-trap concentration in the CCD channel region is less than 4 × 10¹²cm^-3for trap energies in the range between 0.23 and 0.32 eV below the conduction band. This result demonstrates the possibility of developing a 500 × 500 element charge-coupled imaging device which would have satisfactory CTE for signal levels as low as approximately 10 electrons per picture element.