Joris P. Maas

Leakage current modeling of test structures for characterization of dark current in CMOS image sensors

In this paper, we present an extensive study of leakage current mechanisms in diodes to model the dark current of various pixel architectures for active pixel CMOS image sensors. Dedicated test structures made in 0.35-/spl mu/m CMOS have been investigated to determine the various contributions to the leakage current. Three pixel variants with different photodiodes-n/sup +//pwell, n/sup +//nwell/p-substrate and p/sup +//nwell/p-substrate-are described. We found that the main part of the total dark current comes from the depletion of the photodiode edge at the surface. Furthermore, the source of the reset transistor contributes significantly to the total leakage current of a pixel. From the investigation of reverse current-voltage (I-V) characteristics, temperature dependencies of leakage current, and device simulations we found that for a wide depletion, such as n-well/p-well, thermal Shockley-Read-Hall generation is the main leakage mechanism, while for a junction with higher dopant concentrations, such as n/sup +//p-well or p/sup +//n-well, tunneling and impact ionization are the dominant mechanisms.

Frame transfer CCDs for digital still cameras: concept, design, and evaluation

Digital still cameras are becoming a widely used alternative for conventional silver-halide cameras. This paper presents first the concept of frame-transfer CCD imagers designed for consumer digital cameras. Next, the different modes of operation are explained in detail and compared with alternative approaches. Finally, extensive evaluation results on four different imagers using this new concept are presented. It will be demonstrated that the flexible modes of operation, the high dynamic range, and excellent optical properties of FT-CCDs make them very suited for this type of electronic imaging.

Sensitivity improvement in progressive-scan FT-CCDs for digital still camera applications

Sensitivity improvements in a 3.2 M-pixel CCD image sensor developed for digital still camera applications are presented. The introduction of gap-less microlenses increases the sensitivity with 25-30% while the high angular response is maintained. With the binning possibility at the image-storage transition, the sensitivity in monitor mode can be increased. Finally the sensor output amplifier now combines low noise and excellent linearity with a much higher conversion factor. This improvement is obtained by reduced parasitic capacitances around the Floating Diffusion area.