Boukhayma (CEA-Leti) presented a very nice paper about the noise in PPD-based sensors. He modelled the electronic pixel components for their noise performance, and based on this analysis he handed out some guidelines to limit the noise level in the pixels. Although not all conclusions are/were new, it was nice to see them all listed in one presentation and supported by simulation results : lower the FD node capacitance, lower the width of the SF transistor, choose a p-MOS transistor as SF because an n-MOS transistor will give too much 1/f noise, optimize the length of the SF (depending on gate-drain capacitance, gate-source capacitance, FD capacitance and width of the transistor, the formula for the optimum gate length was shown). If the thermal noise of the pixel is dominant, it does not matter whether to use a simple SF in the pixel or to use an in-pixel gain stage. But if the 1/f noise is dominant, one should avoid a standard n-type MOS transistor.
Angel Rodriguez-Vazques (IMSE-CNM, Spain) gave a nice overview of ADC architectures when used for image sensors. It is a pity that for such an overview paper only 20 minutes presentation time were provided. These kind of overview papers deserve to get more time.
Seo (Shizuoka University, Japan) described a pixel without STI, but with isolation between the pixels based on p-wells and p+ channel stops. The omission of the STI has to do with the dark current issues that come together with the STI. The authors showed a very cute lay-out of a 2×2 shared pixel concept (1.75 T/cell). All transistors and transfer gates were ring shaped, located in the center of the 2×2 pixel and with the 4 PPDs at the outside, looks a bit like a spider with only 4 legs. The pixels were pretty large (7.5 x 7.5 um2), in combination with a relatively low fillfactor of 43 %, as well as a low conversion gain of 23 uV/electron. Of course the ring structure of the output transistors consumes a large amount of silicon, and seems to result in a relative large floating diffusion capacitance. The dark current is reduced by a factor of 20 (compared to the STI-based sensor), down to 30 pA/cm2, QE = 68% @ 600 nm.
It is hard to decide who will win the Award for the most artistic pixel lay-out : the hedgehog of Tohoku University or the spider of Shizuoka University ? But it any case, the award goes to a Japanese university. Great work !
February 7th, 2014.