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ccd and cMOs features. they facilitate, however, the mitiga- tion of each technology’s limitations as illustrated in the follow- ing sections. A true merge of CCD and CMOS a major advantage of epc’s BsI concept is the optical detec- tors’ 100% fill factor, independent of the numbers of transis- tors in the pixel. looking at the common requirement of global shuttering, these advantages become evident. cMOs requires additional transistors in each pixel in order to realize global Fig. 3: Pulse response following a 10ns LED pulse with a rise/ shuttering. This usually comes at a loss in fill factor with adverse fall time of 12ns. effects on device responsivity. In the Espros Photonic CMOS process, shuttering is completely implemented in the charge accumulate and handle large charge amounts and the degree of domain and can be controlled in very short timeframes. What’s freedom in pixel design supports the implementation of dedi- more, repetitive shuttering with frame-store functionality in dedi- cated high dynamic range architectures. cated frame buffers is also possible. However, the full CMOS analogue design features including high voltage transistors and Speed EEPrOM blocks remain available for the pixel architecture – all speed is another requirement often stressed in imaging. a of which come with no compromise in the fill factor. Respon- closer differentiation is appropriate in order to reveal the ben- sivity also benefits from the availability of low power high gain efits on speed. First, speed is determined by an efficient photon CMOS amplifiers. And the previously mentioned quantum conversion and the subsequent charge collection mechanisms. efficiency advantage plays a vital supporting role in this respect These mechanisms are limited by diffusion and by drift veloci- by providing higher charges in the first place. ties related to the levels of the applied depletion voltage. higher depletion voltages generate stronger fields and result in faster Dealing with noise drifts and therefore in faster charge separation and detection. When dealing with noise performance, the most suitable design In the Espros Photonic cMOs environment, a combination of measures from both worlds can be applied in a complementary suitable substrate material and high field charge handling lead way. traditionally, cMOs used to be much more prone to noise, to reaction times in the nanosecond range – see figure 3. not in the least due to restrictions resulting from limited real Moving one step further and looking at the speed delivered estate inside the pixel. source follower geometries are limited by the imager as an entity, we are back to the significant advan- by size and complex architectures such as multiple read out tages of cMOs in designing ultra-fast and highly sophisticated stages compromise the requirement to keep noise levels low. system on chip solutions. although, one has to admit that with recent developments in CMOS process technology, the differences are dwindling. In New opportunities and incremental a mixed-process environment, the design engineer can apply improvements ccd pixel designs with all the noise advantages. In-pixel source the above sections illustrate only an arbitrary choice of ex- followers do not suffer size restrictions due to BSI and CMOS amples of how the combination of ccd and cMOs process readout and amplification stages can be deployed where they technologies support better performing optical silicon detectors. deliver the best performance (e.g. in high gain amplifiers). As a In fact, virtually all known trade-offs that used to exist when consequence, the dynamic range will benefit from the ability to considering one particular technology are now obsolete. the in- www.electronics-eetimes.com Electronic Engineering Times Europe September 2012 29


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