Abstract Virtually complete charge transfer can be achieved with charge coupleddevices(CCDs)intheburiedchannelconfigurationbecausetheburiedchannelCCDintroduces no transfer noise. However, conventional charge detectors incorporatedinto CCDs have a comparably large noise, which consists of thermal noise and 1/fnoisefromMOS transistorin theoutputchargedetectors.
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Fig.10.19Photographsof
sensorchipandenlarged
detectorY.Matsunaga
10.3.2FeedbackChargeDetector
Figure10.20showsthedetectorstructure.Thesignalcharge(electron)packetsareclockedalongtheH-CCD,andtransferredintothedetectiontransistorsignalsensingregionundertheFPP.Thedetectedchargesaremovedoutintoaresetdrain(RD)byapplyingapositivepulseonaresetgate(RS).ThesignalchargeinthesensingregionmodulatestheP-channelconductanceinthedetectiontransistor.Anoutputsignalisgeneratedusingtwosource-followerstages,constructedwithn-channelMOStransistors.Aninvertedsignalvoltageisappliedtothefeedbackgateofthedetectiontransistor.Thefeedbackampli erisaninverteranditsnoiseisdesignedtobemuchsmallerthanthatofthesourcefollowersinordernottoaddextranoisebythefeedbackloop.Thecapacitance-coupledfeedbackloopallowsthedynamicrangetobeextended.TheoxidethicknessbetweenthefeedbackgateandtheFPPisapproximately1:0 m.
Figure10.21showstheincreaseinthesaturationchargebythenegativefeedbackoperation.SignalchargeQsisconvertedintovoltagesignalVsbytheverylowsensingcapacitanceCeinthecaseoftheopenloopasshowninFig.10.21a.Onthecontrary,inthefeedbackoperation,theeffectivesensingcapacitanceincreasesntimesbytheinvertedsignalvoltage,whichisappliedtothefeedbackgate,sothattheoutputsignalvoltageiscompressed,asshowninFig.10.21b,andthesaturationchargeincreasesbyafactorofn,asshowninFig.10.21c.Theparameternisequaltothewell-knownfeedbackformula(1CkA),wherekandAarethefeedback
