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.
10Low-NoiseElectronicImagingwithDouble-GateFETs23310.3CCDImageSensorwithDouble-GateFET
ChargeDetector
ThepixelsizeoftheCCDimagesensorhasbeenmadesmallertoreducechipsizeforsmallopticalassemblyandtorealizemegapixelsensorsforHDTVcameras.Itisnotacceptable,however,tocompromisethesensorperformanceforasmallercellsize,andinparticular,thesensitivityandthedynamicrangeremainhigh[1].Incontrast,theburied-channelcharge-coupleddeviceachievescompletechargetransferwithnotransfernoise.Therefore,almostallsolid-stateimagesensorsutilizelow-noiseburied-channelCCDforscanningcircuits.
Inthefollowingwedescribea1/3-ininterlinetransferCCDareaimagesensorwith94-dBdynamicrangeand1.0-electronrmsnoise oatingsurfacedetector.Withcapacitance-couplednegative-feedbackcon guration,thesaturationlevelofthedetectorissuccessfu1lyextendedbyafactorof5,maintainingthesuperiorlow-noisecharacteristics.Moreover,1=fnoiseintheoutputsignal,whichisaveryconspicuousstreaknoiseinthereproducedimageinspiteofitslow-noisepower,issuccessfullysuppressedwithanovelso-calledalternategaininversion(AGI)signalprocessingtechnique.
10.3.1SensorConstruction
Figure10.19showsphotographsofthefabricateddevicechipandthedetectiontransistor.Thechipsizeis7.1mm.H/ 5:1mm(V),andtheimagesizeis
4.8mm.H/ 3:6mm(V),whichissuitablefora1/3-inlensformat.ThedeviceisthemostpopularinterlinetransferCCDcon gurationatpresent,whichconsistsofphotodiodes,verticalCCDs(V-CCDs),andahorizontalCDD(H-CCD).Forlowleakagecurrentnoise,theburiedphotodiodestructure,whichhasahighconcentrationpC-layeronthen-typestorageregion,isadopted[7,8].ThechargedetectorattheendoftheH-CCDisadouble-gate oatingsurfacedetectorwithaneffectivereadoutnoiseof1.0electronrms.Adynamicrangeof94dBisobtainedbyreturningtheinvertedvoltagesignalontoafeedbackgate.Thesaturationnumberofelectronsisextendedto5 104electrons,whichcorrespondstothesaturationchargeofthephotodiode.
Thedeviceisfabricatedinashallowp-wellonann-typesubstrateof47 cmresistivitywith1:0 m nepatterndesignrule.Twoion-implantationprocessesareaddedtotheconventionalCCDfabricationprocess.Oneisahigh-energyphosphorousimplantationforthesensingchannel,andtheotherisasource/drainboronimplantationfortheP-channeldetectiontransistor.
