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.
220Y.Matsunaga10.2Double-GateFETChargeDetector
Theprimaryparameterdominatingthenoiseperformanceofachargedetectoristheeffectivecapacitanceatthegateofthe rsttransistor,asexplainedforexampleinthe rstchapterofthisbook.Forthisreason,asuccessfulstrategyfortherealizationofultra-low-noisephotosensorsisthedesignofchargedetectorswithverylowsensingcapacitance,preferentiallybelow1fF.Thedouble-gateFETstructureisveryusefulforthispurposebecauseofbothitshighsensitivityduetolowsensingcapacitanceanditslownoiseduetocompletechargetransferinthechargedetector.Thedouble-gateFETcanbeimplementedeitherasa oatingwellorasa oatingsurfacedevice.10.2.1FloatingWellType
Anewhigh-sensitivityCCDoutputampli ercalled“double-gate oatingwelldetector”hasbeenfabricatedandevaluated.Atroomtemperature,thenewdetectorachievesacharge/voltageconversiongainof76 V/electronandanoiseequivalentsignalof1.2electronsrmsovera3.58MHzvideobandwidth.
Thisdetectorconsistsofann-typeburiedsensingchannelandaP-typebulkchanneldetectiontransistorcrossingunderneaththesensingchannel,representingthedoublegateFET.ThisstructureisveryusefulforaCCDimagesensorwhichhandlesasmallsignalchargeofafewthousandphotogeneratedelectronsorholes.10.2.1.1DeviceStructure
AlmostallchargedetectorsforCCDimagesensorsare oatingdiffusiondetectors.Figure10.1showsthetopviewofthedetectorincorporatedintotheendofaCCDregister.SignalchargesareclockedalongtheCCDregisterandtransferredoutintoaCCDsensingchannel.Sensingchannelpotentialismodulatedbysignalcharges,andamodulatedpotentialissensedbythedetectionMOStransistor’sgate.Theoutputvoltagesignalsarebufferedbydoublestagesourcefollowers.Thedetectionsensitivityisdeterminedbytheconversiongainforsignalchargestotheoutputvoltages,thatis,bytheeffectivesensingcapacitance.
However,thesensingcapacitanceoftheconventionaldetectorcannotbesuf -cientlysmallforthefollowingreasons:First,thesensingchannelhasacompar-ativelylargeareabecauseitinevitablyhasacontacthole.Second,thedetectiontransistorgatecapacitancemustbeaddedtothesensingcapacitance.Third,thedetectorsuffersfromlargekTCnoise,correspondingtocharge uctuationintheresetgateOFFperiodduetoresetgatechannelthermalnoiseintheONperiod.ThenewlydevelopedchargedetectorhaslowsensingcapacitanceandastructurethatisfreefromkTCnoise.Theprincipleisacombinationofann-typeburiedsensingchannelandaP-channeldetectiontransistoracrossit,asshowninFig.10.2
