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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Output Voltage VOUT [V]Y.Matsunaga
Buried Gate Voltage VIN [V]Fig.10.18Source-followercharacteristics
Thisshowsthatthesensingchannelsarecompletelydepleted.Onthecontrary,thesaturationisnotobservedintype(c).Thesource-followergainoftype(c)issmallerthanthatoftype(a)inspiteofthethickgateoxide,sincetheP-typechannelisalsonot atandthetransistorcharacteristicsaredegraded.
Thesourcefollowergaing1isrepresentedasCBC=.CBCCCGC/,whereCBCandCGCareburied-gatetransistorchannelcapacitanceandgateoxidecapacitance,respectively.However,thecharge-to-voltageconversiongaing2isproportionalto.1=CBCC1=CGC/ CBC=.CBCCCGC/D1=CGC,becausethedepletion-layerdepthunderthesensingchannelreachesto15 m,andthecapacitancebetweentheburiedgateandthesubstrateisnegligiblysmall.So,therelationshipbetweeng1andg2isdescribedbythefollowingequation:
g2Dfg1=.1 g1/gCBC:(10.1)
Equation(10.1)showsthatthecharge-to-voltageconversiongaing2stronglydependsonthesource-followergaing1,g2increasesdramaticallyifg1iscloseto1.Especially,thecharge-to-voltageconversiongainrapidlydecreasesinthesmall-signalregion.Theconversiongainincreasesseventimesbyusingthedouble-gatestructurecomparedtothethin-gate-oxidetransistor.Theconversiongainratioofthedouble-gatetothick-gate-oxidetransistorsis220W31D3:55W0:50,ingoodaccordancewiththeg1=.1 g1/ratioof3.55:0.45.Itisclearfromtheabovediscussionthatthedouble-gatestructurecanrealizesuperiorhighconversiongaindetectors.
