OFET-based Analog Circuits forMicrosystems andRFID-Sensor Transponders
W.‐J. Fischer, Maria-Reiche-Str · 2007
demonstrate that ourorganic electronics technology fulfils therequirements forsuccessful analog designs. The needforsuitable analogfront-ends in Seto 2wilbefy nrduehe mpyd microsystems, inparticular forfuture RFID-sensor Section 2 will briefly introduce theemployed microsystems, e i partiula forga future RFID-enstor technology andits limitations. Thetransistor model will be systems, entirely based onorganic field-effect transistors tetdi eto .Scin4ad5dsussm (OFETs) isaddressed inthis paper. Microsystems are . . . mixed-signal systems having oneormoresensors, analogimportant analogbuilding blocksfortheintended conditioning stages anda digital subsystem. The application fields. A numberofexperimental results are intgraionof * p formid showninsection 6,corroborated bydetailed simulations. integration~ ~ sucacope.ntpsssfrial Finally, afewconclusions arepresented. problems particularly totheanalog subsystem, owing to F thelarge dispersion oftheelectrical characteristics found2OFETTechnology inpresent organic devices. Furthermore, OFET-based Fig.1presents thecross section ofatypical organic analog circuit design isconstrained bytheexistence of field-effect transistor. Bottomgatetransistors arebuilt proper device models, owingtothefact thatorganicusing afour-layer process: aTitan gate isdeposited witha transistors posses certain features notadequately portrayed PVPinsulator layer, whichisolates thegate fromtheactual bytraditional FET-Models. Forthispurpose a novelchannel; boththesource andthedrain, aswellasall other transistor model wasimplemented, involving suchaspectstracks aredefined bythegoldlayerofthesecond astechnology variability andnon-ideal organic transistor metallization; apentacene layer isfinally deposited ontop behaviour. A number ofanalog blocks weredesigned and throughthermalevaporation. An additional thick tested, these madeupthefundament tobuild more cross-over layer(PVP)isolates two crossing or complexcircuits, likeoperational amplifiers, overlapping metal tracks. Inorder toprotect thetransistor analog-digital-c onverters andhigh-gain amplifiers. Inthe against contamination withtheenvironment, apassivation present paper somefundamental analog blocks andtheirlayer maybeapplied overthewholetransistor. Withthe characterization, thetransistor modelemployed and current process onlyp-type transistors andcapacitors can complex circuits, likehigh-gain amplifiers withstablebefabricated. Thelatter arebuilt bymeansofasandwich feedback characteristics (simulated gain50dBwitha structure (MI-gate dielectric -M2).Thegate dielectric is bandwidth of700Hzandaphase margin of700), a4-bitthin enough togenerate anadequately highcapacitance per charge-redistribution analog-digital converter, andothersunit area (0.3 fF/sq. ptm), leading tocapacitors ofmoderate areshown. Working cascode, differential amplifiers and size. No self-alignment of S/D contacts withthe differential-to-single-ended converters, amongotherunderlying gateispossible, sothatrelatively large circuits, featuring unity-gain bandwidths ofover1.4kHzparasitical capacitances aregenerated. withnominal DC-gains of10dB aredemonstrated. Simulation andmeasurement atDC andhigh-frequency 3Transistor Model showverygoodagreement (all measurements fall within OFETmodel[1]g[2] parameters areextracted from the80%confidence interval predicted byourmodels), regular transistor IV-characteristics. Themodelitself is indicating thegoodquality ofthetransistor model subdivided inaphysical submodel, involving allrelevant employed. Thetechnology presently used(evaporated designparameters (geometry), andanumerical submodel, pentacene, mobility 0.2cm2/Vs, 3-layer process, two encapsulating a detailed description ofthetransistor metallayers, Cox0.3fF/um2, minimumchannel lengthbehavior. We usedareference transistor toextract the 4um,glass substrate) proves thatanalog circuits with required measurements, thusdefining areference setof acceptable performance arepracticable. characteristics. Thetransistor current canbedescribed bya single equation (1), hence improving simulation speed and