OFET-based Analog Circuits for Microsystems and RFID-Sensor Transponders

The need for suitable analog front-ends in microsystems, in particular for future RFID-sensor systems, entirely based on organic field-effect transistors (OFETs) is addressed in this paper. Microsystems are mixed-signal systems having one or more sensors, analog conditioning stages and a digital subsystem. The integration of such a complex unit posses formidable problems particularly to the analog subsystem, owing to the large dispersion of the electrical characteristics found in present organic devices. Furthermore, OFET-based analog circuit design is constrained by the existence of proper device models, owing to the fact that organic transistors posses certain features not adequately portrayed by traditional FET-Models. For this purpose a novel transistor model was implemented, involving such aspects as technology variability and non-ideal organic transistor behaviour. A number of analog blocks were designed and tested, these made up the fundament to build more complex circuits, like operational amplifiers, analog-digital-converters and high-gain amplifiers. In the present paper some fundamental analog blocks and their characterization, the transistor model employed and complex circuits, like high-gain amplifiers with stable feedback characteristics (simulated gain 50 dB with a bandwidth of 700 Hz and a phase margin of 70deg), a 4-bit charge-redistribution analog-digital converter, and others are shown. Working cascode, differential amplifiers and differential-to-single ended converters, among other circuits, featuring unity-gain bandwidths of over 1.4 kHz with nominal DC-gains of 10 dB are demonstrated. Simulation and measurement at DC and high-frequency show very good agreement (all measurements fall within the 80% confidence interval predicted by our models), indicating the good quality of the transistor model employed. The technology presently used (evaporated pentacene, mobility 0.2 cm
²
/Vs, 3-Iayer process, two metal layers, Cox 0.3fF/um2, minimum channel length 4 um, glass substrate) proves that analog circuits with acceptable performance are practicable.