Active layer thickness variation in highly-doped amorphous indium-gallium-zinc oxide thin film transistors with molybdenum-chromium contacts is studied to reveal parametric dependencies under both channel length and gate-contact overlap length scaling. Devices with thickness variations from 5 nm to 30 nm, channel length variation from 3 µm to 100 µm and gate-contact overlap length variation from 1 µm to 10 µm were fabricated, characterized and analyzed. Analysis from a field effect transistor perspective show typical thickness variation trends where threshold voltage shifts positively, subthreshold slope improves and I on /I off ratio increases as the active layer thickness is scaled down. Peculiar observations like extremely low saturation voltage, insensitivity towards channel length variations, two different slopes in the subthreshold region and peaks in the transconductance characteristics can be explained only by invoking the principle of Schottky transistor operation. Thinner devices exhibit high field Schottky operation with barrier lowering which causes monotonously increasing transconductance, while thicker devices show low field behavior with no barrier lowering signified by peaks with saturated behavior of transconductance. Channel length modulates this dependence of transconductance on thickness. Sensitivity towards gate-contact overlap length scaling increases with thickness. Devices with active layer thickness of 5 nm can withstand overlap length scaling up to 5 µm without transconductance degradation, while devices of 10 nm thickness can withstand overlap length scaling only upto 10 µm. Devices of 30 nm thickness show contact limited operation even at 10 µm overlap length. Conventional FET operating principles cannot explain these observations and the phenomenon of Schottky contact transistor operation has been invoked. The results point towards a thin borderline in low dimensional transistors, differentiating a FET from Schottky contact transistors, through a field dependent barrier lowering mechanism, which is modulated by active layer thickness, channel length and gate contact overlap length.

Design optimizations like channel length and gate/contact overlap length scaling are generally used for cut-off frequency (fT ) enhancement of thin film transistors (TFT). But channel length scaling leads to mobility degradation due to contact resistance (RC ). RC in TFTs is directly proportional to the transfer length (LT ), and is inversely proportional to the overlap length (LOV ). LT depends upon the metal/semiconductor interfacial resistivity, the vertical bulk resistance and the semiconductor sheet resistance. For the same interfacial contact properties, active layer thickness (Ta ) scaling can be effectively utilized for lowering the vertical bulk resistance thereby lowering LT and enhancing fT . To study the effect of thickness scaling on the bandwidth of TFT based amplifiers, a-IGZO thin film transistor of effective channel thickness of 70nm are first fabricated and characterized. Density of states model of this TFT is extracted and numerical simulations are used to study the influence of active layer thickness scaling. By varying the channel lengths from 10μm to 100μm, transfer line method of contact resistance estimation, is used to extract LT and RC of devices. As the active layer thickness is varied from 70μm to 30μm it is found that LT decreases from 1μm to 60 nm, due to lower vertical bulk resistance. This results in a lower RC and higher μeff, facilitating further scaling of LOV , leading to enhanced fT of the device.

Organic thin film transistors (OTFTs) fabricated with non-destructive patterning techniques are the building blocks in realizing flexible electronic interface for sensing applications. In this study, a single stage differential amplifier is demonstrated with flexible OTFTs. The OTFTs were fabricated with thermally evaporated Pentacene as the semiconducting layer. The electrodes of the OTFTs were realized using a conductive polymer composite, poyaniline:polystyrene sulphonic acid (PANi-PSS), patterned by a modified Parylene lift-off process. Electrical characteristics of bottom contact TFTs with polymeric PANi-PSS electrodes is superior to those with conventional metal electrodes due to a lower charge injection barrier and resultant lower contact resistance. The measurement of the transistor characteristics and the amplifier response reported has been carried out with a wafer probing test jig set up. The differential amplifier realized with p-type Pentacene transistors with mobility, 0.5 cm 2 /Vs, exhibited a voltage gain of 10 dB in a frequency bandwidth of 1 kHz.

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