We report on high performance field-effect transistors fabricated with pentacene as an active material and Al2O3 as a gate
dielectric material grown by atomic layer deposition (ALD). These transistors were operated in enhancement mode with
a zero turn-on voltage and exhibited a low threshold voltage (< -10 V) as well as a low subthreshold slope (< 1
V/decade) and an on/off current ratio larger than 106. Hole mobility values of 1.5 ± 0.2 cm2/Vs were obtained when
using heavily n-doped silicon (n+-Si) as gate electrodes and substrates. Atomic force microscopy (AFM) images of
pentacene films on Al2O3 treated with octadecyltrichlorosilane (OTS) revealed well-ordered island formation, and X-ray
diffraction patterns showed characteristics of a "thin film" phase. Compared with thermally-grown SiO2, Al2O3 gate
insulators have lower surface trap density and higher capacitance density, to which the high performance of pentacene
field-effect transistors can be attributed.
We present a comprehensive study of short channel effects in organic field-effect transistors by measuring the electrical characteristics of devices with fixed channel width and varying channel length. Our studies are conducted on a p-type organic semiconductor, (E,E-2,5-bis-{4'-bis-(4''-methoxy-phenyl)amino-styryl}-3,4-ethylenedioxy-thiophene that is spin-coated from solution to form bottom contact organic field-effect transistors. Drain-source currents from transistors with a channel length of 50 μm show excellent agreement with the square law equations derived for crystalline Si MOSFETs in both the linear and saturation regimes. As the channel length is incrementally reduced to 1 μm, device characteristics such as saturation regime channel conductance, sub-threshold current and threshold voltage, behave in a manner similar to Si MOSFETs of decreasing channel length. Results of these studies indicate the presence of non-destructive current punch-through and in addition, both channel-length modulation and threshold-voltage roll-off, neither of which have previously been reported in OFETs.
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