Very thin (2.3-5.5 nm) self-assembled organic dielectric multilayers have been integrated into organic thin-film transistor structures to achieve sub-1-V operating characteristics. These new dielectrics are fabricated by means of layer-by-layer solution phase deposition of molecular silicon precursors, resulting in smooth, nanostructurally well defined, strongly adherent, thermally stable, virtually pinhole-free, organosiloxane thin films having exceptionally large electrical capacitances (up to Ϸ2,500 nF⅐cm ؊2 ), excellent insulating properties (leakage current densities as low as 10 ؊9 A⅐cm ؊2 ), and single-layer dielectric constant (k) of Ϸ16. These 3D self-assembled multilayers enable organic thin-film transistor function at very low source-drain, gate, and threshold voltages (<1 V) and are compatible with a broad variety of vapor-or solution-deposited p-and n-channel organic semiconductors.gate insulator ͉ molecular multilayer ͉ organic dielectric ͉ self-assembly O rganic thin-film transistors (OTFTs) based on -electron materials are envisioned as critical components of future organic electronics technologies and would enable low-cost solution-processed͞printed logic circuits, displays, and sensors (1-3). Of the two fundamental OTFT components, the semiconductor and the dielectric, the greatest research effort by far has focused on the organic semiconductor, with impressive results achieved in increasing carrier mobility, for both holetransporting (p-type) (2-4) and electron-transporting (n-type) (2, 5, 6) semiconductors, and in developing low-cost fabrication processes (7-9). However, mobilities are still modest by inorganic semiconductor standards and result in transistor function at unacceptably high operating voltages (30-100 V), a serious impediment to useful technologies. A breakthrough would be to develop nano-precise, high-yield growth methodologies enabling state-of-the-art OTFT performance at drastically reduced operating voltages. We report here successful realization of one such approach in which robust, 3D-crosslinked nanoscopic dielectrics are fabricated by means of layer-by-layer deposition of -organosilane modules.Typical ''top-contact'' OTFTs contain a semiconductor layer on top of a dielectric, together with an underlying gate electrode and top charge-injecting͞extracting source and drain electrodes (Fig. 1). Current flowing between source and drain electrodes (I DS ) on application of a drain-source bias (V DS ) is minimal when zero voltage is applied between gate and drain electrodes (V G ϭ 0), in which the device is ''off.'' However, as V G is increased in magnitude, charge carriers are accumulated at the semiconductor-dielectric interface, resulting in a gatemodulated I DS (''on'' state). Parameters characterizing thin-film transistor (TFT) performance include the field-effect mobility () and the current on͞off (I on :I off ) ratio, defining the drainsource current ratio between on and off states. I DS in the linear regime (10) is then expressed by Eq. 1, where W and L are the TFT chan...