Bal Mukund Dhar

Solution-deposited sodium beta-alumina gate dielectrics for low-voltage and transparent field-effect transistors.

Sodium beta-alumina (SBA) has high two-dimensional conductivity, owing to mobile sodium ions in lattice planes, between which are insulating AlO(x) layers. SBA can provide high capacitance perpendicular to the planes, while causing negligible leakage current owing to the lack of electron carriers and limited mobility of sodium ions through the aluminium oxide layers. Here, we describe sol-gel-beta-alumina films as transistor gate dielectrics with solution-deposited zinc-oxide-based semiconductors and indium tin oxide (ITO) gate electrodes. The transistors operate in air with a few volts input. The highest electron mobility, 28.0 cm2 V(-1) s(-1), was from zinc tin oxide (ZTO), with an on/off ratio of 2 x 10(4). ZTO over a lower-temperature, amorphous dielectric, had a mobility of 10 cm2 V(-1) s(-1). We also used silicon wafer and flexible polyimide-aluminium foil substrates for solution-processed n-type oxide and organic transistors. Using poly(3,4-ethylenedioxythiophene) poly(styrenesulphonate) conducting polymer electrodes, we prepared an all-solution-processed, low-voltage transparent oxide transistor on an ITO glass substrate.

Synthesis, Structural Characterization, and Unusual Field-Effect Behavior of Organic Transistor Semiconductor Oligomers: Inferiority of Oxadiazole Compared with Other Electron-Withdrawing Subunits

A new series of heterocyclic oligomers based on the 1,3,4-oxadiazole ring were synthesized. Other electron-deficient cores (fluorenone and fumaronitrile) were introduced to investigate the oligomers as n-channel materials. The physical properties, thin film morphologies, and field-effect transistor characteristics of the oligomers were evaluated. Thin films were deposited at different substrate temperatures and on variously coated Si/SiO(2) for device optimization. Contrary to our expectations, the thin film devices of 4 revealed p-channel behavior, and the average hole mobility was 0.14 cm(2) V(-1) s(-1) (maximum value 0.18 cm(2) V(-1) s(-1)). Compound 11 is the first example of an oxadiazole-containing organic semiconductor (OSC) oligomer in an n-channel organic field-effect transistor (OFET) and shows moderate mobilities. Non-oxadiazole-containing oligomers 9 and 12 showed n-channel OFET behavior on hexamethyldisilazane-treated and Cytop spin-coated SiO(2) in vacuum. These are the first fluorenone- and fumaronitrile-based n-OSCs demonstrated in transistors. However, oxadiazole-core materials 14 and 16 were inactive in transistor devices.

Field-effect-tuned lateral organic diodes

The operation of organic diodes in solar cells and light-emitting displays strongly depends on the properties of the interfaces between hole- and electron-carrying organic semiconductors. Such interfaces are difficult to characterize, as they are usually buried under the surface or exist as an irregular “bulk heterojunction.” Using a unique fluorinated barrier layer-based lithographic technique, we fabricated a lateral organic p-n junction, allowing the first observation of the potential at an organic p-n interface simultaneously with the charge transport measurements. We find that the diode characteristics of the device (current output and rectification ratio) are consistent with the changes in the surface potentials near the junction, and the current-voltage curves and junction potentials are strongly and self-consistently modulated by a third, gate electrode. The generality of our technique makes this an attractive method to investigate the physics of organic semiconductor junctions. The lithographic technique is applicable to a wide variety of soft material patterns. The observation of built-in potentials makes an important connection between organic junctions and textbook descriptions of inorganic devices. Finally, these kinds of potentials may prove to be controlling factors in charge separation efficiency in organic photovoltaics.

Naphthalenetetracarboxylic Diimide Layer-Based Transistors with Nanometer Oxide and Side Chain Dielectrics Operating below One Volt

We designed a new naphthalenetetracarboxylic diimide (NTCDI) semiconductor molecule with long fluoroalkylbenzyl side chains. The side chains, 1.2 nm long, not only aid in self-assembly and kinetically stabilize injected electrons but also act as part of the gate dielectric in field-effect transistors. On Si substrates coated only with the 2 nm thick native oxide, NTCDI semiconductor films were deposited with thicknesses from 17 to 120 nm. Top contact Au electrodes were deposited as sources and drains. The devices showed good transistor characteristics in air with 0.1-1 μA of drain current at 0.5 V of V(G) and V(DS) and W/L of 10-20, even though channel width (250 μm) is over 1000 times the distance (20 nm) between gate and drain electrodes. The extracted capacitance-times-mobility product, an expression of the sheet transconductance, can exceed 100 nS V(-1), 2 orders of magnitude higher than typical organic transistors. The vertical low-frequency capacitance with gate voltage applied in the accumulation regime reached as high as 650 nF/cm(2), matching the harmonic sum of capacitances of the native oxide and one side chain and indicating that some gate-induced carriers in such devices are distributed among all of the NTCDI core layers, although the preponderance of the carriers are still near the gate electrode. Besides demonstrating and analyzing thickness-dependent NTCDI-based transistor behavior, we also showed <1 V detection of dinitrotoluene vapor by such transistors.

Improved morphology and performance from surface treatments of naphthalenetetracarboxylic diimide bottom contact field-effect transistors.

We report bottom contact organic field-effect transistors (OFETs) with various surface treatments based on n-channel materials, specifically, 1,4,5,8-naphthalene-teracarboxylic diimides (NTCDIs) with three different fluorinated N-substituents, systematically studied with a particular emphasis on the interplay between the morphology of the organic semiconductor films and the electrical device properties. The morphological origins of the improvements were directly and dramatically visualized at the semiconductor-contact interface. As a result of a series of treatments, a large range of performances of bottom contact side-chain-fluorinated NTCDI OFETs (mobility from 1 x 10(-6) to 8 x 10(-2) cm(2)/(V s), on/off ratio from 1 x 10(2) to 1 x 10(5)) were obtained. The surface treatments enabled systems that had shown essentially no OFET activity without electrode modification activity to perform nearly as well as top contact devices made from the same materials. In addition, for the fresh bottom contact NTCDI device, the effect of gate bias stress on the tens-of-minutes time scale, during which the threshold voltage (V(t)) shifted and relaxed with similar time constants, was observed.

Silicon-on-insulator (SOI) integration for organic field effect transistor (OFET) based circuits

In this paper, we report the first silicon-on-insulator (SOI) integration technique for organic field effect transistor (OFET) based circuits. Proposed design flow relies on only basic micro-fabrication processes such as photolithography and physical vapor deposition. This novel fabrication technique allows patterning of conductive silicon gate islands on the subtrate and eases the via and interconnect patterning and deposition for a bottom-gate OFET configuration. We fabricated pand n-type transistors, and proof of concept OFET-based complementary circuits such as inverter and NAND-gate. Fabricated CMOS inverters have full rail-to-rail swing, very high gain (up to 58.3 at 60V, and 18.1 at 20V supply voltages), and outstanding noise margins of around 21V symmetric for NMhigh and NMlow at 60V supply voltage.

Improved morphology and bias stress study of a naphthalenetetracarboxylic diimide bottom contact field effect transistor

Organic thin film field effect transistors (OFETs) have attracted considerable interest for use in a number of applications such as flexible active matrix displays, chemical sensors, radio frequency identification tags and labels, smart cards, and large-area logic circuits. OFETs have been studied in one of two configurations- top contact and bottom contact. Historically, many reports have illustrated the characteristics of bottom contact OFETs based on p-channel materials such as pentacene, copper phthalocyanine (CuPc) and sexthiophene in which holes are the majority carriers. However, very few studies have investigated n-channel organic semiconductor growth on substrates with prepatterned OFET metal contacts, relevant to bottom-contact devices. The investigation and development of materials that can be used in n-channel organic transistors, particularly those that can be operated in air, is crucial for the development of practical organic electrics, such as the most power-efficient families of logic elements called “complementary” circuits, in which both hole — carrying (p- channel) and electron carrying (n-channel) semiconductors are required8–10. In this study, bottom contact OFETs with various surface treatments based on 1, 4, 5, 8-naphthalene-teracarboxylic di-imide (NTCDI) derivatives with three different fluorinated N-substituents, systematically investigated with a particular emphasis on the interplay between the morphology of the organic semiconductor films and the electrical device properties. The topography of the NTCDI bottom contact device without any surface treatment was first characterized by AFM (Fig. 1). It can be observed that the growth of NTCDI films on Si/SiO2 substrates is dominated by crystalline grain structures, however their growth on bare gold is dominated by a dewetting resulting in a rough and amorphous film. A clear “gap” is formed at the interface between Si/SiO2 and Au substrates. In order to overcome the morphology limitations, different methods have been studied: 1) surface chemical modifications of Au electrode and Si/SiO2 substrates are applied to improve the morphology in the OFET channel close to the electrode edge. Because SAM preparation normally is time consuming and some SAM-forming thiols have unpleasant odors, we also investigated an alternative means of improving this near-contact NTCDI morphology without SAM modification, namely, spin coating a thin layer of insulating polymer on the gate-gate dielectric substrate. 2) semiconductor-contact thickness ratios are optimized to allow charge injection through larger interface areas. AFM images of the NTCDI bottom contact devices with surface treatment are shown in Fig. 2. It can be observed that the relatively unstructured film is still grown on top of the gold electrodes and the terrace crystal structure was formed on the silicon substrate. However, compared to the AFM image of the untreated F15-NTCDI device, it is very difficult to detect any “gap” area at the interface between SiO2 and the Au electrode. Based on a series of treatments, a large range of performances of bottom contact side-chain-fluorinated NTCDI OFETs (mobility from 1×10-6 to 8×10-2 cm2/Vs, on/off ratio from 102 to 105) were obtained. The surface treatments enabled systems that showed essentially OFET activity to perform nearly as well as top contact devices. In addition, for the fresh bottom contact NTCDI device, the effect of gate bias stress on the tens-of-minutes time scale, during which the threshold voltage (Vt) shifted and relaxed with similar time constants, was observed.