Th. Birendra Singh

Nonvolatile organic field-effect transistor memory element with a polymeric gate electret

Organic field-effect transistors with a polymeric electret as gate insulator and fullerenes as a molecular semiconductor were fabricated. We observed an amplification of the drain–source current Ids on the order of 104 upon applying a gate voltage Vg. Reversing the gate voltage Vg features large metastable hysteresis in the transfer characteristics Ids(Vg) with a long retention time. The observation of a switchable channel current Ids is proposed to originate from charge storage in the organic electret. As such, this device is a demonstration of an organic nonvolatile memory element switchable with the gate voltage.

Fabrication and characterization of solution-processed methanofullerene-based organic field-effect transistors

The fabrication and characterization of high-mobility, n-channel organic field-effect transistors (OFET) based on methanofullerene [6,6]-phenyl C61-butyric acid methyl ester using various organic insulators as gate dielectrics is presented. Gate dielectrics not only influence the morphology of the active semiconductor, but also the distribution of the localized states at the semiconductor-dielectric interface. Spin-coated organic dielectrics with very smooth surfaces provide a well-defined interface for the formation of high quality organic semiconductor films. The charge transport and mobility in these OFET devices strongly depend on the choice of the gate dielectric. The electron mobilities obtained are in the range of 0.05-0.2 cm2 V-1 s-1. Most of the OFETs fabricated using organic dielectrics exhibit an inherent hysteresis due to charge trapping at the semiconductor-dielectric interface. Devices with a polymeric electret as gate dielectric show a very large and metastable hysteresis in its transfer characteristics. The observed hysteresis is found to be temperature dependent and has been used to develop a bistable memory element.

Bio-organic field effect transistors based on crosslinked deoxyribonucleic acid (DNA) gate dielectric

Using DNA-based biopolymers purified from salmon waste, as an insulating layer, bio-organic field effect transistor (BiOFET) devices were fabricated. Such devices exhibit current-voltage characteristics with low operational voltages as compared with using other organic dielectrics. The observed hysteresis in transfer characteristics of such BiOFETs can be reduced using a crosslinking process. Such crosslinked DNA complex is used as a gate dielectric in n-type C60 as well as p-type α-sexithiophene (T6) based BiOFETs.

Organic inverter circuits employing ambipolar pentacene field-effect transistors

Ambipolar transport has been observed in pentacene films grown on polyvinyl alcohol gate dielectric with hole and electron mobilities of 0.3 and 0.04cm2∕Vs, respectively. A simple device structure with Au as source-drain electrode can be used to operate a transistor in both p-channel and n-channel modes without employing low work function metal electrodes for ambipolar charge injection. Using ambipolar pentacene field-effect transistors, we construct a complementarylike inverter with voltage inversion gain of ∼10. These inverters are able to operate both in first and third quadrants of the voltage output to voltage input characteristics which is a unique feature of employing ambipolar transistors.

Correlation of crystalline and structural properties of C60 thin films grown at various temperature with charge carrier mobility

Transistors fabricated from C60 films grown by hot wall epitaxy at higher substrate temperature, showed an order of magnitude increased charge carrier mobility up to 6cm2∕Vs. In this letter, the authors present an extensive study of morphology and crystallinity of the fullerene films using atomic force microscopy and grazing-incidence x-ray diffraction. A clear correlation of crystalline quality of the C60 film and charge carrier mobility was found. A higher substrate temperature leads to a single crystal-like faceted fullerene crystals. The high crystalline quality solely brings a drastic improvement in the charge carrier mobility. A gate voltage independent mobility is also observed in these devices which can be attributed to the highly conjugated nature of the C60 thin film.

Electrical response of highly ordered organic thin film metal-insulator-semiconductor devices

We report a detailed investigation of the electrical properties of organic field-effect transistors (OFETs) and metal-insulator-semiconductor (MIS) capacitors formed from highly ordered thin films of C60 as the active semiconductor and divinyltetramethyl disiloxane-bis(benzocyclobutene) (BCB) as the gate dielectric. Current-voltage measurements show the OFETs to be n-channel devices characterized by a high electron mobility (∼6 cm2/V s). An equivalent circuit model is developed which describes well both the frequency and voltage dependences of the small-signal admittance data obtained from the corresponding MIS capacitors. By fitting the circuit response to experimental data, we deduce that increasing gate voltages increases the injection of extrinsic charge carriers (electrons) into the C60. Simultaneously, the insulation resistance of the BCB decreases, presumably by electron injection into the insulator. Furthermore, the admittance spectra suggest that the capacitance-voltage (C-V) behavior originates ...

Correlation between morphology and ambipolar transport in organic field-effect transistors

Attaining ambipolar charge transport in organic field-effect transistors (OFET) is highly desirable from both fundamental understanding and application points of view. We present the results of an approach to obtain ambipolar OFET with an active layer of organic semiconductor blends using semiconducting polymers in composite with fullerene derivatives. Clear features of forming the super position of both hole and electron-enhanced channels for an applied gate field are observed. The present studies suggest a strong correlation of thin-film nanomorphology and ambipolar transport in field-effect devices.

Solution-processed CdS thin films from a single-source precursor

Herein we outline the in situ formation of a solution processable CdS precursor, which undergoes facile thermal decomposition to form thin films of the metal sulfide. Through the use of 1H and 13C nuclear magnetic resonance spectroscopy (NMR) and single crystal X-ray diffraction (XRD) we examine the speciation of [Cd(EtXn)2] upon dissolution and investigate the decomposition of the xanthate complex with Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis-mass spectroscopy (TGA-MS) and head space gas chromatography mass spectroscopy (HS GC-MS). The effect of using a CdCl2 treatment to promote crystallite growth at various temperatures is elucidated by thin film XRD, scanning electron microscopy (SEM), ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and UV-vis spectroscopy. Field effect transistors are fabricated to measure the mobility of the CdS thin films and spectral photoconductivity measurements reveal optimised deposition conditions give thin films with optoelectronic properties comparable to those of CdS formed by chemical bath deposition.

Switching in C60-fullerene based field effect transistors

We are reporting on the electrical properties of a bottom gate C60-fullerene based n-channel organic field effect transistor. The C60 thin film was epitaxially grown using hot wall epitaxy on top of an organic dielectric divinyltetramethyldisiloxane-bis(benzocyclobutene). The device performance depends on the growth parameters during the C60 film growth. Optimization of the growth parameters leads to a C60 film of a low total number of traps, and the drain-source current is increased by two orders in magnitude. We propose that the high current-densities are caused by space charge limited currents beside the gate induced space charge.

N -Alkyl functionalized barbituric and thiobarbituric acid bithiophene derivatives for vacuum deposited n-channel OFETs

A family of barbituric and thiobarbituric acid end capped small molecule semiconductors were synthesized, characterized and shown to exhibit n-channel organic thin film transistor properties. By changing the N-alkyl substituent from methyl to ethyl, a dramatic increase in electron mobilities was observed with values nearing 0.3 cm2 V−1 s−1.