Analytical device models for disordered organic Schottky diodes and thin-film transistors are presented. The models are developed taking into consideration the strong dependency of the charge mobility on carrier concentration. The drain current expressions are consequently developed in terms of the essential device parameters and applied voltages, to a power exponent of the characteristic temperature associated with the disordered nature of the semiconductor. Upon validation, better agreement of the experimental data is achieved with the disordered model rather than the conventional crystalline equation. Interestingly, under certain conditions, the disordered model reverts back to the conventional model, suggesting the latter to be a special case. Finally, to facilitate the circuit development, alternative design parameters to the mobility term are proposed.

Carbon based materials are considered to be amongst the most promising field emitters for use in displays. Semiconducting polymer (regioregular poly3 octylthiophene) has been found by the authors to have the lowest threshold field reported to date for any carbon based material. The emission process is, we believe, associated with surface irregularities, voids, possibly resulting from solvent evaporation. Field intensification occurs, and as a consequence there is field emission from the top ridges of the voids. The current flowing through these regions is controlled by space charge in traps in the upper half of the energy gap. There is, in addition, current injected from the base of the voids and the two currents mix to give field emission with current IaV1.8 where V is the anode voltage. Measurements on capacitor samples using Aluminium, Gold and Calcium electrodes have been used to examine the conduction processes in the material and these are linked to results of field emission to further illuminate the mechanisms involved and provide a model for the emission process.

We present analytical models for organic thin film transistors (OTFTs) and Schottky diodes based on polycrystalline semiconductors. The OTFT model is developed using a well-established approach previously developed for polysilicon, with slight modification for organics. The model predicts voltage and temperature dependencies on the various device and circuit parameters. A good agreement is obtained with experimental data of TIPS-based OTFTs. Essential parameters such as the characteristic temperature and Meyer-Neldel Energy extracted using the model with TIPS OTFTs data were in agreement with those obtained from Schottky diode measurements.

We have developed an analytical model for polycrystalline-based organic thin-film transistors (OTFTs) that employs, as far as possible, new concepts on carrier injection to the conventional polysilicon model. The drain current equations, both in diffusion and drift regimes, predict the voltage and temperature dependencies on the various device and circuit parameters. Interestingly, upon direct comparison with previously developed disordered model, similarities between the two are not thought to be coincidental. The effect of gate voltage on surface potential is affected by the Fermi level pinning in the grain boundary, which is assumed to consist of mainly disordered material. This work also highlights the problem of using drift mobility, as an organic circuit design parameter, and consequently alternative quantities are proposed for simpler circuits such as an inverter. Upon validation of the model, relatively good fits are obtained with the experimental data on TIPS-based TFTs. The divergence at low drain voltages are thought to be associated with short channel and/or high contact resistance effects.

Organic materials and devices are gaining more and more attention in microelectronics. They are dedicated to low cost applications and easy in fabrications. Device structure and organic material characteristics are key points to achieve targeted products. Considering technology and product specifications, compact organic device modeling and circuit design are necessary to evaluate technology performance in RFID tags. This model is based on variable range hopping theory (VRH), i.e., a carrier may either hop over a small distance with a high activation energy or hop over a long distance with a low activation energy. The model has been developed using a physical basis where the model parameters can easily be extracted and it improves convergence in circuit simulations. It is also suitable for computer aided design (CAD) applications.

A model for predicting the change of currents at the surface of polycrystalline materials for both ohmic and blocking contacts is developed. The model includes electron/hole traps within grain boundaries that are comparable in thickness to that of the dielectric on the surface of the polycrystalline semiconductor. The grains and their interfaces with the dielectric are assumed to be trap free. Account is also taken of the reducing carrier Debye Length as the surface carrier concentration is increased, from its intrinsic value, by the field effect. The net surface conductance is obtained by integrating the carrier density across the surface region through to the back of the material. Four regimes are identified: quasi-drift and quasi-diffusion for the high and low current regimes when there is a good supply of carriers and generation and quasi-diffusion when there is a limited supply of carriers. The analytical relationships are found to give satisfactory agreement with results for the temperature and field dependence of surface conductance in polycrystalline silicon in these regimes. The dependence of surface conductance on field effect voltage is found, at lower currents, to be a means of determining the energy distribution of electron/hole traps.

In this paper, we develop a device model of an organic thin film transistor for a circuit design, more specifically, for organic radio frequency identification applications. This model is based on variable range hopping theory, i.e., a carrier may either hop over a small distance with a high activation energy or hop over a long distance with a low activation energy. The model takes into account all the operating regimes in direct current and transient mode; the transistor symmetry is also considered. The model has been developed using a physical basis where the model parameters can easily be extracted and it improves convergence in circuit simulations. It is also suitable for computer aided design applications.

Carbon is as natural an electronic material as silicon, and it has also the advantages that come from the multiplicity of forms available. We concentrate on the properties and potential uses of carbon in nanotubes, polymers and composites of the two. Unlike silicon, organic molecules can produce light of almost any wavelength and also multiple wavelengths including white. It is likely to be used initially for LCD backplanes. Nanotubes have shown potential for use in cold cathodes. Progress towards using them in displays is slow. However, commercial use in microwave resonators seems to be imminent. There has been substantial progress in understanding the use of conjugated polymers in photovoltaics, where polymer composites with nanotubes or blends of organic semiconductors promise to continue progress to greater efficiency and cost effectiveness. The understanding of fundamental processes has yet to develop to a point where it is of direct value to electronics. Simplifications based on semi-classical device physics appear to be able to short circuit these difficulties.

We have approximated the tail of the Gaussian distribution of states of organic semiconductors with an exponential function. We have used this approach to calculate the carrier concentration in organic materials, and subsequently the charge distribution in the accumulation region of a field effect device, the effective width of an accumulation layer (Debye length), and the space-charge capacitance in accumulation mode. Small signal high frequency capacitance-ramp voltage measurements performed at various temperatures show good agreement with this model and the characteristic temperature of the exponential function has been estimated from these measurements based on the theory developed.

We have approximated the tail of the Gaussian distribution of states of organic semiconductors with an exponential function. We have used this approach to calculate the carrier concentration in organic materials, and subsequently the charge distribution in the accumulation region of a field effect device and the space-charge capacitance in accumulation mode. Small signal high frequency capacitance-voltage measurements performed at various temperatures show good agreement with this model and the characteristic temperature of the exponential function has been estimated from these measurements based on the theory developed.

The negative bias temperature instability (NBTI) of MOS devices is one of the earliest identified reliability problems [l]. It appears in two forms: interface state generation and positive charge formation in the gate oxide [l-31. Some researchers [3] suggested that the generated interface states and positive charges were two products of the same electrochemical reaction. Holes and water-related species were believed to be two of the reactants.

Organic Schottky diodes are widely used in a number of real applications such as OLEDs according to J. H. Burroughhes et al. (1990), G. Gustafsson et al. (1992) and M. Granstrom et al. (1995) and also as an integral part of RF tags. Schottky diodes also provide a simpler way of obtaining the doping density and bulk mobility of the charge carriers in a material. It is thus important to understand and consequently be able to model their behavior. With the aid of experimental data, the behavior of polymer Schottky diodes is analyzed here in detail.

Polymer electronics increasingly needs circuit design tools based on simple but accurate device models. It also needs scaling rules and ultimately its own form of Moore's Law. Such models must include accurate conduction equations. Here we begin the development of device models based on the Universal Mobility Law which is itself the basis of roadmapping. This law is the physical manifestation of variable range hopping and this is widely recognized as a dominant mechanism, except perhaps at the highest doping levels. By interpreting this law in terms of the relationship between mobility and carrier density the gradual channel equation is redeveloped below and above pinch-off. It is immediately apparent that mobility is not an appropriate measure of the speed of circuits. Two new parameters K1 and m are introduced. They can be found from accurate measurements on Schottky barriers and give the maximum possible performance. Real performance is always less than this. The values of K and m for a particular process can be assessed above pinch-off and with stable polymer they can be used to accurately predict the output characteristics.

We present a study of aqueous and plasma anodised aluminium oxide (Al2O3) and its performance in thin film transistors (TFTs). The current through the oxide was measured with aluminium electrodes and with one of the electrode replaced by poly(3-hexylthiophene)(P3HT). The current increased by up to 2 orders of magnitude with P3HT. The current increased further when the polymer was doped with different percentages of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). It was also found to be dependent on the thickness of the polymer film. Surprisingly, the oxide current fell to its initial value when the polymer film was removed. Two mechanisms may explain the behaviour in these devices: charge injection and/or displacement. C-V plots were obtained from the MOS capacitors and were frequency dependent. They also showed substantial hysteresis, with a lateral shift along the voltage axis. This indicates the presence of a mobile species that increases with the concentration of dopant. We deduce that much of the increased gate current is associated with displacement currents induced by ion motion.

There is a need to understand carrier generation polysilicon TFTs. When the generation is stimulated by the depletion of the material, due to the field effect, carriers are drawn to the surface and move along it by a process akin to diffusion. At some gate voltage the supply of carriers to the channel is insufficient to maintain the current flow. This leads to a reverse bias across the drain junction. It is proposed that carrier generation that occurs mainly near to the drain is more likely to be stimulated by increases of drain rather than gate voltage. The temperature dependence of currents at the current minimum, and at current saturation, of the transfer characteristic are described and are used to justify the model for generation and flow.

An increasing range of high-K dielectric is becoming available and it is very worth considering incorporating them into polymer TFTs. One of the benefits is that if metal gates are used then aqueous anodisation provides a very simple approach that is compatible with solution based processing. The details of this process are described. High-K dielectrics reduce threshold voltage and, therefore, increase switching speed. Of particular importance is the problem of bias instability. All the results involve the fractionation and controlled doping of poly-3-hexylthiophene.

Highly regio-regular poly(3-hexylthiophenes) (P3HT) thin films have been produced using the Tzrnadel method. They have head to tail counts approaching very close to 100% and high molecular mass. Thin-film transistors and Schottky diodes have been used to study the effects of counter ion and carrier density on field-effect and bulk mobility, respectively. The density of counter ions was increased using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and had a profound effect on both the field effect and bulk mobility with a ratio between the two of 10(2), in a similar way to DDQ in poly(beta'-dodecyloxy-alpha,alpha',-alpha',alpha'terthienyl) (polyDOT(3)), but with substantially higher drift mobilities. A method is described, using Schottky barriers, of simply and accurately determining carrier drift mobility. The effect of carrier density on hole mobility is believed to be indirect, filling traps in the regions separating the highly ordered domains until trap free conduction and hence high mobility is reached. (C) 2002 American Institute of Physics.

Measurements and theory are presented examining the relationship between mobility and doping in regioregular poly(3-hexylthiophene) (P3HT). Mobility is found to increase super-linearly with doping and is comparable to models reported for other conjugated polymers. Schottky measurements have been used to calculate the doping density and bulk mobility of regioregular P3HT. Aluminium Schottky contacts showed signs of native oxide disrupting current flow through the device. This effect was observed to degrade further with the introduction of dopant into the polymer. Titanium devices show a general shift of the Schottky characteristic to higher current levels with increased dopant. Field effect mobility of P3HT films was also calculated using thin-film transistor (TFT) structures. The field effect mobility values were observed to be more than two orders of magnitude higher than the bulk mobility value. The addition of dopant also increased gate leakage currents in TFT devices. The increased conductivity in doped polymer can increase off currents in the device; this is avoided by using Schottky contacts as the source and drain. Preliminary results on Schottky contact TFTs are also presented as well as a description of the operation of such a device.

By using the Trznadel method, it has been possible to increase molecular weight, the regio-regularity, and probably to decrease the residual catalyst of poly-3-hexylthiophene thin films. The drift mobility of holes, normal to the surface of cast films, in air, has been found using Schottky diodes, and field-effect mobility has been measured with Thin-Film Transistors. Three types of film have been studied using the two methods of assessing mobility. The as-synthesised films are compared with those that have been fractionated. The third set of films involves doping with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The doped films show a field effect mobility of 0.2 cm 2 V -1 s -1 : all others being lower. Field effect mobility is approximately two orders of magnitude greater than that in the bulk normal to the plane of the film. Doping levels in the films are found to be similar, probably because of the process conditions.