S. Rodríguez-Bolívar

Model for the injection of charge through the contacts of organic transistors

A compact model has been employed in organic thin film transistors (OTFTs) to study the electrical characteristics of the contacts, which are formed between the organic layer and source/drain electrodes of the OTFT. The model shows the importance of interrelating different physical phenomena: charge injection, redox reactions at the interface, and charge drift in the organic semiconductor. The model reproduces and explains several features that have been reported for current-voltage curves, ID-VC, at the contacts of OTFTs. The ID-VC curves are extracted from the experimental output characteristics by two techniques. One technique uses a set of transistors with different channel lengths and a simultaneous extraction of the ID-VC curve and the mobility of carriers in the channel of the transistor. When a set of transistors with different channel lengths is not available, we propose an iterative method for the simultaneous extraction of the ID-VC curve and the mobility by changing the gate bias voltages.

Simple analytical valence band structure including warping and non-parabolicity to investigate hole transport in Si and Ge

This work proposes an analytical modelling of non-parabolicity for the valence bands of Si and Ge. With this aim, we obtained piecewise functions that enabled analytical evaluation of the energy after a free flight under drift, avoiding iterative procedures. In addition, particular attention was devoted to solving the problem of discontinuities in the density of states in bulk semiconductors, a shortcoming that had emerged in earlier approaches. Using our analytical functions, we were able to evaluate the mean hole energies at equilibrium and compare them with previous studies based on the pseudopotential method, obtaining an excellent agreement. We also used our analytical expressions in a single-particle Monte Carlo simulator to obtain the drift velocity under an external electric field and ohmic mobilities in pure Si and Ge. These results were compared with the experimental data and showed satisfactory agreement in all cases.

A solution of the effective-mass Schrödinger equation in general isotropic and nonparabolic bands for the study of two-dimensional carrier gases

In this paper we develop a suitable method for solving the effective-mass Schrodinger equation for two-dimensional electron and hole gases in semiconductor structures such as quantum wells using a general nonparabolic band structure. We present two different ways to treat barriers, the first being the exact solution and the second a suitable option when the band structure is not determined inside the gap. As a first application, this procedure was implemented to solve the effective-mass Schrodinger equation for holes in Si and Ge using an analytical valence-band model. Analyzing the results obtained enabled us to demonstrate the importance of nonparabolicity in energy quantization in these systems and to discuss the suitability of each of these two procedures for dealing with barriers.

Dependence of Generation–Recombination Noise With Gate Voltage in FD SOI MOSFETs

A model for computing the generation-recombination noise due to traps within the semiconductor film of fully depleted silicon-on-insulator MOSFET transistors is presented. Dependence of the corner frequency of the Lorentzian spectra on the gate voltage is addressed in this paper, which is different to the constant behavior expected for bulk transistors. The shift in the corner frequency makes the characterization process easier. It helps to identify the energy position, capture cross sections, and densities of the traps. This characterization task is carried out considering noise measurements of two different candidate structures for single-transistor dynamic random access memory devices.

Effects of Gate Oxide and Junction Nonuniformity on the DC and Low-Frequency Noise Performance of Four-Gate Transistors

The effects of imperfections on the electrical performance of four-gate field-effect transistors (G4-FETs) have been studied. Variations in the oxide trap distribution and in the metallurgical boundary of the junction gates impact the low-frequency noise and the static (dc) performance of the G4-FET. By modeling, iterative characterization of published experimental data, and extensive simulations, it is shown that these effects originate from trap distributions in the gate oxides and in the depleted regions of the semiconductor channel. The proposed models are based on established models, such as the unified flicker noise model, with modifications and improvements that extend to trap distributions with gradients, variable frequency slope α of 1/fα noise spectra, and are applicable for gate stacks with high-k dielectrics, such as HfO2 and HfSiON. The characterization procedures allowed for identifying optimum profiles of the metallurgical boundary of junction gates, which simultaneously improve the dc and noise performances of the G4-FET, such as subthreshold swing and low noise. The results indicate the importance of the precise control of depletion and conduction in the channels of multiple-gate FETs.

Implications of nonparabolicity, warping, and inelastic phonon scattering on hole transport in pure Si and Ge within the effective mass framework

Hole mobility over a wide range of temperatures in pure Si and Ge is studied within the framework of effective mass theory using the Monte Carlo method. With this aim, we have implemented a three-band model (heavy, light, and split-off holes) introducing nonparabolicity even for the latter, which is usually considered parabolic in the literature. The warping in the heavy and light bands was taken into account, maintaining a spherical model for the split-off band. We also developed scattering rate expressions to be used in a Monte Carlo procedure with the nonparabolicity and warping effects included explicitly in the scattering rate expressions, an aspect neglected in the literature. In so doing, we calculated exactly the nonparabolicity functions for the valence band from the expressions provided by Kane [J. Phys. Chem. Solids 1, 82 (1956)]. Further, we modeled the acoustic phonons on an inelastic mechanism, generalizing previous work, and applying a temperature-dependent average to obtain typical values ...

Determination of the concentration of recombination centers in thin asymmetrical p-n junctions from capacitance transient spectroscopy

Recombination centers in thin asymmetrical p-n junctions were analyzed in the context of capacitance transient experiments. The combined effect of the thin low-doped region of the junction and the nonzero value of the occupation factor of the recombination center in the depletion layer caused electrons and holes to be simultaneously released from different parts of this layer during an emission transient. The need to introduce modifications in the analytical expressions that determine the parameters of these centers by capacitance transient spectroscopy is demonstrated. A correction formula to determine concentrations of electron or hole traps or recombination centers is proposed.

A Low-Frequency Noise Model for Four-Gate Field-Effect Transistors

In this paper, a model is presented for the low-frequency noise in four-gate FETs (G4-FETs). It combines volume and surface noise sources. The generation recombination noise in the volume of the device originates from fluctuations of trapped charge in the depletion regions. We propose a model for calculating this noise component by evaluating the fluctuation in the cross section of the transistor conducting channel. Drain-current fluctuations due to trapping and detrapping of electrons at interface traps are also incorporated in the model and adapted for mixed surface-volume conduction. The global power spectral density of the drain-current, including both noise sources, is evaluated in different operating modes of the transistor. Our numerical results show good agreement with the experimental results of other authors. A study of the different kinds of center in the semiconductor (traps and recombination centers) allows the interpretation of experimental data. We explain the different trends observed, both numerically and experimentally, in the representation of the total noise current as a function of the drain-current in different operating modes.

Influence of the number of anchoring groups on the electronic and mechanical properties of benzene-, anthracene- and pentacene-based molecular devices.

One of the central issues of molecular electronics (ME) is the study of the molecule-metal electrode contacts, and their implications for the conductivity, charge-transport mechanism, and mechanical stability. In fact, stochastic on/off switching (blinking) reported in STM experiments is a major problem of single-molecule devices, and challenges the stability and reliability of these systems. Surprisingly, the ambiguous STM results all originate from devices that bind to the metallic electrode through a one-atom connection. In the present work, DFT is employed to study and compare the properties of a set of simple acenes that bind to metallic electrodes with an increasing number of connections, in order to determine whether the increasing numbers of anchoring groups have a direct repercussion on the stability of these systems. The conductivities of the three polycyclic aromatic hydrocarbons are calculated, as well as their transmission spectra and current profiles. The thermal and mechanical stability of these systems is studied by pulling and pushing the metal-molecule connection. The results show that molecules with more than one connection per electrode exhibit greater electrical efficiency and current stability.

Miniband structure and photon absorption in regimented quantum dot systems

In this paper, we investigate the physics of electronic states in cubic InAs quantum dot periodic nanostructures embedded in GaAs. This study aims to provide an understanding of the physics of these systems so that they may be used in technological applications. We have focused on the effect of dot densities and dot sizes on the material properties, evaluating the miniband structure of electron states coming from the bulk conduction band, and have calculated the intraband photon absorption coefficient for several light polarizations. Strain is included in this analysis in order to obtain the conduction band offset between the materials by solving the Pikus-Bir 8×8 k·p Hamiltonian. We offer a comparison with approaches used by previous authors and clarify their range of validity. Finally, we draw our conclusions and propose future technological applications for these periodic arrangements.