J. A. Jiménez Tejada

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.

Unified model for the injection and transport of charge in organic diodes

This paper presents a unified model for charge injection and transport in organic or polymeric diodes. A special focus is placed on charge injection from the contacts, using existing models such as thermionic emission and tunnel injection. These are combined to produce a compact model that includes effects associated with oxidation/reduction reactions at the electrodes. Charges are formed at the electrodes by oxidation or reduction in a process governed by the Nernst equation, and these charges modulate the injection barrier. A current-voltage relationship for organic or polymeric diodes that incorporates these injection phenomena, as well as the charge transport by the drift mechanism, is presented and discussed. The voltage ranges in which these effects are dominant are estimated. Finally, the resulting current-voltage and current-temperature relations are used to explain published experimental data.

Charge transfer processes at the semiconductor/electrolyte interface for solar fuel production: insight from impedance spectroscopy

Knowledge of the nature of charge transfer processes at the semiconductor/electrolyte interface is crucial for the optimization of semiconductors used for solar fuel production. In the literature, there are two types of charge transfer mechanisms: (i) direct hole transfer from the valence band and (ii) indirect hole transfer via surface states. In this paper, we discuss both processes in the steady state regime through full drift-diffusion simulations considering the concomitant influence of the electric field and surface states at the semiconductor/electrolyte interface. We discuss the role of surface states and valence band holes in the photo-anodic current. We subsequently analyze both hole transfer processes in a dynamic regime via the impedance spectroscopy (IS) method. We provide a solid criterion to discriminate both mechanisms and discuss some experimental examples from the literature.

Compact Modeling and Contact Effects in Thin Film Transistors

A compact model for the current-voltage characteristics of organic thin-film transistors (OTFTs), which includes the effects of the contact regions, is proposed. Different physical and morphological aspects of contacts with organic or other emerging materials such as graphene, semiconducting dichalcogenides such as MoS2, or NW devices are described. The electrical behavior of the contacts is studied in OTFTs, and circuit models that describe them are reviewed. Two trends are observed in the current-voltage curves of the contacts of different OTFTs: linear and nonlinear, and different models are used to explain them. A unified model for the contact region that reproduces both trends and gathers the different physical and structural features of the contacts is developed. It is described by a single parameter and introduced in a generic analytical model for TFTs. The variability in OTFT structures, materials, and fabrication approaches gives rise to a strong variability in the values of the parameters of the model. In this regard, a characterization technique to determine the value of the parameters of the model from experimental data is also developed. Different physical tests are proposed to validate the results of the technique. The procedure is applied to recent experimental data for different pentacene-based transistors. The good agreement between the experimental data and our analytical results provides a way to relate the parameters of the model with the physical or geometrical origin of the contact effects in OTFTs.

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.

Evaluation of the charge density in the contact region of organic thin film transistors

This paper presents a procedure to evaluate the charge density in the low conductivity regions between the metal and the accumulated intrinsic channel of an organic thin film transistor (OTFT). This charge links different physical mechanisms in the contacts of OTFTs. The charge density is evaluated in transistors with different metal-organic barriers to study its dependence with the voltage, temperature and the materials forming the contact.

Theory of microplasma fluctuations and noise in silicon diode in avalanche breakdown

A theory that explains physically the mechanism of microplasma switching from nonconducting to conducting state (turn-on process) and from conducting to nonconducting state (turn-off process) is presented. This theory describes the microplasma phenomenon in avalanche breakdown, and it provides relations between the microplasma properties (size, current density, and turn-on-off mechanism), noise properties (amplitude and wave form), and semiconductor properties (impurity concentration, lifetime constants, and diffusion coefficients). By use of this theory, the current-fluctuation [McIntyre, IEEE Trans. Electron Devices ED-13, 164 (1966)] and probability-dependent [Haitz, J. Appl. Phys. 35, 1370 (1964)] theories can be combined, and a very attractive characterization tool for investigation of semiconductor junctions in breakdown is developed.

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.

High Doping Density/High Electric Field, Stress and Heterojunction Effects on the Characteristics of CMOS Compatible p-n Junctions

This paper critically reviews the different mechanisms impacting the current-voltage and capacitance voltage characteristics of complementary metal oxide semiconductor (CMOS) compatible p-n junctions. Special attention is given to the influence of high doping density/high electric fields, mechanical stress and the presence of a hetero-junction either at the junction or in the depletion region. The basic mechanisms reported in the literature are checked for their validity for state-of-the-art structures and processing techniques. Critical issues are pointed out and illustrated for advanced CMOS compatible hetero-junctions, where high-field effects, like trap-assisted tunneling (TAT) and band-to-band-tunneling (BTBT) play a prominent role. The presence of an isotype hetero-junction gives rise to frequency dispersion in the depletion layer capacitance, which becomes more pronounced in combination with grown-in or processing-induced defects at the hetero-interface. Finally, the challenges and opportunities for future devices are addressed.

Effects of oxygen related defects on the electrical and thermal behavior of a n+-p junction

This study examines the electrical and temperature behavior of two of the levels in Czochralski-grown silicon that are most detected by different authors. A comparison between an analytical expression of the generation recombination noise in p−n junctions with experimental data taken from other authors was used as a tool for determining capture cross sections and densities of oxygen related traps in silicon. The parameters found in the literature for a deep level located at EC−0.43 eV are verified in this work. Parameters for a shallower level, a Coulomb trap with an activation energy of 0.17 eV, are also obtained. To correlate our theoretical results, obtained by noise analysis, with those of other authors, obtained with different techniques, thermally activated and field enhanced transitions between the latter center and the conduction band are considered. A thorough theoretical study of a silicon p−n junction with these levels shows a mutable electrical operation of the shallower center, acting as an e...