Nonlinear electro-thermal modeling and field-simulation of OLEDs for lighting applications I: Algorithmic fundamentals

Large area OLEDs aimed at lighting applications should provide homogeneous luminance—homogeneity is one of the quality metrics of such devices. Local light generation depends on both the local temperature and the local voltage drop across the light emitting polymer(s) in the device. Therefore the thermal and electrical engineering of OLEDs aimed at lighting applications is critical. Due to the large area of these devices the coupled electrical and the thermal simulation problem is of distributed nature. Electrical characteristics of organic semiconductor materials used in OLED devices are highly nonlinear, and their nonlinear temperature-dependence is significant. In our present approach to distributed electro-thermal field simulation we address special needs of OLEDs, which is not yet the case with widely used, commercially available simulation tools. In this paper we present the latest version of our SUNRED electro-thermal field solver algorithm capable of handling coupled, non-linear electro-thermal problems. The new features of the algorithm are demonstrated by modeling some research OLED samples available to us in the Fast2Light project—this way simulation results are compared against measured data.     

Shape reconstruction from PO multifrequency scattered fields viathe singular value decomposition approach

This paper deals with the problem of determining the shape of unknown perfectly conducting infinitely long cylinders, starting from the knowledge of the scattered electric far field under the incidence of plane waves with a fixed angle of incidence and varying frequency. The problem is formulated as a nonlinear inverse one by searching for a compact support distribution accounting for the objects contour. The nonlinear unknown to data mapping is then linearized by means of the Kirchhoff approximation, which reduces it into a Fourier transform relationship. Then, the Fourier transform inversion from incomplete data is dealt with by means of the singular value decomposition (SVD) approach and the features of the reconstructable unknowns are investigated. Finally, numerical results confirm the performed analysis     

Simulating the dynamic electrothermal behavior of power electroniccircuits and systems

The simulator solves for the temperature distribution within the semiconductor devices, packages, and heat sinks (thermal network) as well as the currents and voltages within the electrical network. The thermal network is coupled to the electrical network through the electrothermal models for the semiconductor devices. The electrothermal semiconductor device models calculate the electrical characteristics based on the instantaneous value of the device silicon chip surface temperature and calculate the instantaneous power dissipated as heat within the device. The thermal network describes the flow of heat from the chip surface through the package and heat sink and thus determines the evolution of the chip surface temperature used by the semiconductor device models. The thermal component models for the device silicon chip, packages, and heat sinks are developed by discretizing the nonlinear heat diffusion equation and are represented in component form so that the thermal component models for various packages and heat sinks can be readily connected to one another to form the thermal network     

A DC behavioral electrical model for quasi-linear spin-valve devices including thermal effects for circuit simulation

An advanced model for quasi-linear spin-valve (SV) structures is presented for circuit simulation purposes. The model takes into account electrical and thermal effects in a coupled way in order to allow a coherent representation of the sensor physics for design purposes of electronics applications based on these sensor devices. The model was implemented in Verilog-A and used in a commercial circuit simulator. For testing the model, different SV structures have been specifically fabricated and measured. The characterization included DC measurements as well as steady-state and transient thermal analysis. From the experimental data, the parameters of the model have been extracted. The model reproduces correctly the experimental measurements obtained for devices with diverse sizes in different electrical and thermal operation regimes.     

Experimental Observation of Chaotic Dynamics in Two Coupled Diode Lasers Through Lateral Mode Locking

The nonlinear and chaotic phenomena in lateral coupled diode lasers have been widely studied theoretically. In this work an experimental analysis of the complex nonlinear and chaotic dynamic regimes experimentally observed in these devices and the mechanisms that produces it is made. This analysis is set by means of the RIN electrical spectrum and of the high resolution Fabry–Perot optical spectrum. A mapping of the nonlinear occurrences in these devices is obtained and a study of the relation between the relaxation oscillation frequency and the lateral locking frequency is achieved.      

Transient distributed parameter electrical analogous model of TE devices

This paper describes a one-dimensional distributed parameter transient model for thermoelectric devices implemented using analogies between the thermal and electrical domains, where thermal variables are described by their electrical analogues. The resulting electrical network can be tested by means of an electrical simulation tool such as SPICE. This approach facilitates simulation of a thermoelectric module and its interconnections with electronic control circuits and other thermal elements under varying boundary and initial conditions. Capabilities of the model are illustrated from simulations carried out for a free-standing thermoelectric element during the pulse cooling operation. Simulation results fit well with those obtained using other models reported in the literature as well as with numerical solutions.     

On the analysis of nonlinear resistive networks considering the effect of temperature

In this correspondence a new method is presented for the analysis of nonlinear resistive networks considering the effect of the dissipated-power-dependent inner temperature of electronic devices on the electrical behaviour of circuits. It is shown that this consideration is only possible if the equations of the electrical network are extended by equations of the corresponding thermal network of the circuit and if all equations together are solved simultaneously. The temperature is introduced as variable and not as a parameter.     

Genetic algorithms as applied to the numerical computation ofelectromagnetic scattering by weakly nonlinear dielectric cylinders

This paper deals with the application of an optimization procedure based on a genetic algorithm (GA) to the prediction of the electromagnetic fields scattered by weakly nonlinear dielectric objects. Starting by an integral approach and describing the nonlinearities of the constitutive parameters by the Volterra-type integrals, the nonlinear scattering problem is numerically solved by an iterative procedure developed for the minimization of a suitable defined cost function. A GA is applied in order to deal with a large number of unknowns related to the harmonic components of the nonlinear internal electromagnetic field. In a preliminary stage, the behavior of typical parameters of the GA is analyzed; then numerical solutions are carried out and compared with those provided by other methods. Finally, some considerations are made concerning the rate of convergence of the iterative procedure     

Numerical methods for the charge transfer analysis of charge-coupled devices

Charge transfer phenomenon in charge-coupled devices is characterized by a nonlinear partial differential equation of the parabolic type, usually coupled with a very undesirable nonlinear boundary condition. In this study, special treatment is made to the boundaries such that the nonlinearity of the boundary condition does not appear in the final calculation. Four possible finite-difference schemes for this problem are described and results compared. Through numerical experimentations, the linearized Crank-Nicolson scheme is proved to exhibit superior quality and is recommended for the exclusive use in studying the charge transfer phenomenon in CCD. Using this scheme, the charge transfer phenomenon of a two phase overlapping gate CCD has been studied and numerical results are presented. Special emphasis is directed toward the relative importance of the self-induced drift, fringing field drift and thermal diffusion currents. Also, the usefulness of approximating a spatial fringing field pattern by a constant value to the charge transfer phenomenon is discussed.     

Requirements for accurate MOS-SOI device simulations

Simulations of the electrical behavior of MOS-SOI devices pose a difficult numerical problem due to the floating substrate region. The numerical analysis techniques required to solve the floating region problem are discussed. Models for the carrier mobilities and lifetime variation with depth into the silicon film are introduced to fit measured SOS device data. The current-voltage characteristics of SOS transistors, including the kink, are accurately simulated and compared to measurements. The floating potential variation with applied gate and drain bias predicted by the simulation is discussed     

Simulation and Design of Microwave Class-C Amplifiers through Harmonic Analysis

A method for analyzing microwave class-C amplifiers is proposed which satisfies the requirements of a wide application field, and, at the same time, operates with a fast runing time and without convergence problems. It is based on the partitioning of the circuit into linear and nonlinear subnetworks for which, respectively, frequency-domain and time-domain equations are written. Then, taking into account that the time-domain and frequency-domain representations are related by the Fourier series, the circuit behavior is described by means of a system of nonlinear equations whose unknowns are the harmonic components of the incident waves at all the connections. To overcome the numerical problems arising in the search for the solution of this system when strong nonlinearities are involved, a special step-by-step procedure is adopted. The problem is transformed into the search for the solution of a sequence of well-conditioned systems of equations corresponding to a sequence of well-chosen circuits obtained from the original one through progressive changes of the input signal starting from 0 up to the nominal value. The program which implements the method is also described and the results of the analysis relative to a class-C amplifier are compared with measured values.     

Thermal resistance calculation of AlGaN-GaN devices

We present an original accurate closed-form expression for the thermal resistance of a multifinger AlGaN-GaN high electron-mobility transistor (HEMT) device on a variety of host substrates including SiC, Si, and sapphire, as well as the case of a single-crystal GaN wafer. The model takes into account the thickness of GaN and host substrate layers, the gate pitch, length, width, and thermal conductivity of GaN, and host substrate. The model's validity is verified by comparing it with experimental observations. In addition, the model compares favorably with the results of numerical simulations for many different devices; very close (1%-2%) agreement is observed. Having an analytical expression for the channel temperature is of great importance for designers of power devices and monolithic microwave integrated circuits. In addition, it facilitates a number of investigations that are not practical or possible using time-consuming numerical simulations. The closed-form expression facilitates the concurrent optimization of electrical and thermal properties using standard computer-aided design tools.     

An iterative Newton-Raphson method to solve the inverse admittivityproblem

By applying electrical currents to the exterior of a body using electrodes and measuring the voltages developed on these electrodes, it is possible to reconstruct the electrical properties inside the body. This technique is known as electrical impedance tomography. The problem is nonlinear and ill conditioned meaning that a large perturbation in the electrical properties far away from the electrodes produces a small voltage change on the boundary of the body. This paper describes an iterative reconstruction algorithm that yields approximate solutions of the inverse admittivity problem in two dimensions. By performing multiple iterations, errors in the conductivity and permittivity reconstructions that result from a linearized solution to the problem are decreased. A finite-element forward-solver, which predicts voltages on the boundary of the body given knowledge of the applied current on the boundary and the electrical properties within the body, is required at each step of the reconstruction algorithm. Reconstructions generated from numerical data are presented that demonstrate the capabilities of this algorithm     

Superconductive Traveling-Wave Photodetectors: Fundamentals and Optical Propagation

This paper studies the theory of superconductive traveling-wave photodector devices (STWPDs), as a general platform for ultrafast, ultrasensitive, and ultralow-noise optoelectronic functions such as detection, modulation, photomixing, high-frequency electrical signal generation and amplification. Principles of operation of STWPDs are discussed based on the kinetic-inductance theory of superconductive thin films and a thorough investigation of the guiding mechanism of light within this class of devices is presented. The transfer matrix method is introduced to model the superconductive optical waveguide in TWDs and an efficient numerical method based on the Cauchy integral method and the argument principle method are developed for the analysis and design of the waveguides. Moreover, several regimes of device operation will be distinguished in terms of the modal characteristics of the optical waveguide and their effects on the overall performance of the device will be highlighted.     

Small-signal operation of semiconductor devices includingself-heating, with application to thermal characterization andinstability analysis

A rigorous mathematical treatment of dynamic self-heating in semiconductor devices is presented. Two formulations for the admittance parameters are given. The thermal behavior of the device is referred to device temperature in the first formulation, and to ambient temperature in the second. Contrary to previous work, nonlinear thermal effects are included. An analytical model for the thermal resistance is derived which confirms the relevance of these effects. Applications of the above results to device modeling and thermal characterization are studied in detail by means of numerical simulations. Possible sources of inaccuracies are evidenced. Finally, it is shown that the differential analysis of thermal feedback provides a general and rigorous means to determine the conditions for the onset of thermally-induced instabilities     

Fully coupled dynamic electro-thermal simulation

Fully coupled dynamic electro-thermal simulation on chip and circuit level is presented. Temperature dependent thermal conductivity of silicon is taken into account, thus solving the nonlinear heat diffusion equation. The numerical solution is carried out by using the industry-standard simulator SABER, therefore for electro-thermal simulations we are able to use the common electrical compact models by adding a heat source and thermal pins to them. The application of this technique and need for electro-thermal simulation is illustrated with the simulation of a current control circuit built into a multiwatt package     

Analysis and simulation of the effects of distributed nonlinearities in microwave superconducting devices

This paper presents a comprehensive study of microwave nonlinearities in superconductors, with an emphasis on intermodulation distortion and third-harmonic generation. It contains the analysis of various resonant and nonresonant test devices and its validation using numerical simulations based on harmonic balance (HB). The HB simulations made on test devices show that the closed-form equations for intermodulation and third-harmonic generation are only valid at low power levels. The paper also contains examples of application of HB to illustrate that this technique is useful to simulate superconductive devices other than simple test devices, and that the validity of the simulations is not restricted to low drive power levels. Most of the analyses and simulations of this paper are based on electrical parameters that describe the nonlinearities in the superconducting material. These parameters are compatible with many existing models of microwave nonlinearities in superconductors. We discuss the particulars on how to relate these electrical parameters with one of the existing models that postulates that the nonlinear effects are due to a dependence of the penetration depth on the current density in the superconductor.     

Thermal modeling of power gallium arsenide microwave integratedcircuits

Manufacturers are developing power devices for ever higher frequencies using GaAs MESFETs and heterojunction bipolar devices constructed with III-V compounds on GaAs substrates, as well as integrated power devices on monolithic microwave integrated circuits (MMICs). A problem with the technology is the low thermal conductivity of gallium arsenide, giving rise to thermal design problems that must be solved if good reliability is to be achieved. A three-dimensional numerical simulator is used to study this problem. In particular, the approximations which are possible in performing realistic assessments of the thermal resistance of typical GaAs power device structures under steady-state conditions are examined     

General noise analysis of nonlinear microwave circuits by thepiecewise harmonic-balance technique

This paper presents a self-consistent set of algorithms for the numerical computation of noise effects in forced and autonomous nonlinear microwave circuits. The analysis relies upon the piecewise harmonic-balance method, and thus retains all the peculiar advantages of this technique, including general-purposeness in the widest sense. The noise simulation capabilities include any kind of forced or autonomous nonlinear circuit operated in a time-periodic large-signal steady state, as well as microwave mixers of arbitrary topology. The limitations of the traditional frequency-conversion approach to noise analysis are overcome. The analysis takes into account the thermal noise generated in the passive subnetwork, the noise contributions of linear and nonlinear active devices, and the noise injected by sinusoidal driving sources of known statistical properties. The nonlinear noise models of two representative families of microwave devices (FET's/HEMT's and Schottky-barrier diodes) are discussed in detail, and several applications are illustrated