Large-signal device simulation in time- and frequency-domain: a comparison

The aim of this paper is to compare the most common time- and frequency-domain numerical techniques for the determination of the steady-state solution in the physics-based simulation of a semiconductor device driven by a time-periodic generator. The shooting and harmonic balance (HB) techniques are applied to the solution of the discretized drift-diffusion device model coupled to the external circuit embedding the semiconductor device, thus providing a fully nonlinear mixed mode simulation. The comparison highlights the strong and weak points of the two approaches, basically showing that the time-domain solution is more robust with respect to the initial condition, while the HB solution provides a more rapid convergence once the initial datum is close enough to the solution itsel

Extraction of Mobility from Quantum Transport Calculations of Type-II Superlattices

Type-II superlattices (T2SLs) are being investigated as an alternative to traditional bulk materials in infrared photodetectors due to predicted fundamental advantages. Subject to significant quantum effects, these materials require the use of quantum transport methodologies, such as the nonequilibrium Green’s function (NEGF) formalism to fully capture the relevant physics without uncontrolled approximations. Carrier mobility is a useful parameter that affects carrier collection in photodetectors. This work investigates the application of mobility extraction methodologies from quantum transport simulations in the case of T2SLs exemplified using an InAs/GaSb midwave structure. In a resistive region, the average velocity can be used to calculate an apparent mobility that incorporates both diffusive and ballistic effects. However, the validity of this mobility for predicting device properties is limited to cases of diffusive limited transport or when the entire device can be included in the simulation domain. Two methods that have been proposed to extract diffusive limited mobility, one based on approximating the ballistic component of transport and the other which considers the scaling of resistance with simulation size, were also studied. In particular, the resistance scaling approach is demonstrated to be the method most physically relevant to predicting macroscopic transport. We present a method for calculating the mobility from resistance scaling considerations that accounts for carrier density variation between calculations, which is particularly relevant in the case of electrons. Finally, we comment on the implications of applying the different mobility extraction methodologies to device property predictions. The conclusions of this study are not limited to T2SLs, and may be generally relevant to quantum transport mobility studies

A generalized drift-diffusion model for rectifying Schottky contact simulation

We present a discussion on the modeling of Schottky barrier rectifying contacts (diodes) within the framework of partial-differential-equation-based physical simulations. We propose a physically consistent generalization of the drift-diffusion model to describe the boundary layer close to the Schottky barrier where thermionic emission leads to a non-Maxwellian carrier distribution, including a novel boundary condition at the contact. The modified drift-diffusion model is validated against Monte Carlo simulations of a GaAs device. The proposed model is in agreement with the Monte Carlo simulations not only in the current value but also in the spatial distributions of microscopic quantities like the electron velocity and concentratio

Quale insegnamento dalla "epopea" dell'amianto?

What lessons can be drawn from the “epic history” of asbestos? How this emblematic case can help the reflection on prevention in the workplace? The subjects of primary prevention, risk management, laws and regulation protecting safety and health in the workplace are discussed from different points of view, such as history of medicine, epidemiology, occupational and preventive medicine, labour law, organization theory, science methodology. These collected papers are aimed at proposing prevention strategies allowing to avoid mistakes and pitfalls like the ones that characterized the history of asbestos

Modeling the Conductor Losses of Thick Multiconductor Coplanar Waveguides and Striplines: A Conformal Mapping Approach

A conformal-mapping approach to model the skin-effect conductor losses of thick multiconductor coplanar waveguides and multiconductor coplanar strips is proposed. The model allows for arbitrary strip or slot number and widths and is accurate for conductor thicknesses up to around 40% of the minimum strip and slot width. Examples are presented to demonstrate the accuracy of the approach when compared to the results from a finite-element method numerical code

Modeling of single-mode high-power VCSEL arrays

In this work we investigate different architectures to realize a single-mode VCSEL array for high power applications. This is done by simulating a large active-area VCSEL with either a metallic or a grating-relief-based patterning at the outcoupling aperture. The investigated designs are compared, highlighting possible strategies to improve the fiber coupling by improving the far field (FF) profile. This contribution also presents a satisfactory match between the simulations and the FF experimental data obtained from the measurements of a VCSEL array structure