Andrés M. Roldán

A SPICE Compact Model for Unipolar RRAM Reset Process Analysis

A physically based circuit model is proposed for SPICE simulation of thermally assisted reset transitions in resistive switching devices. The model allows the simulation of conductive filaments with complex structures, such as a main filament with several subfilaments attached forming a tree structure, or several filaments interlaced between them. The model has been validated by comparing with experimental data and the simulations obtained with a previously published simulation tool. A study of the influence of different subfilaments configurations on the variability of resistive random access memory I-V reset curves is also presented.

An in-depth study on WENO-based techniques to improve parameter extraction procedures in MOSFET transistors

WENO-based techniques, along with some particular polynomial interpolation procedures, have been employed to improve parameter extraction in Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), in particular for the determination of the threshold voltage. The limitations detected in conventional numerical methods to calculate derivatives of experimental data are overcome with this new application of WENO-based techniques. The numerical noise that comes up in the experimental and simulated data usually employed to characterize MOSFETs transistors is strongly reduced. The need for an accurate determination of the threshold voltage motivates the use of this advanced numerical approach that solves many of the issues that affect the conventional parameter extraction procedures currently in use in the microelectronics industry. In addition, also the influence of DIBL effects on the threshold voltage in short channel MOSFETs has been analyzed with this smart weighted ENO procedure.

Magnetic Tunnel Junction (MTJ) sensors for integrated circuits (IC) electric current measurement

We report on MgO Magnetic Tunnel Junction (MTJ) devices focusing on their potential application in the measurement of electrical current at the integrated circuit level. Single devices and full bridges have been specifically developed for this purpose. A sort of different designs regarding their geometry arrangement as well as the number of constitutive elements have been tested. Experimental characterization has been performed and results including impedance and sensitivity measurements are given.

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.

Stability behavior of composite magnetorheological fluids by an induction method

In this work, we study the stability behavior of composite magnetorheological fluids consisting of magnetic (iron) and non-magnetic (poly(methylmethacrylate)) particles dispersed in mineral oil. Because of the opacity of the suspensions, optical methods traditionally employed for evaluation of the gravitational settling in colloidal suspensions are not suitable for sedimentation follow-up in this case. For this reason, we use an alternative method based on the evaluation of the resonant frequency of the inductance of a thin coil surrounding the sample. The movement of the coil along the height of the container at specified steps and time intervals allows obtaining information about the local volume fraction of particles inside the tube. The obtained successive profiles for the multi-component suspensions show a decrease of the iron particle settling and of the initial rate of settling as the poly(methylmethacrylate) volume fraction is increased. Finally, the increase of the poly(methylmethacrylate) concentration gives rise to an improvement of the rheological properties upon magnetic field application for a given concentration of iron. Both a strong rheological response and a good colloidal stability are essential for practical applications.

Original article: A comprehensive characterization of the threshold voltage extraction in MOSFETs transistors based on smoothing splines

In this work we propose a method to obtain the MOSFET transistor threshold voltage, which is known to be an essential magnitude from the modeling viewpoint. Generally, there are a large number N of experimental data, and the use of smoothing splines leads to resolution of linear systems of size N+d, where d depends on the degree of the splines used. The computational effort could be reduced by decomposing the original problem into subproblems and using an appropriate method to combine the corresponding solutions. We adopt this idea, considering for simplicity the decomposition into two subsets and using a boolean sum based method for combining the intermediate spline approximants.

Polynomial pattern finding in scattered data

A new numerical procedure to extract the threshold voltage in MOSFET transistors has been developed by means of polynomial pattern recognition. The technique proposed here is based on the use of particular properties of discrete orthogonal Chebyshev polynomials, it allows the extraction of polynomial curves of different degrees within a set of experimental or simulated data. For the MOSFET threshold voltage determination we have detected linear patterns in the logarithmic representation of MOSFET transfer characteristics (drain current versus gate voltage curves). The results have been compared with the threshold voltage obtained with a classical technique, the transconductance change method, where the maximum of the drain current second derivative is assumed as the threshold voltage. Reasonable and comparable results are obtained. The new technique has shown more immunity to numerical and measurement noise, which is an important feature in the current industrial context.

Monitoring of Carbon Fiber-Reinforced Old Timber Beams via Strain and Multiresonant Acoustic Emission Sensors

This paper proposes the monitoring of old timber beams with natural defects (knots, grain deviations, fissures and wanes), reinforced using carbon composite materials (CFRP). Reinforcement consisted of the combination of a CFRP laminate strip and a carbon fabric discontinuously wrapping the timber element. Monitoring considered the use and comparison of two types of sensors: strain gauges and multi-resonant acoustic emission (AE) sensors. Results demonstrate that: (1) the mechanical behavior of the beams can be considerably improved by means of the use of CFRP (160% in bending load capacity and 90% in stiffness); (2) Acoustic emission sensors provide comparable information to strain gauges. This fact points to the great potential of AE techniques for in-service damage assessment in real wood structures.

An in-depth noise model for giant magnetoresistance current sensors for circuit design and complementary metal–oxide–semiconductor integration

Full instrumentation bridges based on spin valve of giant magnetoresistance and magnetic tunnel junction devices have been microfabricated and experimentally characterized from the DC and noise viewpoint. A more realistic model of these devices was obtained in this work, an electrical and thermal model previously developed have been improved in such a way that noise effects are also included. We have implemented the model in a circuit simulator and reproduced the experimental measurements accurately. This provides a more realistic and complete tool for circuit design where magnetoresistive elements are combined with well-known complementary metal–oxide–semiconductor modules.

Quasi-static magnetoresistive sensor modeling for current-time conversion circuit applications

In this paper we report a current-to-time converter (CTC) suitable for current sensor monitoring in low power applications. Based on a discrete resistence-to-frequency converter and a Giant MagnetoResistance (GMR) current sensor. Simulations have been done using a quasi-static electrical Verilog-A model for the GMR current sensor. A reduced set of parameters has been extracted to characterize the GMR sensors behavior. The application has been analyzed making use of different sensors, whose device parameters were previously extracted. Finally, the accuracy of the models has been tested by comparing with experimental transient measurements.