J.L. Sebastian

Analysis of the influence of the cell geometry, orientation and cell proximity effects on the electric field distribution from direct RF exposure.

This paper shows the importance of using a cell model with the proper geometry, orientation and internal structure to study possible cellular effects from direct radiofrequency exposure. For this purpose, the electric field intensity is calculated, using the finite element numerical technique, in single- and multilayer spherical, cylindrical and ellipsoidal mammalian cell models exposed to linearly polarized electromagnetic plane waves of frequencies 900 and 2450 MHz. An extensive analysis is performed on the influence that the cell geometry and orientation with respect to the external field have in the value of the electric field induced in the membrane and cytoplasm. We also show the significant role that the cytoplasmic and extracellular bound water layers play in determining the electric field intensity for the cylindrical and ellipsoidal cell models. Finally, a study of the mutual interactions between cells shows that polarizing effects between cells significantly modify the values of field intensity within the cell.

A study of the electric field distribution in erythrocyte and rod shape cells from direct RF exposure.

This paper shows the importance of using realistic cell shapes with the proper geometry and orientation to study the mechanisms of direct cellular effects from radiofrequency (RF) exposure. For this purpose, the electric field distribution within erythrocyte, rod and ellipsoidal cell models is calculated by using a finite element technique with adaptive meshing. The three cell models are exposed to linearly polarized electromagnetic plane waves of frequencies 900 and 2450 MHz. The results show that the amplification of the electric field within the membrane of the erythrocyte shape cell is more significant than that observed in other cell geometries. The results obtained show the dependence of the induced electric field distribution on frequency, electrical properties of membrane and cytoplasm and the orientation of the cell with respect to the applied field. The analysis of the transition of an erythrocyte shape to an ellipsoidal one shows that a uniformly shelled ellipsoid model is a rough approximation if a precise simulation of bioeffects in cells is desired.

High-frequency modeling of GaN/SiC blue light-emitting diodes

We report on this work a model to accurately predict the electrical behavior of double-heterostructure GaN/SiC blue light-emitting diodes up to microwave frequencies. A procedure to extract the series resistance (R-s) from the reflection coefficient is suggested. This procedure offers the advantage of using measurements without any bias current and therefore the obtained values of R-s are influenced neither by the device heating nor by inaccuracies in the calculation of the ideality factor. The junction capacitance and conductance measured in the range 1 kHz-10 MHz shows two different relaxation mechanisms, and the total capacitance can be fitted very accurately to a double Lorentzian function. Blue light-emitting diodes and lasers based on gallium nitride (GaN) semiconductor compounds represent one of the most important breakthroughs in electronics and optoelectronics of recent years. The combination of silicon carbide (SiC) and GaN has recently enabled low-cost blue-emitting diodes to be introduced in industry

Electric field distribution and energy absorption in anisotropic and dispersive red blood cells

We have studied the influence of the anisotropic and dispersive nature of the red blood cell structure on the energy absorption and electric field distribution within the cell exposed to electromagnetic fields of frequencies in the range from 50 kHz to 10 GHz. For this purpose we have generated a realistic model of a multilayered erythrocyte cell from a set of parametric equations in terms of Jacobi elliptic functions. The effect of dipole relaxations and anisotropic conductivities is taken into account in the dispersion equations for the conductivity and permittivity of each layer (cytoplasmic and extra-cellular bound waters, membrane, cytoplasm and external medium). Using a finite element numerical technique, we have found that the electric field distribution and the energy absorbed in the membrane show well-defined maxima for both normal and parallel orientations of the external field with respect to the symmetry axis of the cell. The normal and tangential conductivities and permittivities of the membrane are shown to be responsible for the different peak amplitudes and frequency shifts of the maxima. A previously unnoticed effect is that the cell shape combined with the dispersion of the membrane permittivity and the influence of bound water layers leads to a very high amplification factor (greater than 300) of the electric field in the membrane at frequencies in the megahertz range.

Influence of the calibration kit on the estimation of parasitic effects in HEMT devices at microwave frequencies

In this paper, we investigate how critical the calibration kit is in an accurate estimation of microwave device parasitic elements. The semiempirical cold FET method has been applied to the extraction of the small signal equivalent circuit of several HEMT devices. Two different measurements were made on the same devices by using two calibration kits. The first kit is a commercial one based on the LRM method, whereas the second kit was designed by the authors and fabricated on the same chip of the devices. The discrepancies found in the calculated parasitic elements provided information on the sensitivity of the elements with respect to the calibration kit, and, therefore, on the physical origin of the parasitics. These discrepancies. show that it is possible to evaluate the influence of the contact pads on the electrical behavior of on-chip semiconductor devices by making measurements with different calibration standards.

Analysis of radiofrequency energy stored in the altered shapes: Stomatocyte-echinocyte of human erythrocytes.

The aim of this study is to analyze the electromagnetic energy stored in stomatocyte, erythrocyte and echinocyte cells exposed to a linearly polarized electromagnetic plane wave at 900, 1800 and 2450MHz radiofrequency signals. This analysis can provide a better understanding of the order of appearance of altered shapes of erythrocytes (RBC) in the stomatocyte-echinocyte transition under radiofrequency exposure in terms of the deposited electromagnetic energy. For this purpose we use a realistic geometrical cell model based on parametric equations that allow for continuous transformations between normal erythrocytes and three stomatocyte subclasses with different degree of invagination and also between normal erythrocytes and echinocytes with an arbitrary number of spicules. We use a finite element technique with adaptive meshing for calculating the electromagnetic energy deposited on the different regions of the cell models. It is found that the echinocyte cell stores the minimum electromagnetic energy and therefore from an energetic point of view it would be the most stable and preferred cell state when this electromagnetic energy is the predominant energy component.

Analysis of the electric field induced forces in erythrocyte membrane pores using a realistic cell model

We calculate the induced electric stress forces on transient hydrophobic pores in the membrane of an erythrocyte exposed to an electric field. For this purpose, we use a finite element numerical technique and a realistic shape for the biconcave erythrocyte represented by a set of parametric equations in terms of Jacobi elliptic functions. The results clearly show that the electrical forces on the base and sidewalls of the pore favour the opening of the pore. A comparison of the force densities obtained for an unstretched flat membrane and for the realistic erythrocyte model shows that the thinning and curvature of the membrane cannot be neglected. We also show that the pore deformation depends strongly on the orientation of the pore with respect to the external field, and in particular is very small when the field is tangent to the membrane surface.

Noise performance of submicron HEMT channels under low power consumption operation

We have investigated the noise performance of HEMT devices for low noise operation with the aim of developing a noise model valid for low power biasing. Analytical expressions useful for CAD models have been derived for the calculation of the Pospieszalski gate and drain temperatures, and have been verified from near pinchoff conditions up to usual bias voltages. An overshoot in the drain temperature as a function of the drain voltage has been observed at low drain currents in deep submicron gate length devices.

Influence of the minimization of self-scattering events on the Monte Carlo simulation of carrier transport in III-V semiconductors

This paper presents a procedure to improve the algorithm of Sangiorgi, Riccoand Venturi for the calculation of the time of flight in Monte Carlo simulations. The method is used to efficiently optimize the step function in which the total scattering probability is discretized. The optimization criterion suggested in this work can reduce the self-scattering events to less than 30% in a fairly wide range of temperatures, applied fields and doping levels. Different examples are presented to illustrate the advantages of the method.

Drain Temperature Dependence on Ambient Temperature for a Cryogenic Low Noise C-Band Amplifier

A comparison between predicted and measured noise temperatures for cryogenic HEMT amplifiers is presented by using the Pospieszalskis noise model. A good agreement between predicted and measured amplifiers noise performance is obtained both at room and cryogenic temperatures. However, the predicted values overestimate noise temperature in the center part of the measured temperature range (50K - 230K). A parabolic dependence for the drain temperature with ambient temperature is proposed to obtain a better fitting to the experimental results.