Dejan Križaj

FOXFET structure — device modelling and analysis

A FOXFET structure for biasing AC coupled detector structures has been analyzed by a two-dimensional device simulation. For this purpose, a floating strip junction with zero current boundary condition has been applied. The floating strip voltage increase is analyzed from depletion layer spreading through three charge regions: electron accumulation surface region, hole current flow region and depleted bulk region. As a result, the floating strip potential increases approximately as the square root of the drain/backside reverse voltage. Strip potential saturation is observed for oxide charge densities larger than 5 × 1011 cm−2 and results in a weaker gate control and oxide thickness influence. Current conduction mechanisms are critically discussed and drift-diffusion injection from the floating strip junction is proposed instead of the thermionic emission model. Strip potential increase by an additionally injected strip current is due to the effect of space-charge-limited-current (SCLC) for an injected strip current larger than approximately 10−9 A/μm. The dynamic resistance calculated from numerically obtained strip current/voltage curves has a slope of 0.85 for a lower injected strip current (Is<10−10 A/μm) and decreases to 0.6 for a larger one.

Lumped series resistance of solar cells as a result of distributed sheet resistance

Abstract An analysis of solar cell distributed sheet resistance is performed by solving the nonlinear Poisson equation for the surface potential. Two different approaches to lumped series resistance are discussed: equivalent series resistance R Seq obtained from the cells equivalent circuit that satisfies the actual current of the cell (all other parameters in the equivalent circuit except the series resistance are kept constant) and Joule series resistance R SJ obtained from the Joule losses in the emitter of the cell. It is observed that the I ( U ) characteristic obtained from the equivalent circuit that includes R SJ generally disagrees with the actual I ( U ) characteristic of the solar cell. An additional series resistance R Sadd should be introduced in series with R SJ . Series resistances R SJ , Sadd and R Seq are analyzed numerically in one and two dimensions for different conditions of terminal voltage, illumination and weighted sheet resistance R sh b 2 , where b is related to the geometry of the analyzed cell. Following the derivations and the results of the numerical analysis it can be concluded that wherever R SJ varies as a function of terminal voltage, R Sadd should be taken into consideration.

Capacitive-type counter of nanoparticles in air

A capacitive-type counter of nanoparticles in air, based on measurement of the capacitance of a capacitor with a variable dielectric part, has been developed. The method is suitable for detection of aerosols in a wide concentration range, regardless of shape or chemical composition. The method is demonstrated experimentally and verified by numerical simulations. The counter allows for construction of a simple, portable, and cheap nanoparticle detector for air quality control.

A Strip-Type Microthrottle Pump: Modeling, Design and Fabrication

A novel design for a strip-type microthrottle pump with a rectangular actuator geometry is proposed, with more efficient chip surface consumption compared to existing micropumps with circular actuators. Due to the complex structure and operation of the proposed device, determination of detailed structural parameters is essential. Therefore, we developed an advanced, fully coupled 3D electro-fluid-solid mechanics simulation model in COMSOL that includes fluid inertial effects and a hyperelastic model for PDMS and no-slip boundary condition in fluid-wall interface. Numerical simulation resulted in accurate virtual prototyping of the proposed device only after inclusion of all mentioned effects. Here, we provide analysis of device operation at various frequencies which describes the basic pumping effects, role of excitation amplitude and backpressure and provides optimization of critical design parameters such as optimal position and height of the microthrottles. Micropump prototypes were then fabricated and characterized. Measured characteristics proved expected micropump operation, achieving maximal flow-rate 0.43 mL·min−1 and maximal backpressure 12.4 kPa at 300 V excitation. Good agreement between simulation and measurements on fabricated devices confirmed the correctness of the developed simulation model.

Numerical analysis of edge effects in side illuminated strip detectors for digital radiology

Abstract The influence of edge defects on side illuminated X-ray strip detectors for digital radiology is investigated by numerical device modeling. By assuming positive fixed oxide charges on side and top surfaces simulations have shown strong curvature of the equipotential lines in the edge region. A fraction of the edge generated current surpasses the edge guard-ring junction and is collected by the readout strips. As a consequence, strips cannot be placed close to the edge of the structure and collection efficiency is reduced. An n-on-n instead of a p-on-n strip detector is proposed enabling collection of edge generated carriers by a very narrow guard-ring junction and placement of the readout strip close to the edge without increase of the strip leakage current.

Design and Development of a Portable WiFi enabled BIA device

A bioimpedance device (BIA) for evaluation of sarcopenia – age related muscle mass loss – is designed, developed and evaluated. The requirements were based on lightweight design, flexible and user enabled incorporation of measurement protocols and WiFi protocol for remote device control, full internet integration and fast development and usage of measurement protocols. The current design is based on usage of a microcontroller with integrated AD/DA converters. The prototype system was assembled and the operation and connectivity to different handheld devices and laptop computers was successfully tested. The designed BIA device can be accessed using TCP sockets and once the connection is established the data transfer runs successfully at the specified speed. The accuracy of currently developed prototype is about 5% for the impedance modulus and 5 deg. for the phase for the frequencies below 20 kHz with an unfiltered excitation signal and no additional amplifiers employed.

Basics of Numerical Simulations of Bioimpedance Phenomena

Bioimpedance measurement technique is based on injecting AC current into a biological material and from comparison of voltage and current signals (amplitude and phase) extract (bio)impedance between the electrodes. Several details such as electrode shape, positioning and their electrical properties, and electrical and geometrical properties of the measured biological materials influence the measurements. Many of these influences can be foreseen and analyzed by performing a numerical simulation of AC current conduction through the analyzed structure. Usually it is sufficient to numerically solve Poisson equation for complex potential by taking into account appropriate boundary conditions and material parameters (both dielectric and conductive) and from known potential distribution extract other quantities of interest such as electric field, current density, current through the electrodes, complex power, and finally (bio)impedance. For basic demonstration purposes, free computer simulation tools can be used to perform this task as presented in this chapter by introducing free simulation software FEMM.

Measuring impedance using an open-source instrumentation platform

The article presents development and design of a precision LCR meter based on an affordable open-source instrumentation platform. The design of the instrument has been performed by an interdisciplinary group of students. A project-based approach has been used in order to make a practical use of engineering knowledge within the group of participants. The results cover all the necessary project results achieved by the participants from the state-of-the art analysis in the domain of LCR meters to dissemination of project results. These may serve as a guide for similar project specifications. Alternatively, the presented results may be used in preparing student assignments in electrical impedance measurements based on affordable hardware.

Analysis of a Single Supply Constant Current Source for Bioimpedance Measurements

Constant current source is one of crucial components of every bioimpedance measurement device. Here we present an analysis of a single supply current source based on a modified Howland circuit. The analysis of track to track capacitances has been performed by finite element numerical device simulation and compared to measurements on developed PCB without soldered elements. Capacitances of the order of 1 pF were simulated/measured the largest being the voltage source to ground capacitance (3 pF). The measured transconductance bandwith was about 3 MHz at 100 Ω load and 100 kHz at 100 kΩ. Circuit analysis with Spice resulted in larger bandwidths compared to the measured ones.

Bioimpedance Method with a Conductive Floating Electrode

Bioimpedance method is based on measurement of impedance at separated current injection and voltage measurement electrodes. An alternative or variation of a method is proposed by usage of a floating conductive electrode. The principle relies on measurements of the (bio)impedance with and without usage of the floating electrode that should be placed close to the area of “interest”. The difference in measured impedance depends on the conductive properties of the area in the close vicinity of the floating electrode and thus enables analysis of the changes of the conductivity of this area.