Composant semi-conducteur | EPFL Graph Search

vignette|Aperçu de quelques dispositifs semi-conducteurs encapsulés Un composant semi-conducteur est un composant électronique dont le fonctionnement repose sur les propriétés électroniques d'un matériau semi-conducteur (principalement le silicium, le germanium et l'arséniure de gallium, ainsi que des semi-conducteurs organiques). Sa conductivité se situe entre les conducteurs et les isolants. Les composants semi-conducteurs ont remplacé les tubes à vide dans la plupart des applications. Ils conduisent le courant électrique à l'état solide, plutôt que sous forme d'électrons libres dans le vide (généralement libérés par émission thermoionique) ou d'électrons libres et d'ions dans un gaz ionisé. Les composants semi-conducteurs sont fabriqués à la fois sous forme de dispositifs discrets individuels et de puces de circuits intégrés, qui consistent en deux dispositifs ou plus, dont le nombre peut aller de centaines à des milliards, fabriqués et interconnectés sur une seule tranche de semi-

Contribution to the active generator principle for high power electric supply

Massimiliano Visintin

In recent years the use of solid state frequency converters is rapidly increasing in the industrial and power plants where electric machines are installed, since it allows variable speed operations for the electric system, thus becoming a key factor which is capable of increasing the overall efficiency, as well as the global plant flexibility. The benefits that can be reached when electric motors are under investigation can be so enticing, in particular from the perspective of the operating machine coupled to the motor, that drawbacks take a back seat and can be overcome. On the contrary, power electronic for variable speed is generally not used in power generation, since the gains brought out by the present technology are not sufficient to attract anyone's interest. There are some exceptions in the wind- and the hydro power plant businesses, where there are some cases of variable speed already introduced. However in the former applications the output power is limited to some MW, while in the latter the converter is sized only to a fraction of the whole generating power, since it feeds the machine's rotor.This dissertation investigates on a possible electronic converter for power generation in the 40 MW range and above, which is also attracting from both the cost's and the efficiency's point of view, as well as from the operation reliability perspective and thus may be proposed as a breakthrough in this field. Among the possible frequency converters, those ones have been selected which perform natural commutations only, i.e. those that employ thyristors as the semiconductor devices, since the efficiency, the robustness and the relatively high performances are actually the key success factors to be addressed from the beginning. Well known matrix converter topologies will be referred to, as well as new matrix converter arrangements will be brought out and analyzed: their behaviours will be targeted by the dissertation, in particular when different strategies are adopted for controlling the operations. Specifically comparisons will be carried out when the same converter topology is used, but driven by either the well-known cycloconverter strategy or the newly brought out "active generator" one. In addition several generator winding arrangements will be proposed and analyzed, with the aim of increasing the actual feasibility of the proposed solution. Finally some test results on a sized down experimental rig will be analyzed and compared to the simulation outcomes.

EPFL2006

Diamond on GaN integration for power semiconductor devices with efficient thermal management

Reza Soleiman Zadeh Ardebili

The international actions against global warming demands reductions in carbon emission and more efficient use of energy. Energy efficiency in the conversion and use of electricity, as an important form of energy in the modern life, has strong environmental and economical impacts. Power electronic devices determining roles in the overall system efficiency in many applications such as power converters and inverters. Gallium Nitride (GaN) devices have emerged as an superior alternative to the silicon devices and enabled unprecedented efficiencies. GaN-based high electron-mobility transistors (HEMTs) offer excellent characteristics such as high switching speed, low switching and conduction losses, small device size and high power density capabilities. However, such significant increases in the power density and the reduction of device size comes at the price of dramatic increases in the heat fluxes and appearance of hot spots in the device footprint that cannot be addressed using conventional methods. The integration of diamond and GaN has been highly pursued for thermal management purposes as well as combining their exceptional complementary properties for power electronics applications and novel semiconductor heterostructures. In this thesis, we discuss the key hindrances towards realisation of diamond on GaN substrates and devices, and we propose novel solutions for improving the feasibility of diamond-on-GaN integration for efficient thermal management of hot spots in GaN devices and development of diamond devices integrated with GaN devices in the same chip.The growth of diamond on GaN is challenging due to the high lattice and thermal expansion mismatches. The weak adhesion of diamond to GaN and high residual stresses after the deposition often result in the diamond film delamination or development of cracks, which prevents the subsequent device fabrication. In this thesis, a new seed dibbling method is presented for seeding and growing high-quality diamond films on cost-effective GaN-on-Si substrates, which result in significantly larger diamond grains and lower residual stresses (as low as 0.2 GPa) compared to conventional methods. Moreover, the excellent adhesion obtained by this method enabled a reliable polishing of the as-grown diamond films on GaN-on-Si without any delamination, resulting in smooth diamond-on-GaN substrates with sub-nanometer roughness. The high temperature hot spots in GaN devices are addressed by the near-junction heat spreaders using diamond films deposited on GaN. An analytical model for the heat spreading is presented to analyse the performance and optimize the heat spreaders, considering the geometry and thermal conductivity of the heat spreader and the substrate. Experimental demonstration of diamond heat spreaders on vertical GaN diodes together with the thermal characterization of their behaviour shows significant reduction of thermal spreading resistances, hot spot temperatures and temperature gradients from the device footprint, which highlights the impact of such heat spreaders for the thermal management of high power density GaN devices.The diamond films deposited on GaN were also used to demonstrate high-performance diamond transistors integrated on AlGaN/GaN-on-Si substrates with characteristics such as high breakdown voltage, high current density, and high on/off ratio, which opens many new possibilities for the development of power ICs with complementary logics, gate drivers and power

EPFL2021

Dynamics of Electronic and Ionic Charges in Cyanine Organic Semiconductor Devices

Sandra Christina Jenatsch

Organic semiconductor materials have been widely applied in optoelectronic devices to replace their inorganic counterparts and explore new fields of applications. Due to their high extinction coefficients, chemical tunability and solubility, cyanine dyes are an interesting class of molecules to be employed in organic semiconductor devices. Their beneficial properties have been demonstrated in organic solar cells (OSC), photodetectors and recently in organic light-emitting electrochemical cells (OLEC). The focus of this thesis is to analyse different aspects regarding the dynamics of electronic and ionic charges in cyanine based devices. Models and conclusions are not restricted to cyanine based devices but can potentially be applied in various systems where combined ionic and electronic charges are present. In order to focus on electronic processes in OSCs, a combination of steady-state, transient and impedance analysis is used. It is resolved that trapped charges can be introduced at the dye/hole extracting layer interface by using specific solvents for the cyanine layer fabrication. These measurements also reveal the low hole mobility of cyanine films which in turn limits the possible upper layer thickness in OSCs. A frequently used way to enhance the conductivity of organic semiconductors is the addition of extra charges by chemical doping. This concept is employed for a near-infrared absorbing dye which is used as donor in a bilayer solar cell. The dependence of all figures of merit versus doping concentration and layer thickness is studied in detail. It is found that the performance of the optimised cell is not further improved by doping, but that it presents a good option if thicker layers are required, e.g. on rough substrates. Furthermore, the stability and degradation pathway of the p-doped species is analysed. This information is of general relevance as they also occur after charge transfer at the heterojunction interface of an OSC and during operation of OLECs. The dynamics of ionic charge carriers is separated into two contributions. In a first step, the steady-state diffusion profile of the counter ion within a cyanine/fullerene bilayer structure is scrutinised using different profiling techniques. An unexpected constant ion profile is found throughout the C60 layer. Simultaneously, the potential within the acceptor is measured by the Kelvin probe technique. A mechanism of combined electron and ion transfer from the C60 to the dye and vice versa is proposed which is in agreement with the experimental results. A prerequisite for OLEC operation is the mobility of ionic charge carriers. Their dynamics in cyanine salts under the effect of an external field is therefore studied in such devices. Several methods are developed and applied to characterise the dynamic p-i-n structure in detail, i.e. the position and width of the intrinsic region as well as the doping concentrations in p- and n-doped regions. The combination of all these methods helps to understand the transient behaviour of these cyanine devices but can also be adapted to other OLEC systems. A major drawback of cyanine dyes as emissive material is their low fluorescence quantum yield in solid films. To overcome this restriction and improve the OLEC efficiency, host-guest systems with various visibly emitting dyes are studied. The gain in fluorescence quantum yield in the film can directly be transferred to an increase in external quantum efficiency of the device.

EPFL2017