An introduction to heat transfer

One of the most frequently used methods for characterizing thin films is UV–Vis absorption. The near-edge region can be fitted to a simple expression in which the intercept gives the band-gap and the fitting exponent identifies the electronic transition as direct or indirect (see Tauc et al., Phys. Status Solidi 15, 627 (1966); these are often called “Tauc” plots). While the technique is powerful and simple, the accuracy of the fitted band-gap result is seldom stated or known. We tackle this question by refitting a large number of Tauc plots from the literature and look for trends. Nominally pure zinc oxide (ZnO) was chosen as a material with limited intrinsic deviation from stoichiometry and which has been widely studied. Our examination of the band gap values and their distribution leads to a discussion of some experimental factors that can bias the data and lead to either smaller or larger apparent values than would be expected. Finally, an easily evaluated figure-of-merit is defined that may help guide more accurate Tauc fitting. For samples with relatively sharper Tauc plot shapes, the population yields Eg(ZnO) as 3.276 ± 0.033 eV, in good agreement with data for single crystalline material.

Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system

In the past decade graphene has been one of the most studied material for several unique and excellent properties. Due to its two dimensional nature, physical and chemical properties and ease of manipulation, graphene offers the possibility of integration with the exiting semiconductor technology for next-generation electronic and sensing devices. In this context, the understanding of the graphene/semiconductor interface is of great importance since it can constitute a versatile standalone device as well as the building-block of more advanced electronic systems. Since graphene was brought to the attention of the scientific community in 2004, the device research has been focused on the more complex graphene transistors, while the graphene/semiconductor junction, despite its importance, has started to be the subject of systematic investigation only recently. As a result, a thorough understanding of the physics and the potentialities of this device is still missing. The studies of the past few years have demonstrated that graphene can form junctions with 3D or 2D semiconducting materials which have rectifying characteristics and behave as excellent Schottky diodes. The main novelty of these devices is the tunable Schottky barrier height, a feature which makes the graphene/semiconductor junction a great platform for the study of interface transport mechanisms as well as for applications in photo-detection, high-speed communications, solar cells, chemical and biological sensing, etc. In this paper, we review the state-of-the art of the research on graphene/semiconductor junctions, the attempts towards a modeling and the most promising applications.

Graphene Schottky diodes: an experimental review of the rectifying graphene/semiconductor heterojunction

A novel fault diagnosis technique for photovoltaic systems based on artificial neural networks

Graphene Schottky diodes: An experimental review of the rectifying graphene/semiconductor heterojunction

In this paper the control of a grid-tied PV (Photovoltaic) Cascaded H-bridge inverter is discussed. The operation principle of the considered circuit is analyzed and a particular attention is paid to the different modulation techniques well-suited for this configuration. Moreover, some critical implementation issues are addressed, while a fair comparison between the proposed modulation scheme and the Phase Shifted PWM (Pulse Width Modulation) is carried out. The adopted control strategy is based upon a sorting algorithm in order to modulate only one power cell (i.e. H-bridge) per sorting cycle while the others are in fixed state. This technique allows to reduce power switching losses, thus increasing overall system efficiency. The numerical results reported in the paper permit to confirm the feasibility of the proposed modulation strategy and the better performance with respect to PS-PWM in terms of power efficiency.

Modulation technique for grid-tied PV multilevel inverter

A new test structure for the recombination lifetime profile measurement has been designed and applied, for the first time, to characterize very thin (4 /spl mu/m) silicon epitaxial layers. The results of our analysis have shown how the lifetime behavior, at room temperature, is clearly position dependent its value being influenced by different recombination centers. Moreover, two distinct recombination centers have been identified, the first one related to the dopant (arsenic in our case) and the second one induced by the process steps required to realize the test structure itself.

Recombination centers identification in very thin silicon epitaxial layers via lifetime measurements

Overtemperature occurring in silicon solar panels subject to partial shadowing are reduced by means of a new bypass circuit. The circuit exploits a series connected power MOSFET which sustains part of the reverse voltage developing across the shaded solar cell. Neither power supply nor control logic are required, resulting in an appealing advantage with respect the other active bypass circuits. Experiments performed on a commercial mono-crystalline solar panel evidenced a reduction of the reverse voltage across a shaded cell, if compared with the traditional bypass diode, of about 50%, corresponding to a temperature decease approaching 22°C.

A new bypass circuit for hot spot mitigation

Small shadows are often neglected in the prediction of energy producible by photovoltaic systems. This is mostly due to the lack of specific tools available in commercial design software packages, which neither allow realistic drawings nor can account for individual solar cells currents. In this paper the effects of small shadows are accurately analyzed with reference to two special cases, a TV antenna and self-shading. The analysis is based on the interaction between highly specialized and powerful software, AUTOCAD, MATLAB and PSpice, which allow an utmost degree of accuracy.

Accurate analysis of small shadows effects on photovoltaic systems yield

Silicon carbide (SiC) metal-oxide-semiconductor field effect transistor (MOSFETs) are gradually replacing silicon power devices in many applications because of the higher performances of the material. Even if the technology for SiC MOSFET has been improved in the last years, the very high interface SiO2/SiC trap density is still a problem that affects the present SiC MOSFET generations. This issue is still not addressed in technology computer aided design (TCAD) simulations supporting the devices development. In this work we demonstrate how an accurate calibration of the TCAD model of a commercial SiC MOSFET is only possible by considering a non-uniform trap distribution along the SiO 2 SiC interface.

Santolo Daliento

Papers

Modulation technique for grid-tied PV multilevel inverter

Recombination centers identification in very thin silicon epitaxial layers via lifetime measurements

A new bypass circuit for hot spot mitigation

Accurate analysis of small shadows effects on photovoltaic systems yield

TCAD model calibration for the SiC/SiO 2 interface trap distribution of a planar SiC MOSFET