Avram Bar-Cohen

Bio: Avram Bar-Cohen is an academic researcher from University of Maryland, College Park. The author has contributed to research in topics: Heat transfer & Heat sink. The author has an hindex of 50, co-authored 329 publications receiving 8329 citations. Previous affiliations of Avram Bar-Cohen include Auburn University & DARPA.

Experimental demonstration of thermal management using thermoelectric generation

Abstract: It has been well established that thermal management of electronic equipment is a critical need for the continued success of the microelectronics industry. Portable electronic devices, such as notebook computers and cellular telephones, require that the thermal solution be small, light, and energy efficient. Small-scale thermoelectric (TE) modules can be used to generate electricity from the waste heat of the microprocessor. The electricity can then be used to power the thermal solution. The concept of using a TE module to generate electricity from the waste heat of a microprocessor in a notebook computer class device is discussed by Solbrekken, et. al. In that study, the 'shunt attach' design configuration is introduced and parametrically explored. Using the models developed in that study, a prototype is designed such that a 25 W processor is kept below 85/spl deg/C in a 25/spl deg/C ambient using 'heat-driven cooling'. The prototype built using off-the-shelf TE modules verifies that a 25 W microprocessor can be kept below 85/spl deg/C in a 25/spl deg/C ambient, cooled solely by a fan driven with TE generated electricity.

Thermo-structural behavior of underfilled flip-chips

Abstract: The continuing drive towards high-density, low-profile integrated circuit packaging has accelerated the spread of flip-chip technology. The use of an area array of solder bumps for the electrical and mechanical, as well as thermal attachment of the chip to the substrate provides flip-chip technology with considerable advantages in cost, density, and electrical performance, relative to the use of conventional single-chip packages. Unfortunately, however the difference in the coefficients of thermal expansion, between the silicon die and the substrate, leads to substantial, thermally-induced strains, which can result in fatigue failure of the solder joints. This problem is exacerbated by the use of the larger chips now becoming available. In recent years it has been found possible to obtain dramatically-higher reliability and to operate successfully with much larger chip sizes by "underfilling" the gap between the chip and substrate with epoxy. The present effort is aimed at exploring the thermo-structural behavior of such underfilled chips and more firmly establishing the physical basis for the improved reliability of this packaging technology. The thermo-structural behavior of an underfilled flip-chip package has been evaluated using the structural FEM code NIKEDP and employing an axisymmetric model of a typical flip-chip structure. In the course of this effort, numerical simulations were performed for underfill materials of varying thermo-structural properties and two solder bump heights. The results were used to examine the parametric sensitivity of the thermal strain in the solder joints and the axial, as well as shear, stress in the underfill material. The Coffin-Manson relation is used to relate the increased number of cycles-to-failure to the solder strain reduction associated with the use of underfilling.

Light-emitting diodes

Abstract: Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

A review of Ga2O3 materials, processing, and devices

Abstract: Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

Low loss dielectric materials for LTCC applications: a review

Abstract: Small, light weight and multifunctional electronic components are attracting much attention because of the rapid growth of the wireless communication systems and microwave products in the consumer electronic market. The component manufacturers are thus forced to search for new advanced integration, packaging and interconnection technologies. One solution is the low temperature cofired ceramic (LTCC) technology enabling fabrication of three-dimensional ceramic modules with low dielectric loss and embedded silver electrodes. During the past 15 years, a large number of new dielectric LTCCs for high frequency applications have been developed. About 1000 papers were published and ∼500 patents were filed in the area of LTCC and related technologies. However, the data of these several very useful materials are scattered. The main purpose of this review is to bring the data and science of these materials together, which will be of immense help to researchers and technologists all over the world. The comme...