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.

Adhesion strength and inversely-determined nonlinear Young's moduli for die-attach adhesives

Abstract: The use of loaded epoxy adhesives in the microelectronics industry is widespread. Integrated circuit (IC) chips are often adhesively bonded to die-pads, substrates, leadframes, and/or heatspreaders. In addition to silicon, a variety of adherends including IC passivation coatings (such as polyimide), copper and copper alloys (plated with precious metals and non-plated), aluminum, and ceramics (such as alumina (Al2O3)) are encountered in situations such as these. The adhesion strength as well as the mechanical properties of the adhesives used in these configurations are unfortunately not well known. In order to obtain an estimate of the values of the parameters that are of interest it is necessary to experimentally investigate the adhesives in the configuration under consideration. This was done for a number of commercially available loaded epoxy adhesives. A full factorial experimental study was conducted, using double lap shear test specimens (DLSTSs), in order to determine the effect of moisture and temperature on the adhesion strength of these adhesives. An Instron mechanical test system was used to generate force-displacement (F-d) curves for each of the adhesives that were studied. The adhesives that were examined consisted of both 'low stress' and high strength materials and they were obtained from four different vendors. The experimental results were also used in conjunction with a Finite Element (FE) model of the DLSTS in order to determine the non-linear Young's moduli of the adhesives as a function of strain, moisture, and temperature.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A Hybrid Multi-gate Model of a Gallium Nitride (GaN) High Electron Mobility Transistor (HEMT) Device Incorporating GaN-substrate Thermal Boundary Resistance

Abstract: : This report describes a pseudo-analytical thermal model of gallium nitride (GaN) high electron mobility transistors (HEMTs), which combines analytical heat spreading models with spreading width boundary conditions derived from two-dimensional finite element thermal simulations We successfully produced an accurate GaN HEMT hybrid model capable of evaluating the impact of thermally important device parameters on junction and individual layer temperatures A parameter space investigation, covering GaN-substrate thermal boundary resistance (TBR), gate pitch, substrate thickness, substrate thermal conductivity, and GaN thickness validated the hybrid model against full finite element numerical analysis and provided insight into device thermal behavior This modeling showed near junction thermal resistance contributions from the GaN and interface TBR stay relatively constant with gate number and pitch down to 5 m Alternatively, the thermal profiles in the substrate layers and below show strong interaction between gates; the magnitude of those components scale directly with gate number and increase significantly with decreasing gate pitch Also finite substrate and GaN thicknesses produce a minimum temperature rise dependent on downstream thermal resistance Finally, increasing substrate thermal conductivity, by replacing a silicon carbide (SiC) substrate with higher thermal conductivity diamond, appears to only be advantageous if the TBR does not increase substantially beyond the SiC range

Hierarchical Reliability Model for Life Prediction of Actively Cooled LED-Based Luminaire

Abstract: The interest in light-emitting diodes (LEDs) for illumination applications has been increasing continuously over the last decade due to two key attributes of long lifetime and low energy consumption compared to the conventional incandescent light and compact fluorescent light. Although LEDs are attractive for lighting applications due to the aforementioned advantages, unique technical challenges, such as the extreme sensitivity of luminous output and useful lifetime to LED junction temperature, need to be overcome for their large-scale commercialization.

Application of Thermally-Enhanced Thermoplastics to Seawater-Cooled Heat Exchangers

Abstract: In this study, the potential benefits of using thermally-enhanced polymers in high-performance seawater heat exchangers are assessed. The thermal and mechanical properties of commercially available, thermally-conductive resins are reviewed and compared with those of polymer heat exchangers described in the literature, as well as those of metals commonly used in compact heat exchangers, including in seawater-based cooling systems (i.e., Cu-Ni alloys, stainless steel, and titanium). This survey reveals that engineered thermoplastics have sufficiently high thermal conductivities to compete with their metal counterparts. The thermo-fluid performance of a conceptual, doubly-finned plate liquid-liquid heat exchanger module, is analytically evaluated using the -NTU method. The heat transfer rate and coefficient of performance (COP) of this heat exchanger are parametrically assessed for various fin spacings, wall thicknesses, and a near 3 orders-of magnitude range of wall thermal conductivities. This analysis shows that the thermal conductivities achievable with enhanced thermoplastics, 20 W/mK, can provide approximately half the heat transfer rate of an aluminum heat exchanger operating under the same conditions, and 80% of the heat transfer rate provided by a corrosion-resistant, metallic heat exchanger. This study indicates that thermally conductive thermoplastics offer a promising alternative to the use of conventional and/or corrosion-resistant metals in compact, high performance heat exchangers in seawatercooled applications. While conventional metal heat exchangers are generally incapable of providing reliable long-term service with seawater (and other corrosive fluids), to date the cost, complexity, and the restricted availability of exotic corrosion-resistant materials have limited the use of seawater as the ultimate heat sink for energy conversion processes. Currently available polymer heat exchangers, fabricated with thermally un-enhanced thermoplastics, are an alternative option, but are limited to relatively low heat transfer rates. In this study, the potential benefits of using thermally-conductive polymers in high-performance seawater heat exchangers are assessed. These advanced materials could provide reduced weight, greater resistance to corrosion and fouling, and reduced energy of formation and fabrication, as well as greater geometric flexibility and ease of manufacturing, relative to the conventional material technologies in use today. On the premise that such materials have the required thermal and mechanical performance characteristics, they could facilitate the development of seawater heat exchangers for the power industry, naval applications, and coastal petroleum refineries. In this paper, commercial, prototype, and research polymer heat exchangers described in the literature are firstly discussed, with an emphasis on their performance limits and physical characteristics. A review of commercially available, thermally-conductive resins is then presented, with a comparison of their thermal and mechanical properties to those of metals commonly used in compact heat exchangers, including in seawater-based cooling systems (i.e. Cu-Ni alloys, stainless steel, and titanium). Based on this review, the thermo-fluid performance of a notional, doubly-finned plate liquid-liquid heat exchanger module is analytically evaluated using the e-NTU method. The heat transfer rate and coefficient of performance (COP) 1 of this heat exchanger are assessed for various fin spacings, wall thicknesses, and a near 3 orders-of magnitude range of wall thermal conductivities.

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...