Computational analyses of die-embedded microchannels for high electron mobility transistors considering thermal, hydrodynamic and structural behavior

Abstract

The continuous endeavors towards the miniaturization of electronics introduce new challenges in the thermal management of devices with high power density. Especially for high electron mobility transistors (HEMTs), which are widely used in aerospace, radar, and automotive applications for radio frequency (RF) power amplification, highly localized heat sources cause extremely high-power density. Hence, spreading the heat generated by localized heat sources or hotspots in an effective way is of utmost importance to enhance the operational efficiency and lifespan of power amplifier (PA) HEMTs. Furthermore, gallium nitride (GaN) semiconductor material provides outstanding electrical and thermal properties and is compatible with HEMT devices. However, even though GaN HEMT PAs provide outstanding electrothermal performance, wide bandgap and full capacity operations of such electronics cause diminished reliability due to many reasons, such as concentrated heat dissipation, mechanical stress, and electron collapse. To surmount the thermal limitations of GaN HEMT-based devices, a plethora of active cooling solutions have been thoroughly examined and investigated in scholarly literature. One of the most auspicious technologies for thermal management at the chip level is the utilization of microchannel heat sinks. It is evident that embedding the microfluidic cooling channels in the vicinity of the heat sources displays the potential to handle concentrated heat within a confined region in high-power dissipating microelectronics. Notwithstanding the microchannels' effective thermal performance, the employment of small hydraulic diameter channels engenders significant pressure drops and, consequently, energy consumption. In addition, integrating complex microfluidic interior architectures to augment flow mixing and wetted surface area increases the pressure drop, leading to an exorbitant demand for pumping power. Despite the fact that miniaturization enables microchannels to be positioned in close proximity to heat sources, rendering effective cooling performance, such endeavors remain inconsequential and detrimental in the absence of energy source management. This is because the absence of such measures results in a wasteful expenditure of energy sources. In this context, this thesis delves into the benchmarking of cooling and hydrodynamic performance by investigating various microfluidic wall shapes, including straight, wavy, hybrid straight-wavy, converging-diverging and diverging-converging walls. Moreover, along with assessing the performance of diverse microchannel wall shapes, this study suggests a collection of microfluidic cooling structures that are embedded in the die, featuring microfluidic interior walls and micro pin-fins combined with sidewall ribs, to mitigate hotspots in high-power density devices. Aside from evaluating the performance of the microfluidic paths, this research also compares the suggested single-layer and double-layer microchannel configurations. The flow direction is examined in double-layer microchannels to ascertain the most efficient flow direction between the parallel and counter flow. The findings demonstrate that implementing the suggested hybrid and converging-diverging microchannels can effectively decrease the maximum temperatures of gate fingers compared to conventional straight microchannels, without causing a significant increase in pressure drop within the microchannel. Such results indicate the potential of these solutions to address the thermal management and pumping energy consumption challenges in die-embedded microchannels having extremely small hydraulic diameters. Additionally, the findings demonstrate that microchannels with straight microfluidic interior walls exhibit a local Nusselt number that is more than twice as high as that of conventional straight microchannels. In comparison to microchannels featuring straight interior microfluidic walls, the incorporation of microfluidic interior walls with a wavy shape within microfluidic structures embedded in the silicon substrate can potentially yield a local Nusselt number improvement of up to 43% at the elevated Reynolds numbers. In addition to examining wall separators, this study also investigates micro pin-fins and sidewall ribs. The findings suggest that elliptical micro pin-fin structures exhibit better performance compared to circular and diamond-shaped pin-fins. Moreover, at higher flow velocity, the utilization of ellipsoid pin-fins has been shown to significantly reduce pressure drop, thereby reducing pumping power requirement by up to 20%, as compared to circular pin-fins. Consequently, circular pin-fins of this scale exhibit poor performance in both thermal and hydrodynamic aspects. Therefore, it is advisable to employ more streamlined pin-fins in order to enhance the overall effectiveness of embedded microchannels. Alongside examining the thermal and hydrodynamic aspects of microfluidic structures, the structural characteristics of these components are also subject to scrutiny. In this context, the objective is to examine the hydro-mechanical performance of internal microfluidic structures that are susceptible to structural failure. This situation will be accomplished by utilizing the fluid-structure interaction (FSI) phenomenon. The research findings indicate that elliptical pin-fins, even those with higher aspect ratios, display outstanding performance within the Reynolds number range of 200 to 1000. This is attributed to the streamlined ellipsoidal shape of these structures. Finally, an evaluation is conducted to determine if temperature fluctuations result in elevated stress and strain levels on microfluidic devices. Given that conventional straight microchannels exhibit the highest temperature rise and resultant thermal expansion, they are selected for the numerical examination. The findings in this study reveal that the investigated microfluidic shapes can withstand potential thermo-mechanical stresses. The strategies and geometries proposed in this thesis provide valuable insights into the miniaturization of microelectronics. The geometries and techniques recommended in this research contribute to decreasing device temperatures, validating structural durability, and diminishing energy consumption. The results of this study possess the potential to develop more efficient microfluidic cooling systems integrated into the substrate of high-power microelectronics.