Investigation of Thermal Characteristics of Multilayer Heterostructures for Enhanced GaN HEMT Reliability

Gallium nitride high-electron-mobility transistor (GaN HEMT) power devices are favored in various scenarios due to their high-power density and efficiency. However, with the significant increase in the heat flux density, the junction temperature of GaN HEMT has become a crucial factor in device reliability. Since the junction temperature monitoring technology for GaN HEMT based on temperature-sensitive electrical parameters (TSEPs) is still in the exploratory stage, the TSEPs’ characteristics of GaN HEMT have not been definitively established. In this paper, for the common steady-state TSEPs of GaN HEMT, the variation rules of the saturation voltage with low current injection, threshold voltage, and body-like diode voltage drop with temperature are investigated. The influences on the three TSEPs’ characteristics are considered, and their stability is discussed. Through experimental comparison, it is found that the saturation voltage with low current injection retains favorable temperature-sensitive characteristics, which has potential application value in junction temperature measurement. However, the threshold voltage as a TSEP for certain GaN HEMT is not ideal in terms of linearity and stability.

Soil profile method for soil thermal diffusivity, conductivity and heat flux: Comparison to soil heat flux plates

The surface heat flux provides important constraints on the present-day thermal state of the lunar interior. In situ surface heat flux measurements, performed during the Apollo program, obtained surface heat flux values of 21±3 and 14±2 mW/m2 at the landing sites of Apollo 15 and 17, respectively [1]. Recently, a peak heat flux of ~180 mW/m2 was inferred by [2] from the Chang’E 1 and 2 data at the Compton-Belkovich location. A lower bound for global lunar heat flux of ~6 mW/m2, based on measurements of the Diviner Lunar Radiometer Experiment onboard LRO, has been suggested for Region 5, a location at the eastern edge of Haworth crater [3]. In addition to heat flux measurements, global thermal expansion/contraction estimates can be used to constrain the thermal state of the interior throughout lunar history [4].Differences between the surface heat flux measurements, even for geographically close regions, indicate a high heat flux variability on the Moon. Previous work investigated regional combinations of crustal thermophysical properties (thickness, density, thermal conductivity), distribution of heat producing elements (HPEs), and heat flux coming from the mantle to explain the differences between the two Apollo measurements [5,6]. The results of these regional-scale models suggested  heat-flux values of 7-13 mW/m2  and 12-14 mW/m2  at the crust-mantle interface and for the global average, respectively. These studies also highlighted the need to include regional-scale scenarios within global thermal evolution models and investigate key aspects (e.g., the geographical extent of a subsurface KREEP layer), in order to relate local heat flux measurements and regional-scale estimates to the global properties of the Moon. Here, we construct 3D thermal evolution models using the fluid solver GAIA [7]. The interior dynamics of the Moon are simulated over 4.5 billion years, by solving the conservation equations of mass, linear momentum and thermal energy. Similar to [8], we account for  spatially variable crustal thickness, derived from gravity and topography data [9] (Fig. 1a). We consider higher HPEs abundances in the PKT and crust compared to the mantle, and compute the interior dynamics (Fig. 1b) using an Arrhenius viscosity for diffusion creep similar to [11]. This setup is designed to allow for an extensive investigation of lunar crustal properties and their spatial variability.In our models we vary critical parameters such as the location, size, thermal conductivity, and HPE enrichment of the PKT region, but also the location of a putative KREEP layer (i.e., within or below the crust). Finally, we consider various thermal conductivity values and HPE enrichment of the anorthositic crust, and initial temperature profile of the interior. In addition to present-day surface heat flux, we compute the heat flux at the crust-mantle interface and also estimate the time-variable thermal expansion/contraction during lunar evolution. These values are compared to all available constraints [1-6] to select best-fit models.Our models that are best compatible with the Apollo measurements are in agreement with [5] and [6]. These models have a bulk uranium content of 25 ppb [11], and show average and standard deviation surface and crust-mantle interface heat flux values of ~14±5 and ~8±1  mW/m2, respectively (Fig. 2a and b). For the Compton-Belkovich anomaly, none of our models can reproduce the high heat flux value proposed by [2]. Therefore, we conclude that the inferred heat flux value of ~180 mW/m2 indicates either very specific local processes (e.g., overly high radiogenic enrichment) [2] or large measurement uncertainties.We find that, without considering 3D lateral variability in thermal conductivity across the lunar surface, measurably lower heat flux values at Region 5 [3] with respect to Apollo 15 & 17 require for KREEP material to extend at least partly beneath mare Serenitatis. In this case, the Apollo 15 measurement would be representative of the PKT average heat flux, while Apollo 17 would lie on its edge (Fig. 2c). On the other hand, a smaller PKT could imply that the low heat flux at Region 5 is caused by different phenomena, for instance a laterally variable thermal conductivity. Heat flux measurements that will be performed by the Farside Seismic Suite [12] at Schrödinger crater, if successful, will provide key information that would help to exclude one of these two scenarios, and thus potentially constrain the extent on the KREEP layer underneath PKT region.The full spatial variations of the surface heat flux on the Moon obtained by our models are an important step forward towards understanding the thermal history and present-day state of the lunar interior. Such models can help to put current and future heat flux measurements into a global context and to constrain the distribution of the KREEP layer beneath the PKT region. Future work will include a laterally variable thermal conductivity of the crust to test the effects of the spatial distribution of lunar regolith on the surface heat flow and subsurface temperature variations.

This study investigates the origin of gate degradation in AlGaN/GaN HEMTs under high-temperature reverse bias (HTRB) conditions and proposes a buffer engineering strategy to mitigate this degradation. Transmission electron microscopy (TEM) analysis reveals the presence of a thin oxide layer between the gate and AlGaN contact. It is found that the holes generated through impact ionization (under high electric fields of HTRB operation), are directed toward the gate due to the intense local electric field under the gate and the field plate overhang, and are accumulated under this energetic barrier. Consequently, these trapped holes cause gate degradation and increased gate leakage. To address this issue, impact ionization, as the initial forcing mechanism of the degradation, is suppressed via thinning the channel GaN (C-GaN) layer. This improvement is attributed to the suppression of the local electric field near the gate region in thin C-GaN HEMTs. Additionally, the impact of C-GaN thinning on breakdown characteristics and RF performance is discussed. Overall, the findings provide insights into the root cause of gate degradation and offer a buffer engineering strategy to minimize gate degradation under deep off-state stress. This approach enhances the reliability of future high-power and high-frequency GaN HEMTs, contributing to their long-term performance in demanding applications.

Subject of Study. The paper deals withthermophysical properties of a new polydimethylsiloxane potting compound. We study the principles of thermal conductivity and heat capacity changing depending on composition of materials in the temperature range of 25-175 ° C. The search of optimal composition of the compound is carried out providing thermal conductivity not less than 0.5 W/(м·К) and data set for the change prediction of thermo-physical properties depending on a compound composition. Methods. All the samples were produced by FSUE “Institute of Synthetic Rubber” on the basis of polymers used in serial production. All the compounds are two- or three-component materials based on a low molecular weight (liquid) polydimethylsiloxane caoutchouc made by a cold solidification method. Polyethylsiloxane PES-5, aluminum hydroxide, TiN, boron, zinc oxide are used as additives. The research of thermal conductivity and heat capacity are carried out by meters λ-400 and С-400by dynamic method of monotonic warming. Main Results. We have obtained new experimental data about the thermal conductivity and heat capacity of the polysiloxane compound, depending on the temperature and the concentrations of various fillers. With increasing of the filler concentration the thermal conductivity of the samples increases as well and the heat capacity decreases. It is shown that with increasing of temperature the thermal conductivity of compounds is falling by about 15%, and the heat capacity increases by about 60-70%. Practical Relevance. The retrieved data give the possibility to find the optimal composition of compound material that guarantees the following operational properties: thermal conductivity not less than 0.5 W/(м·К), long thermostability in the temperature range of -60 - +200 °C, the density of hydrogen nucleus not less than 1∙1014 nucleus/cm2 and the summary absorbed gamma radiation dose up to 300 Gy. The developed compound is being tested and can be applicable in the development of neutron shielding for the transportation of the used nuclear fuel in the nuclear fuel containers.

A paradigm shift in liquid cooling by multitextured surface design

The main idea in this work, is to evaluate the advantage of Graphene transistors. In addition, we investigate the thermal stability of nano devices. In the first step, we study the ability of our proposed model to predict the thermal conduction in nano MOSFET devices. On the other hand, we studied the nano-heat conduction process in Graphene transistors. The ballistic-diffusive equation (BDE), was numerically solved to assess the heat transport in the nano transistors. The proposed model is used to report the thermal conductivity through the nature of scattering mechanism. It is found that our predictive model is able to describe phonons scattering in MOS structures. Furthermore, the Graphene MOSFET is more thermally stable than the Si MOSFET devices.

This study presents the result of the estimation of heat flow from six (6) wells (Well X:001 to 006) in South-Western Niger Delta using values of Geothermal gradient (GG), Geothermal heat flux (Q) and thermal conductivity (K) computed from Sonic and continuous temperature log data for each well. Geothermal gradient was computed from continuous temperature logs using the simple gradient method while geothermal heat flux and thermal conductivity of the rocks in the wells were computed from the sonic log data, using the Relative Heat Flow Model and Fourier One-dimensional Heat Flow Law respectively. The results were analysed and interpreted to investigate the thermal structure and pattern of heat flow distribution of the basin. Results showed that geothermal gradient ranges from 1.45 0 C/100m to a value of 1.61 0 C/100m, with a simple average of 1.55 0 C/100m. Geothermal gradient contour map computed from this result, showed a low thermal gradient at the northern part of the study area where we have Well X-006 and increases outwards in all direction as we move further offshore. These differences reflect changes in thermal conductivity of rocks, ground water movement and endothermic reaction during diagenesis, since geothermal gradient is influenced by lithology or differential rate of sedimentation. Therefore, it was inferred that sediments with a relatively high geothermal gradient (1.55 to 1.61 0 C/100m) will mature earlier (low oil window) than those with low thermal gradient values. By implication, a high geothermal gradient enhances the early formation of oil at relatively shallow burial depths, but causes the depth range of the oil window to be narrow, while low geothermal gradient causes the first formation of oil to begin at fairly deep subsurface levels, but makes the oil window broad. Geothermal heat flux estimated from subsurface temperature and one-way sound travel time, shows heat flux varying between 33.16 mWm -2 to 72.73 mWm -2 with a simple average of 48.43 mWm -2 . Low heat flux was observed at the central part of the study area which increases towards the western and eastern parts of the area with Well X-005 characterized by a higher geothermal heat flux. Therefore, it was inferred that the western and eastern parts of the study area with higher heat flux values may be characterized as zones with maximum sediment thickness and are characterized as having depressions (gravity low) on the geoid which is characteristics of a basin, while the central part of the study area with low heat flux values correspond with zones of minimum sediment thickness. Also, thermal conductivity of rocks in the study area computed directly from heat flux and geothermal gradient results, ranges from 2.28W/m 0 C to 4.76 W/m 0 C with an average of 3.19 W/m 0 C. Thermal conductivity contour map computed from this result, showed low thermal conductivity values observed at the central part of the study area, and increases outwards towards the west and eastern parts. This pattern of thermal conductivity variation suggests probably there exists heavy crude oil at the central part of the study area and lighter crude oil as we move outward in all direction. It was also observed that within each well, thermal conductivity increased with depth and decreased with porosity which may be caused by difference in lithology and fluid content, due to the fact that all pore fillers (i:e gases and liquids) are poor conductors. The estimated values of geothermal gradient, heat flux and thermal conductivity obtained in this study are similar to the results obtained from previous studies in the region and with other passive continental margins of the world. Keywords: Heat flow, Geothermal Gradient, Geothermal Heat Flux, Thermal Conductivity, Continuous Temperature and Sonic log. DOI: 10.7176/JETP/10-2-04 Publication date: April 30 th 2020

Thermal management of GaN HEMT devices using subcooled flow boiling in an embedded manifold microchannel heat sink

AlN/GaN/AlN high electron mobility transistors (HEMTs) have demonstrated exceptional potential for surpassing the electrical limitations of conventional AlGaN/GaN HEMTs. This study investigates the thermal performance of two types of AlN/GaN/AlN HEMTs with homoepitaxial AlN buffer layers grown on AlN substrates: an AlN/GaN/AlN single-crystal HEMT (AlN XHEMT) featuring a pseudomorphic/thin GaN channel and a conventional structure with a relaxed/thick GaN channel. Frequency- and time-domain thermoreflectance measurements reveal bulk-like thermal conductivity in the homoepitaxial AlN buffer layer, with negligible thermal boundary resistance at the AlN buffer/substrate interface. Consequently, Raman thermometry demonstrates that the AlN XHEMT with a thin (∼20 nm) pseudomorphically strained GaN channel exhibits better thermal performance than identical HEMT layer structures grown on a 4H-SiC substrate, despite 4H-SiC possessing a higher thermal conductivity. In addition, the AlN XHEMT exhibits a 22% lower channel temperature under 14 W/mm power density than the AlN/GaN/AlN-on-AlN HEMT that employs a thick (275 nm) relaxed GaN channel. These findings highlight that AlN XHEMTs offer not only electrical but also thermal advantages for high-power and high-frequency applications.

AlN has the largest bandgap in the wurtzite III‐nitride semiconductor family, making it an ideal barrier for a thin GaN channel to achieve strong carrier confinement in field‐effect transistors, analogous to silicon‐on‐insulator technology. Unlike /Si/, AlN/GaN/AlN can be grown fully epitaxially, enabling high carrier mobilities suitable for high‐frequency applications. However, developing these heterostructures and related devices has been hindered by challenges in strain management, polarization effects, defect control, and charge trapping. Here, the AlN single‐crystal high electron mobility transistor (XHEMT) is introduced, a new nitride transistor technology designed to address these issues. The XHEMT structure features a pseudomorphic GaN channel sandwiched between AlN layers, grown on single‐crystal AlN substrates. XHEMTs demonstrate RF performance on par with the state‐of‐the‐art GaN HEMTs, achieving 5.92 W/mm output power and 65% peak power‐added efficiency at 10 GHz under 17 V drain bias. These devices overcome several limitations present in conventional GaN HEMTs, which are grown on lattice‐mismatched foreign substrates that introduce undesirable dislocations and exacerbated thermal resistance. With the recent availability of 100‐mm AlN substrates and AlN's high thermal conductivity (340 W/), XHEMTs show strong potential for next‐generation RF electronics.

Dynamical consequences in the lower mantle with the post-perovskite phase change and strongly depth-dependent thermodynamic and transport properties

Calculation and analysis of lattice thermal conductivity in tungsten by molecular dynamics

The heat release process in a rotating detonation combustor (RDC) exhibits highly transient characteristics, posing significant demands on the thermal protection and management of the rotating detonation engine (RDE). In this work, the wall heat transfer characteristics of the RDC supplied by H2/air were experimentally examined with different equivalence ratios, mass flow rates, and initial wall temperatures. High-speed photography and dynamic pressure transducers were used to determine the propagation mode of the rotating detonation wave, while the wall temperature and heat flux were monitored by thermocouples. The results showed that the wall temperature and heat flux decreased along the axial direction. A parabolic temperature variation occurs when equivalence ratio increases from 0.8 to 1.3, and the extreme value appears at Φ = 1.2. The same trend happens between heat flux and equivalence ratio. The mass flow rate increase leads to the overall increase in the temperature and heat flux, with the spatial distributions remaining unchanged. The higher initial wall temperature leads to the increase in the combustor outer wall temperature, a reduction in the spatial variation of temperature distribution, a decrease in heat flux, and a reduction in the spatial variation of heat flux. Furthermore, an empirical model was developed to estimate the heat transfer characteristics. Valid calculations show that the temporal and spatial temperature function results in lower errors of peak temperature prediction by approximately 50% and higher spatial resolution compared to a constant heat flux boundary condition. The research findings provide a theoretical foundation for the RDE thermal protection issues.

Performance analysis of a medium concentrated photovoltaic system thermally regulated by phase change material: Phase change material selection and comparative analysis for different climates