Search

Recent years have witnessed the increasing popularity of electric buses (e-buses) around the globe due to their environment friendly nature. However, various factors, such as the prohibitive purchasing costs and the scarcity of large-scale charging facilities, hinder the wider adoption of e-buses. Thus, to effectively cut the cost of building and maintaining urban e-bus systems, we optimize the dispatching strategy for urban e-bus systems to satisfy public transportation demands with the minimum e-bus fleet. Specifically, we propose to systematically exploit at city-scale cross-line dispatching, a smart dispatching strategy allowing one bus to serve multiple bus lines when necessary. Technically, we construct a novel and generalizable graph-theoretic model for urban e-bus systems integrating e-buses non-negligible charging time, the spatio-temporal constraints of bus trips, and various other real-world factors. We prove that it is NP-hard, and has no $(2-\epsilon)$ -approximation algorithm. Next, we propose a polynomial-time algorithm solving the problem with a guaranteed approximation ratio. Furthermore, we conduct extensive experiments on a large-scale real-world bus dataset from Shenzhen, China, which validate the effectiveness of our algorithms. As shown by our experimental results, to serve 300 bus lines, our dispatching strategy needs 38.2\% less e-buses than the one currently used in practice.

β-phase gallium oxide (Ga2O3) is an emerging ultrawide bandgap (UWBG) semiconductor (EG ∼ 4.8 eV), which promises generational improvements in the performance and manufacturing cost over today's commercial wide bandgap power electronics based on GaN and SiC. However, overheating has been identified as a major bottleneck to the performance and commercialization of Ga2O3 device technologies. In this work, a novel Ga2O3/4H-SiC composite wafer with high heat transfer performance and an epi-ready surface finish has been developed using a fusion-bonding method. By taking advantage of low-temperature metalorganic vapor phase epitaxy, a Ga2O3 epitaxial layer was successfully grown on the composite wafer while maintaining the structural integrity of the composite wafer without causing interface damage. An atomically smooth homoepitaxial film with a room-temperature Hall mobility of ∼94 cm2/Vs and a volume charge of ∼3 × 1017 cm-3 was achieved at a growth temperature of 600 °C. Phonon transport across the Ga2O3/4H-SiC interface has been studied using frequency-domain thermoreflectance and a differential steady-state thermoreflectance approach. Scanning transmission electron microscopy analysis suggests that phonon transport across the Ga2O3/4H-SiC interface is dominated by the thickness of the SiNx bonding layer and an unintentionally formed SiOx interlayer. Extrinsic effects that impact the thermal conductivity of the 6.5 μm thick Ga2O3 layer were studied via time-domain thermoreflectance. Thermal simulation was performed to estimate the improvement of the thermal performance of a hypothetical single-finger Ga2O3 metal-semiconductor field-effect transistor fabricated on the composite substrate. This novel power transistor topology resulted in a ∼4.3× reduction in the junction-to-package device thermal resistance. Furthermore, an even more pronounced cooling effect is demonstrated when the composite wafer is implemented into the device design of practical multifinger devices. These innovations in device-level thermal management give promise to the full exploitation of the promising benefits of the UWBG material, which will lead to significant improvements in the power density and efficiency of power electronics over current state-of-the-art commercial devices.

Heteroepitaxy of β-phase gallium oxide (β-Ga2O3) thin films on foreign substrates shows promise for the development of next-generation deep ultraviolet solar blind photodetectors and power electronic devices. In this work, the influences of the film thickness and crystallinity on the thermal conductivity of (201)-oriented β-Ga2O3 heteroepitaxial thin films were investigated. Unintentionally doped β-Ga2O3 thin films were grown on c-plane sapphire substrates with off-axis angles of 0° and 6° toward ⟨1120⟩ via metal-organic vapor phase epitaxy (MOVPE) and low-pressure chemical vapor deposition. The surface morphology and crystal quality of the β-Ga2O3 thin films were characterized using scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. The thermal conductivities of the β-Ga2O3 films were measured via time-domain thermoreflectance. The interface quality was studied using scanning transmission electron microscopy. The measured thermal conductivities of the submicron-thick β-Ga2O3 thin films were relatively low as compared to the intrinsic bulk value. The measured thin film thermal conductivities were compared with the Debye-Callaway model incorporating phononic parameters derived from first-principles calculations. The comparison suggests that the reduction in the thin film thermal conductivity can be partially attributed to the enhanced phonon-boundary scattering when the film thickness decreases. They were found to be a strong function of not only the layer thickness but also the film quality, resulting from growth on substrates with different offcut angles. Growth of β-Ga2O3 films on 6° offcut sapphire substrates was found to result in higher crystallinity and thermal conductivity than films grown on on-axis c-plane sapphire. However, the β-Ga2O3 films grown on 6° offcut sapphire exhibit a lower thermal boundary conductance at the β-Ga2O3/sapphire heterointerface. In addition, the thermal conductivity of MOVPE-grown (201)-oriented β-(AlxGa1-x)2O3 thin films with Al compositions ranging from 2% to 43% was characterized. Because of phonon-alloy disorder scattering, the β-(AlxGa1-x)2O3 films exhibit lower thermal conductivities (2.8-4.7 W/m·K) than the β-Ga2O3 thin films. The dominance of the alloy disorder scattering in β-(AlxGa1-x)2O3 is further evidenced by the weak temperature dependence of the thermal conductivity. This work provides fundamental insight into the physical interactions that govern phonon transport within heteroepitaxially grown β-phase Ga2O3 and (AlxGa1-x)2O3 thin films and lays the groundwork for the thermal modeling and design of β-Ga2O3 electronic and optoelectronic devices.

GaN radio frequency (RF) power amplifiers offer many benefits including high power density, reduced device footprint, high operating voltage, and excellent gain and power-added efficiency. Accordingly, these parts are enabling next-generation technologies such as fifth-generation (5G) base transceiver stations and defense/aerospace applications such as high-performance radar and communication systems. However, these benefits can be overshadowed by device overheating that compromises the performance and reliability. In response to this, researchers have focused on GaN-on-diamond integration during the past decade. However, manufacturability, scalability, and long-term reliability remain as critical challenges toward the commercialization of the novel device platform. In this work, a diamond-incorporated flip-chip integration scheme is proposed that takes advantage of existing semiconductor device processing and growth techniques. Using an experimentally validated GaN-on-SiC multifinger device model, the theoretical limit of the cooling effectiveness of the device-level thermal management solution has been evaluated. Simulation results show that by employing a $\sim 2-\mu \text{m}$ diamond passivation overlayer, gold thermal bumps, and a commercial polycrystalline carrier wafer, the power amplifier’s dissipated heat can be effectively routed toward the package, which leads to a junction-to-package thermal resistance lower than GaN-on-diamond high electron mobility transistors (HEMTs). Furthermore, simulation results show that this approach is even more promising for lowering the device thermal resistance of emerging ultra-wide bandgap devices based on $\beta $ -Ga2O3 and AlGaN, below that for today’s state-of-the-art GaN-on-diamond HEMTs.

Radio frequency (RF) microelectromechanical systems (MEMS) based on Al1-xScxN are replacing AlN-based devices because of their higher achievable bandwidths, suitable for the fifth-generation (5G) mobile network. However, overheating of Al1-xScxN film bulk acoustic resonators (FBARs) used in RF MEMS filters limits power handling and thus the phone's ability to operate in an increasingly congested RF environment while maintaining its maximum data transmission rate. In this work, the ramifications of tailoring of the piezoelectric response and microstructure of Al1-xScxN films on the thermal transport have been studied. The thermal conductivity of Al1-xScxN films (3-8 W m-1 K-1) grown by reactive sputter deposition was found to be orders of magnitude lower than that for c-axis-textured AlN films due to alloying effects. The film thickness dependence of the thermal conductivity suggests that higher frequency FBAR structures may suffer from limited power handling due to exacerbated overheating concerns. The reduction of the abnormally oriented grain (AOG) density was found to have a modest effect on the measured thermal conductivity. However, the use of low AOG density films resulted in lower insertion loss and thus less power dissipated within the resonator, which will lead to an overall enhancement of the device thermal performance.

Companies today increasingly rely on microservice architectures to deliver service for their large-scale mobile or web applications. However, not all developers working on these applications are distributed systems engineers and therefore do not anticipate partial failure: where one or more of the dependencies of their service might be unavailable once deployed into production. Therefore, it is paramount that these issues be raised early and often, ideally in a testing environment or before the code ships to production. In this paper, we present an approach called service-level fault injection testing and a prototype implementation called Filibuster, that can be used to systematically identify resilience issues early in the development of microservice applications. Filibuster combines static analysis and concolicstyle execution with a novel dynamic reduction algorithm to extend existing functional test suites to cover failure scenarios with minimal developer effort. To demonstrate the applicability of our tool, we present a corpus of 4 real-world industrial microservice applications containing bugs. These applications and bugs are taken from publicly available information of chaos engineering experiments run by large companies in production. We then demonstrate how all of these chaos experiments could have been run during development instead, and the bugs they discovered detected long before they ended up in production.

Aluminum gallium nitride (AlGaN) high electron mobility transistors (HEMTs) are candidates for next-generation power conversion and radio frequency (RF) applications. AlxGa1-xN channel HEMT devices (x = 0.3, x = 0.7) were investigated using multiple in-situ thermal characterization methods and electro-thermal simulation. The thermal conductivity, contact resistivity, and channel mobility were characterized as a function of temperature to understand and compare the heat generation profile and electro-thermal transport within these devices. In contrast to GaN-based HEMTs, the electrical output characteristics of Al0.70Ga0.30 N channel HEMTs exhibit remarkably lower sensitivity to the ambient temperature rise. Also, during 10kHz pulsed operation, the difference in peak temperature between the AlGaN channel HEMTs and GaN HEMTs reduced significantly.

Nowadays, crowdsourcing has become an increasingly popular paradigm for large-scale data collection, annotation, and classification. Today’s rapid growth of crowdsourcing platforms calls for effective worker selection mechanisms, which oftentimes have to operate with a priori unknown worker reliability. We discover that the empirical entropy of workers’ results, which measures the uncertainty in the final aggregated results, naturally becomes a suitable metric to evaluate the outcome of crowdsourcing tasks. Therefore, this paper designs a worker selection mechanism that minimizes the empirical entropy of the results submitted by participating workers. Specifically, we formulate worker selection under sequentially arriving tasks as a combinatorial multi-armed bandit problem, which treats each worker as an arm, and aims at learning the best combination of arms that minimize the cumulative empirical entropy. By information theoretic methods, we carefully derive an estimation of the upper confidence bound for empirical entropy minimization, and leverage it in our minimum entropy upper confidence bound (ME-UCB) algorithm to balance exploration and exploitation. Theoretically, we prove that ME-UCB has a regret upper bound of O(1), which surpasses existing submodular UCB algorithms. Our extensive experiments with both a synthetic and real-world dataset empirically demonstrate that our ME-UCB algorithm outperforms other state-of-the-art approaches.

The β-gallium oxide (Ga 2 O 3 ) material system offers the potential to dramatically improve the electrical performance and cost-effectiveness of next-generation power electronics. This is because of its ultra-wide bandgap (~4.8 eV) and the availability of high-quality single-crystal bulk substrates. However, the low thermal conductivity of Ga 2 O 3 (11-27 W/m-K) implies that significant thermal challenges need to be overcome to commercialize Ga 2 O 3 devices. In the present work, a single crystal (010) Ga 2 O 3 wafer was integrated with a 4H-SiC substrate via fusion bonding to address this concern of poor thermal conductivity. A differential steady-state thermoreflectance method was established to measure the thermal boundary resistance at the Ga 2 O 3 /SiC interface (100 m2K/GW), which has yet to be reported due to the limited probing depth of conventional frequency- and time-domain thermoreflectance techniques.

Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gallium nitride (GaN) with an aim to accomplish an improvement in size, weight, and power (SWaP) of power electronics beyond current devices based on silicon (Si). However, the increased operating power densities and reduced areal footprints of WBG device technologies result in significant levels of self-heating that can ultimately restrict device operation through performance degradation, reliability issues, and failure. Typically, self-heating in WBG devices is studied using a single measurement technique while operating the device under steady-state direct current (DC) measurement conditions. However, for switching applications, this steady-state thermal characterization may lose significance since high power dissipation occurs during fast transient switching events. Therefore, it can be useful to probe the WBG devices under transient measurement conditions in order to better understand the thermal dynamics of these systems in practical applications. In this work, the transient thermal dynamics of an AlGaN/GaN high electron mobility transistor (HEMT) were studied using thermoreflectance thermal imaging and Raman thermometry. Also, the proper use of iterative pulsed measurement schemes such as thermoreflectance thermal imaging to determine the steady-state operating temperature of devices is discussed. These studies are followed with subsequent transient thermal characterization to accurately probe the self-heating from steady-state down to sub-microsecond pulse conditions using both thermoreflectance thermal imaging and Raman thermometry with temporal resolutions down to 15 ns.

The extremely high concentration of the electric field near the drain corner of the gate is the major cause for premature breakdown and local hotspot formation of AlGaN/GaN high electron mobility transistors (HEMTs). This hotspot negatively impacts the device performance, reliability, lifetime, and limits the operability envelope. Metal field plates have been used to mitigate this electric field concentration. However, it has been proven difficult to reach the theoretical breakdown voltage of GaN using conventional field plate structures. In this work, a novel “resistive field plate” structure based on amorphous indium-gallium-zinc-oxide (a-IGZO) was constructed to minimize the electric field concentration by imposing a linear voltage boundary condition on the device surface, between the drain and source electrodes. Standard electrical testing and state-of-the-art optical thermography techniques were used to understand the electro-thermal interactions behind the improvement in electrical and thermal performance of AlGaN/GaN metal-oxide-semiconductor HEMTs (MOS-HEMTs) employing the a-IGZO resistive field plate. The novel field plated design resulted in ∼25% increase in the device breakdown voltage (V B ) and ∼24% reduction in the channel peak temperature as compared to a control sample employing no field plate structures. Key knowledge obtained from this study provides insight into enhancing the performance and reliability of AlGaN/GaN HEMTs and also suggests a viable solution that may enable the full exploitation of the potential of wide bandgap and ultra-wide bandgap lateral power electronic devices.

Floquet topological insulators (FTIs) have been used to study the topological features of a dynamic quantum system within the band structure. However, it is difficult to directly observe the dynamic modulation of band structures in FTIs. Here, we implement the dynamic Su–Schrieffer–Heeger model in periodically curved waveguides to explore new behaviors in FTIs using light field evolutions. Changing the driving frequency produces near-field evolutions of light in the high-frequency curved waveguide array that are equivalent to the behaviors in straight arrays. Furthermore, at modest driving frequencies, the field evolutions in the system show boundary propagation, which are related to topological edge modes. Finally, we believe curved waveguides enable profound possibilities for the further development of Floquet engineering in periodically driven systems, which ranges from condensed matter physics to photonics.

The motion of domain walls in lead zirconate titanate (PZT) produces both nonlinearity and hysteresis. While measurements of the resulting self-heating under an electric field drive are well known in bulk ferroelectric ceramics, self-heating effects in PZT films may differ from those in bulk ceramics due to a combination of reduced domain wall motion, differences in heat dissipation associated with substrates or passive elastic layers, and differences in typical drive fields. Here, it is shown that the thermal imaging of the test structures of PZT piezoelectric microelectromechanical systems using techniques such as infrared thermography and thermoreflectance thermal imaging suffers from motion-induced artifacts. These limitations were overcome via nanoparticle-assisted Raman thermometry with a spatial resolution of ∼1 μm. To acquire the local temperature distribution quantitatively, anatase nanoparticles were distributed across the electrodes and actuating PZT diaphragm. The temperature rise of the test structures increased as the operating frequency, voltage amplitude, and slew rate increased. As expected, the largest temperature rises were induced due to self-heating associated with domain switching under bipolar operation. In addition, a higher voltage amplitude testing revealed non-uniform temperature distributions across the piezoelectric actuator, suggesting that AC Joule heating can induce significant heat generation (ΔT ∼ 30 K) under high electric fields (∼390 kV/cm).

Ultra-wide bandgap β-gallium oxide (Ga2O3) devices are of considerable interest with potential applications in both power electronics and radio frequency devices. However, current Ga2O3 device technologies are limited by the material's low intrinsic electron mobility and thermal conductivity. The former problem can be addressed by employing modulation-doped β-(AlxGa1−x)2O3/Ga2O3 heterostructures in the device architecture. In this work, (AlxGa1−x)2O3/Ga2O3 modulation-doped field effect transistors (MODFETs) have been investigated from a thermal perspective. Thermoreflectance thermal imaging was used to characterize the self-heating of the MODFETs. The (Al0.18Ga0.82)2O3 thermal conductivity (3.1–3.6 W/mK) was determined using a frequency domain thermoreflectance technique. Electro-thermal modeling was used to discern the effect of design parameters such as substrate orientation and channel length on the device self-heating behavior. Various thermal management schemes were evaluated using the electro-thermal device model. From an electro-thermal co-design perspective, the improvement in electrical performance followed by the mitigation of self-heating was also studied. For example, by employing a Ga2O3-on-SiC composite wafer, which was fabricated in this work, a 50% increase in power handling capability can be achieved as compared to a homoepitaxial device. Furthermore, flip-chip heterointegration and double-sided cooling approaches can lead to more than 2× improvement in the power handling capability. Using an augmented double-sided cooling design that includes nanocrystalline diamond passivation, a 5× improvement in the power handling capability can be accomplished, indicating the potential of the technology upon implementation of a suitable thermal management scheme.

Improvements in radio frequency and power electronics can potentially be realized with ultrawide bandgap materials such as aluminum gallium nitride (AlxGa1−xN). Multidimensional thermal characterization of an Al0.30Ga0.70N channel high electron mobility transistor (HEMT) was done using Raman spectroscopy and thermoreflectance thermal imaging to experimentally determine the lateral and vertical steady-state operating temperature profiles. An electrothermal model of the Al0.30Ga0.70N channel HEMT was developed to validate the experimental results and investigate potential device-level thermal management. While the low thermal conductivity of this III-N ternary alloy system results in more device self-heating at room temperature, the temperature insensitive thermal and electrical output characteristics of AlxGa1−xN may open the door for extreme temperature applications.