Design and performance analysis of Tri-gate GaN HEMTs

GaN-based high electron mobility transistors (HEMT) are promising devices for radio frequency (RF) and high-power electronics and are already in use for RF power amplifiers and for power switches. Commonly, these devices are normally-on transistors, i.e., they are in the on-state at zero applied gate voltage, what limits their suitability for various applications, such as fail-safe power switches and RF amplifiers with single-polarity power supply. Unfortunately, in contrast to GaAs- and InP HEMTs, achieving normally-off operation, i.e., a positive thresh-old voltage, for GaN heterostructures is difficult due to the high density of the polarization-induced two-dimensional electron gas (2DEG) at the barrier/buffer interface. For fast RF HEMTs, short gates are required. However, HEMTs with aggres-sively scaled gate length frequently suffer from short-channel effects caused by a degraded control of the gate over the channel. This leads to a deterioration of the transistors off-state performance (increased subthreshold swing and drain-induced barrier lowering) and on-state behavior (increased drain conductance). The tri-gate design has recently been applied to MOSFETs and HEMTs to improve the gate control and suppress short channel effects. Experimental tri-gate transistors show excellent down-scaling characteristics, improved performance, and, in particular for GaN tri-gate HEMTs, a significant shift of the threshold voltage toward positive values. On the other hand, tri-gate GaN normally-off HEMTs are still suffering from increased parasitics causing degraded RF performance (particularly in terms of cutoff frequency) compared to their planar counterparts. Improving the RF performance of GaN tri-gate HEMTs by reducing the parasitics is essential, but this requires a deep understanding of device physics and a thorough analysis of the root causes. In the present work, in-depth theoretical investigations of GaN tri-gate HEMT operation are performed and extensive simulation studies for these devices are conducted. As a result of these efforts, improved insights in the physics of GaN tri-gate HEMTs are achieved, the potential of this transistor type is assessed, design guidelines are elaborated, and advantageous designs are developed. It is shown that the 2DEG sheet density decreases by shrinking the body width, that the threshold voltage of GaN tri-gate HEMTs strongly depends on the width of AlGaN/GaN bodies, and that solely by decreasing the body width a transition from normally-on to normally-off operation can be achieved. The separation between adjacent bodies is shown to have less impact on threshold voltage. The results also show that for wide bodies (> 200 nm) the channel is controlled by both the top-gate and the sidewall gates, while for decreasing body width the control by top-gate gradually diminishes and the channel will be only controlled by side-gates. Furthermore, the impact of AlGaN barrier design (Al content, thickness) is studied, and the results show a limited dependency of the threshold voltage on the barrier design for very narrow bodies. The tri-gate concept enables normally-off operation, provides improved on-state performance (higher transconductance), and effectively suppresses short-channel effects in the off-state. Moreover, the simulation results show that GaN tri-gate HEMTs can exhibit higher breakdown voltages and operate closer to the theoretical limit for GaN devices than their planar counterparts. Moreover, the simulations indicate that the RF performance of GaN tri-gate HEMTs with optimized body designs can be superior to that of conventional planar devices. A means to improve the RF performance is the reduction of the body etch height, leading to a decreased parasitic coupling between the sidewalls and the source/drain electrodes. Thus, reducing the body height leads to a decreased overall gate capacitance and an improved RF performance. Another way to reduce the overall gate capacitance is to cover the body sidewalls with a dielectric (e.g. SiN). This reduces the fringing capacitance components since the gap between neighboring bodies that is filled with gate metal is narrower compared to the case without dielectrics. Finally, the polarization charge at the barrier/channel interface and thus the electron density in the 2DEG) can be increased either by increasing the aluminium content of the AlGaN barrier or by using a different barrier material (e.g., lattice matched In0.17 Al0.83 N). In the frame of a joint DFG project, GaN tri-gate HEMTs designed based on the improved insights in the physics of these devices have been fabricated and characterized at Fraunhofer IAF. These devices having a gate length of 100 nm are by far the fastest GaN tri-gate HEMTs worldwide and show record performance in terms of cutoff frequency (120 GHz) and maximum frequency of oscillation (300 GHz).HEMTs (high electron mobility transistors) auf GaN-Basis besitzen großes Potenzial für die HF- (Hochfrequenz) und Leistungselektronik und werden bereits in HF-Leistungsverstärkern und als Leistungsschalter verwendet. Üblicherweise sind GaN HEMTs Normally-On Transistoren (d.h. Transistoren, die sich bei einer Gatespannung von 0 V im Ein-Zustand befinden), was für Anwendungen wie Fail-Safe-Leistungsschalter und HF-Verstärker mit nur einer Versorgungsspannung nachteilig ist. Es schwierig, GaN HEMTs mit Normally-Off-Charakteristik (HEMTs mit positiver Schwellspannung) zu realisieren, da in diesen Transistoren die Dichte des sich an der Grenzfläche Barriere/Puffer ausbildenden 2DEG (zweidimensionales Elektronengas) auf Grund starker Polarisationseffekte erheblich größer als in GaAs und InP HEMTs ist. Die Realisierung schneller HF-HEMTs erfordert kurze Gates. Allerdings leiden Transistoren mit sehr kurzen Gates häufig unter Kurzkanaleffekten und einer reduzierten Steuerwirkung des Gates, was zu einer Verschlechterung des Verhaltens im Aus-Zustand (erhöhte Werte für den Subthreshold Swing und das Drain-Induced Barrier Low-ering) und im Ein-Zustand (erhöhter Drainleitwert) führt. In jüngster Zeit wird bei MOSFETs und HEMTs das Tri-Gate-Design angewendet, um die Gatesteuerwirkung zu verbessern und Kurzkanaleffekte zu unterdrücken. So wurden bereits Tri-Gate-Transistoren mit ausgezeichnetem Skalierungsverhalten, verbesserten Eigenschaften und, speziell im Fall von GaN Tri-Gate-HEMTs, positiver Schwellspannung, demonstriert. Auf der anderen Seite leiden GaN Tri-Gate-HEMTs mit Normally-Off-Charakteristik jedoch unter großen Parasitäten, die das HF-Verhalten (insbesondere die Transitfrequenz) beeinträchtigen. Die Verbesserung des HF-Verhaltens und eine Reduzierung der Parasitäten von GaN Tri-Gate-HEMTs ist daher dringend nötig. Das erfordert jedoch ein tiefes Eindringen in die Physik dieser Bauelemente. In der vorliegenden Arbeit werden umfassende theoretische Untersuchungen und Bauelementesimulationen zu GaN Tri-Gate-HEMT beschrieben, die zu einem deutlichen verbesserten Verständnis der Wirkungsweise von GaN Tri-Gate-HEMTs führten. So konnten das Potential dieses Transistortyps bewertet, Designregeln erarbeitet und vorteilhafte Transistordesigns entwickelt werden. In der Arbeit wird gezeigt, dass eine Verringerung der Bodyweite bei gegebener Gatespannung zu einer Verringerung der Ladungsträgerdichte im 2DEG führt, dass die Schwellspannung maßgeblich von der Bodyweite bestimmt wird und dass bei hinreichend geringer Bodyweite der Übergang vom Normall-On- zum Normally-Off-Betrieb erfolgt. Es wird auch gezeigt, dass der Abstand zwischen benachbarten Bodies nur einen geringen Einfluss auf die Schwellspannung hat. Darüber hinaus wird demonstriert, dass im Fall weiter Bodies (> 200 nm) der Kanal sowohl durch das Top-Gate als auch durch die Seiten-Gates gesteuert wird, während bei schmaleren Bodies die Steuerwirkung durch das Top-Gate geringer wird und die Verhältnisse im Kanal im Wesentlichen durch das Seiten-Gates bestimmt werden. In der Arbeit wird weiterhin Rolle des Designs der AlGaN-Barriere (Al-Gehalt, Dicke) untersucht und demonstriert, dass die Gestaltung der Barriere bei schmalen Bodies nur einen begrenzten Einfluss auf die Schwellspannung hat. Die Untersuchungen zeigen deutlich, dass das mit dem Tri-Gate-Konzept Normally-Off-Transistoren realisierbar sind, dass das Transistorverhalten im Ein-Zustand verbessert (höhere Steilheit) wird, und dass Kurzkanaleffekte im Aus-Zustand wirkungsvoll unterdrückt. Es wird auch demonstriert, dass GaN Tri-Gate HEMTs höhere Durchbruchspannungen zeigen und näher an der theoretischen Grenze für GaN-Bauelemente arbeiten als planare GaN HEMTs. Ein weiteres Ergebnis der vorliegenden Arbeit ist der Nachweis, dass GaN Tri-Gate-HEMTs mit sorgfältig optimiertem Design den planaren HEMTs auch hinsichtlich des HF-Verhaltens überlegen sind. Ein Mittel zur Verbesserung des HF-Verhaltens ist die Reduzierung der Body-Ätzhöhe, die zur Verringerung der parasitären Kopplung zwischen den Body-Seitenwänden und den Source/Drain-Elektroden und somit zu einer geringeren Gatekapazität führt. Eine weitere Maßnahme zur Reduzierung der Gatekapazität ist die Beschichtung der Body-Seitenwände mit einem Dielektrikum (z.B. SiN). Das verringert die Streukapazität, da jetzt die mit dem Gatemetall gefüllte Lücken zwischen benachbarten Bodies schmaler sind. Schließlich wird gezeigt, dass die Polarisationsladung an der Grenzfläche Barrier/Kanal und somit die Elektronendichte im 2DEG durch Erhöhung des Al-Gehalts der AlGaN-Barriere oder durch Nutzung eines anderen Materials für die Barriere (z.B. gitterangepasstes In0.17 Al0.83 N) gesteigert werden kann

A single-phase compact-sized matrix converter with symmetrical bipolar buck and boost output voltage control

The development of single-phase symmetrical bipolar voltage gain matrix converters (MC) is growing rapidly as they find their application in power systems for dynamic restoration of line voltages, high voltage AC–DC converters, and variable frequency controllers for many industrial processes. However, the existing trend in matrix converter technology is a buck–boost operation that has inherently serious issues of high voltage and current surges or stresses. This is a big source of the high voltage and current rating of semiconductor switching devices. There is also a problem of high ripples both for voltage as well for current, requiring large size of filtering capacitors and inductors. The non-symmetrical control of the voltage gain increases the control complication. A large count of operating transistors is critical regarding their cost, size, and power conversion losses, as the space and cost required by their gate control circuits are much larger than the size and cost of the switching transistors. Thus, in this research work, a new single-phase MC is introduced only employing six fully controlled switching devices, ensuring similar operation or outputs as is obtained from the existing topologies that require the use of eight or more fully controlled switching devices, and the reduction by two or more switching transistors helps to compact the overall size and lower the overall cost. The separation in its voltage buck and boost operation enables smooth control of the voltage gain through duty cycle control. The low values of the voltage and current surges reduce the power rating and losses of the switching devices. The flow of the current in the filtering inductor is kept unidirectional to avoid the current interruption and reversal problem once the operation of the converter is abruptly switched from inverting to non-inverting and vice versa. All these factors are comprehensively detailed through the circuit’s description and comparative analysis. Simulation and practical results are presented to confirm the effectiveness of the developed circuit topology

Introducing adaptive machine learning technique for solving short-term hydrothermal scheduling with prohibited discharge zones

The short-term hydrothermal scheduling (STHTS) problem has paramount importance in an interconnected power system. Owing to an operational research problem, it has been a basic concern of power companies to minimize fuel costs. To solve STHTS, a cascaded topology of four hydel generators with one equivalent thermal generator is considered. The problem is complex and non-linear and has equality and inequality constraints, including water discharge rate constraint, power generation constraint of hydel and thermal power generators, power balance constraint, reservoir storage constraint, initial and end volume constraint of water reservoirs, and hydraulic continuity constraint. The time delays in the transport of water from one reservoir to the other are also considered. A supervised machine learning (ML) model is developed that takes the solution of the STHTS problem without PDZ, by any metaheuristic technique, as input and outputs an optimized solution to STHTS with PDZ and valve point loading (VPL) effect. The results are quite promising and better compared to the literature. The versatility and effectiveness of the proposed approach are tested by applying it to the previous works and comparing the cost of power generation given by this model with those in the literature. A comparison of results and the monetary savings that could be achieved by using this approach instead of using only metaheuristic algorithms for PDZ and VPL are also given. The slipups in the VPL case in the literature are also addressed

Dragonfly algorithm-based optimization for selective harmonics elimination in cascaded H-bridge multilevel inverters with statistical comparison

Harmonics worsen the quality of electrical signals, hence, there is a need to eliminate them. The test objects under discussion are single-phase versions of cascaded H-bridge (CHB) multilevel inverters (MLIs) whose switching angles are optimized to eliminate specific harmonics. The Dragonfly Algorithm (DA) is used to eradicate low-order harmonics, and its statistical performance is compared to that of many other optimization techniques, including Particle Swarm Optimization (PSO), Accelerated Particle Swarm Optimization (APSO), Differential Evolution (DE), and Grey Wolf Optimization (GWO). Various scenarios of the algorithms’ search agent population for inverters with seven, nine, and eleven levels of output voltages are comprehensively addressed in this research. No algorithm shows total dominance in every scenario. The DA is least impacted by the change in dimensions of the narrated problem

Effects of hospital facilities on patient outcomes after cancer surgery: an international, prospective, observational study

Background Early death after cancer surgery is higher in low-income and middle-income countries (LMICs) compared with in high-income countries, yet the impact of facility characteristics on early postoperative outcomes is unknown. The aim of this study was to examine the association between hospital infrastructure, resource availability, and processes on early outcomes after cancer surgery worldwide.Methods A multimethods analysis was performed as part of the GlobalSurg 3 study-a multicentre, international, prospective cohort study of patients who had surgery for breast, colorectal, or gastric cancer. The primary outcomes were 30-day mortality and 30-day major complication rates. Potentially beneficial hospital facilities were identified by variable selection to select those associated with 30-day mortality. Adjusted outcomes were determined using generalised estimating equations to account for patient characteristics and country-income group, with population stratification by hospital.Findings Between April 1, 2018, and April 23, 2019, facility-level data were collected for 9685 patients across 238 hospitals in 66 countries (91 hospitals in 20 high-income countries; 57 hospitals in 19 upper-middle-income countries; and 90 hospitals in 27 low-income to lower-middle-income countries). The availability of five hospital facilities was inversely associated with mortality: ultrasound, CT scanner, critical care unit, opioid analgesia, and oncologist. After adjustment for case-mix and country income group, hospitals with three or fewer of these facilities (62 hospitals, 1294 patients) had higher mortality compared with those with four or five (adjusted odds ratio [OR] 3.85 [95% CI 2.58-5.75]; p<0.0001), with excess mortality predominantly explained by a limited capacity to rescue following the development of major complications (63.0% vs 82.7%; OR 0.35 [0.23-0.53]; p<0.0001). Across LMICs, improvements in hospital facilities would prevent one to three deaths for every 100 patients undergoing surgery for cancer.Interpretation Hospitals with higher levels of infrastructure and resources have better outcomes after cancer surgery, independent of country income. Without urgent strengthening of hospital infrastructure and resources, the reductions in cancer-associated mortality associated with improved access will not be realised

Isolation Improvement of Parasitic Element-Loaded Dual-Band MIMO Antenna for Mm-Wave Applications

A dual-band, compact, high-gain, simple geometry, wideband antenna for 5G millimeter-wave applications at 28 and 38 GHz is proposed in this paper. Initially, an antenna operating over dual bands of 28 and 38 GHz was designed. Later, a four-port Multiple Input Multiple Output (MIMO) antenna was developed for the same dual-band applications for high data rates, low latency, and improved capacity for 5G communication devices. To bring down mutual coupling between antenna elements, a parasitic element of simple geometry was loaded between the MIMO elements. After the insertion of the parasitic element, the isolation of the antenna improved by 25 dB. The suggested creation was designed using a Rogers/Duroid RT-5870 laminate with a thickness of 0.79 mm. The single element proposed has an overall small size of 13 mm × 15 mm, while the MIMO configuration of the proposed work has a miniaturized size of 28 mm × 28 mm. The parasitic element-loaded MIMO antenna offers a high gain of 9.5 and 11.5 dB at resonance frequencies of 28 GHz and 38 GHz, respectively. Various MIMO parameters were also examined, and the results generated by the EM tool CST Studio Suite® and hardware prototype are presented. The parasitic element-loaded MIMO antenna offers an Envelop Correlation Coefficient (ECC) < 0.001 and Channel Capacity Loss (CCL) < 0.01 bps/Hz, which are quite good values. Moreover, a comparison with existing work in the literature is given to show the superiority of the MIMO antenna. The suggested MIMO antenna provides good results and is regarded as a solid candidate for future 5G applications according to the comparison with the state of the art, results, and discussion

Global variation in postoperative mortality and complications after cancer surgery: a multicentre, prospective cohort study in 82 countries

Background: 80% of individuals with cancer will require a surgical procedure, yet little comparative data exist on early outcomes in low-income and middle-income countries (LMICs). We compared postoperative outcomes in breast, colorectal, and gastric cancer surgery in hospitals worldwide, focusing on the effect of disease stage and complications on postoperative mortality. Methods: This was a multicentre, international prospective cohort study of consecutive adult patients undergoing surgery for primary breast, colorectal, or gastric cancer requiring a skin incision done under general or neuraxial anaesthesia. The primary outcome was death or major complication within 30 days of surgery. Multilevel logistic regression determined relationships within three-level nested models of patients within hospitals and countries. Hospital-level infrastructure effects were explored with three-way mediation analyses. This study was registered with ClinicalTrials.gov, NCT03471494. Findings: Between April 1, 2018, and Jan 31, 2019, we enrolled 15 958 patients from 428 hospitals in 82 countries (high income 9106 patients, 31 countries; upper-middle income 2721 patients, 23 countries; or lower-middle income 4131 patients, 28 countries). Patients in LMICs presented with more advanced disease compared with patients in high-income countries. 30-day mortality was higher for gastric cancer in low-income or lower-middle-income countries (adjusted odds ratio 3·72, 95% CI 1·70–8·16) and for colorectal cancer in low-income or lower-middle-income countries (4·59, 2·39–8·80) and upper-middle-income countries (2·06, 1·11–3·83). No difference in 30-day mortality was seen in breast cancer. The proportion of patients who died after a major complication was greatest in low-income or lower-middle-income countries (6·15, 3·26–11·59) and upper-middle-income countries (3·89, 2·08–7·29). Postoperative death after complications was partly explained by patient factors (60%) and partly by hospital or country (40%). The absence of consistently available postoperative care facilities was associated with seven to 10 more deaths per 100 major complications in LMICs. Cancer stage alone explained little of the early variation in mortality or postoperative complications. Interpretation: Higher levels of mortality after cancer surgery in LMICs was not fully explained by later presentation of disease. The capacity to rescue patients from surgical complications is a tangible opportunity for meaningful intervention. Early death after cancer surgery might be reduced by policies focusing on strengthening perioperative care systems to detect and intervene in common complications. Funding: National Institute for Health Research Global Health Research Unit

Global variation in postoperative mortality and complications after cancer surgery: a multicentre, prospective cohort study in 82 countries

© 2021 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY-NC-ND 4.0 licenseBackground: 80% of individuals with cancer will require a surgical procedure, yet little comparative data exist on early outcomes in low-income and middle-income countries (LMICs). We compared postoperative outcomes in breast, colorectal, and gastric cancer surgery in hospitals worldwide, focusing on the effect of disease stage and complications on postoperative mortality. Methods: This was a multicentre, international prospective cohort study of consecutive adult patients undergoing surgery for primary breast, colorectal, or gastric cancer requiring a skin incision done under general or neuraxial anaesthesia. The primary outcome was death or major complication within 30 days of surgery. Multilevel logistic regression determined relationships within three-level nested models of patients within hospitals and countries. Hospital-level infrastructure effects were explored with three-way mediation analyses. This study was registered with ClinicalTrials.gov, NCT03471494. Findings: Between April 1, 2018, and Jan 31, 2019, we enrolled 15 958 patients from 428 hospitals in 82 countries (high income 9106 patients, 31 countries; upper-middle income 2721 patients, 23 countries; or lower-middle income 4131 patients, 28 countries). Patients in LMICs presented with more advanced disease compared with patients in high-income countries. 30-day mortality was higher for gastric cancer in low-income or lower-middle-income countries (adjusted odds ratio 3·72, 95% CI 1·70–8·16) and for colorectal cancer in low-income or lower-middle-income countries (4·59, 2·39–8·80) and upper-middle-income countries (2·06, 1·11–3·83). No difference in 30-day mortality was seen in breast cancer. The proportion of patients who died after a major complication was greatest in low-income or lower-middle-income countries (6·15, 3·26–11·59) and upper-middle-income countries (3·89, 2·08–7·29). Postoperative death after complications was partly explained by patient factors (60%) and partly by hospital or country (40%). The absence of consistently available postoperative care facilities was associated with seven to 10 more deaths per 100 major complications in LMICs. Cancer stage alone explained little of the early variation in mortality or postoperative complications. Interpretation: Higher levels of mortality after cancer surgery in LMICs was not fully explained by later presentation of disease. The capacity to rescue patients from surgical complications is a tangible opportunity for meaningful intervention. Early death after cancer surgery might be reduced by policies focusing on strengthening perioperative care systems to detect and intervene in common complications. Funding: National Institute for Health Research Global Health Research Unit

Effects of hospital facilities on patient outcomes after cancer surgery: an international, prospective, observational study

© 2022 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY 4.0 licenseBackground: Early death after cancer surgery is higher in low-income and middle-income countries (LMICs) compared with in high-income countries, yet the impact of facility characteristics on early postoperative outcomes is unknown. The aim of this study was to examine the association between hospital infrastructure, resource availability, and processes on early outcomes after cancer surgery worldwide. Methods: A multimethods analysis was performed as part of the GlobalSurg 3 study—a multicentre, international, prospective cohort study of patients who had surgery for breast, colorectal, or gastric cancer. The primary outcomes were 30-day mortality and 30-day major complication rates. Potentially beneficial hospital facilities were identified by variable selection to select those associated with 30-day mortality. Adjusted outcomes were determined using generalised estimating equations to account for patient characteristics and country-income group, with population stratification by hospital. Findings: Between April 1, 2018, and April 23, 2019, facility-level data were collected for 9685 patients across 238 hospitals in 66 countries (91 hospitals in 20 high-income countries; 57 hospitals in 19 upper-middle-income countries; and 90 hospitals in 27 low-income to lower-middle-income countries). The availability of five hospital facilities was inversely associated with mortality: ultrasound, CT scanner, critical care unit, opioid analgesia, and oncologist. After adjustment for case-mix and country income group, hospitals with three or fewer of these facilities (62 hospitals, 1294 patients) had higher mortality compared with those with four or five (adjusted odds ratio [OR] 3·85 [95% CI 2·58–5·75]; p<0·0001), with excess mortality predominantly explained by a limited capacity to rescue following the development of major complications (63·0% vs 82·7%; OR 0·35 [0·23–0·53]; p<0·0001). Across LMICs, improvements in hospital facilities would prevent one to three deaths for every 100 patients undergoing surgery for cancer. Interpretation: Hospitals with higher levels of infrastructure and resources have better outcomes after cancer surgery, independent of country income. Without urgent strengthening of hospital infrastructure and resources, the reductions in cancer-associated mortality associated with improved access will not be realised. Funding: National Institute for Health and Care Research