Development of power semiconductor silicon carbide technology - a breakthrough and epoch-making steps

There is a continuous demand for improvements in the quality of switching power devices, such as higher switching frequency, higher withstand voltage capability, larger current-handling capability, and lower conduction losses. However, for single-conduction-mechanism devices (SCRs, GTOs, BJTs, FETs), possessing all these features is probably unrealizable for physical reasons. An attractive solution appears to be double-mechanism devices, in which the features of both a minority carrier device (BJT or SCR) and a majority carrier device (MOSFET) are embedded. Both IGBTs (insulated-gate bipolar transistors) and MCTs (MOS-controlled thyristors) belong to this family of double-mechanism devices and promise to have a major impact on converter circuit signs. The authors deal with the major features of these two devices, pointing out those that are most critical to the design of power converter topologies. In particular, the two devices have been tested both in a chopper and in two resonant link converter topologies, and the experimental results are reported. >

The three-phase buck-type rectifier has advantages as front-end converter for high efficiency power supplies in telecommunication and data centers. In this paper, the different commutation types of a three-phase buck rectifier with a freewheeling diode are analyzed through experiments using different semiconductor devices. Further, the switching loss of the converter is modeled and calculated for four space vector modulation schemes. It is shown that when the switches include minority carrier devices, such as Si PiN diode, IGBT and Reverse Blocking IGBT (RB-IGBT), more switching loss will occur in the commutation between two switches than between a switch and the freewheeling diode. This difference can be reduced if majority carrier devices, such as SiC Schottky diodes, are used in series with the switches. The modulator can be arranged to eliminate the specific transition which has the most switching loss. According to the analysis, each modulation scheme has its own field for high efficiency application. The advantageous modulation scheme is given for different device combinations in this paper.

In this paper, diffusion capacitance of a uniformly doped high barrier Schottky barrier diode and its behavior for different device parameters are studied. When barrier height is low, Schottky barrier diode acts as a majority carrier device. But, a high barrier Schottky diode injects minority carrier charge at forward bias. For this reason minority carrier charge is stored in the drift region. This stored charge gives rise to diffusion capacitance which affects the device speed remarkably. In the past, there has been some notable work in this regard. In this work, a new approach is taken to find out the analytical expression of diffusion capacitance. Solving a second order nonlinear differential equation, minority carrier charge profile is obtained for all level of injections in our previous work. Using this expression, analytical expressions of both total stored minority carrier charge and diffusion capacitance are obtained. Once the expression of diffusion capacitance is obtained, then its dependence on various device parameters such as device length, bias voltage and effective surface recombination velocity has been studied. In this work, both majority and minority carrier current and their drift and diffusion components are considered. Recombination of carriers in the epitaxial layer is also neglected.

Introduction: Atrial fibrillation is a major cause of stroke, heart failure, and sudden death and is often associated with valve disease. The Maze procedure using the unipolar and bipolar radiofrequency devices is often combined with valvular surgery, especially mitral valve, to treat atrial fibrillation.
 Method: Cross-sectional descriptive study; 40 patientsss were treated for atrial fibrillation by COX-MAZE procedure combined with heart valve surgery in 2020-2021 at Cardiovascular Center- E Hospital.
 Result: In 40 patients, 23 males (57.5%), 17 females (42.5%), mean age 53.6 ± 8.3 (36-68) years. Mean time to dêtct atrial fibrillation 3.27 (1-9) years. 05 cases (12.5%) were preopration stroke history. Mean left atrial diameter before surgery 49.2 ± 10.1 (21-70) mm, thrombus in left atrial appendage was 8 cases (20%). All patients had mitral valve disease, accompanied by aortic valve disease 4 cases (10%) and tricuspid valve regurgitation 13 cases (32.5%). 100% of patients underwent COX-MAZE procedure. The rates of free from AF and sinus recovery after surgery 31 cases (77.5%). 02 patients (5%) sinus node dysfunction and 04 patients (10%) atrioventricular block I-II degree but they did not require temporary pacemaker placement, and recovered in 1-3 weeks.
 Conclusion: The Maze procedure by unipolar devices alone and combination with bipolar devices are effective and safe in patient atrial fibrillation combined with valvular surgery, especially mitral valve surgery.

This paper presents the effect of recombination-induced stacking faults on the drift based forward conduction and leakage currents of high voltage 4H-SiC power devices. To show the effects, IV characteristics of a 4H-SiC 10 kV DMOSFET and a 4H-SiC 4 kV BJT have been evaluated before and after the induction of stacking faults in the drift epilayer. For both devices, significant increases in forward voltage drops, as well as marked increases in leakage currents have been observed. The results suggest that injection of minority carriers in majority carrier devices should be avoided at all times.

The low temperature power electronics are needed for the interface between the superconducting energy system and the conventional power system, since the latter has been developed for the high voltage and low current, and for the alternating current, while the former has been oriented for the high current and low voltage, and for the direct current. As for the power switching devices for the converter between ac(conventional technology) and dc(superconducting technology), we have experimented the semiconductor unipolar and bipolar devices and their models for the computer simulation have been derived. The unipolar devices among which power MOS FET's represent the capability of low loss and high density current conduction in the liquid nitrogen temperature, although in the liquid helium temperature the freeze out of carriers has been observed. However the bipolar devices have the problem of the freeze out of carriers in low temperatures. The computer simulation based on the derived model are in good agreement with the experimental results and the optimal design parameters have been discussed. The operation in the liquid nitrogen temperature of power switching devices will be well consistent with the high temperature superconductors which have been recently discovered.

Raising operation voltage of power conversion circuit is effective in reducing conduction loss. It requires power switching device to have high breakdown voltage. High voltage Si power device requires bipolar operation to reduce conduction loss sufficiently. However, bipolar operation worsens the switching characteristics of the device especially in turn off operation; such as reverse recovery for PiN diode and tail current for IGBT. The high critical electric field of SiC semiconductor realizes high break down voltage majority carrier device with substantially low on resistance. It can achieve fast switching capability for it only requires conducting and blocking majority carrier in the device for switching operation. This paper demonstrates the fast high voltage switching operation of SiC Schottky barrier diode (SBD) and MOSFET with comparing to the conventional Si device.

Linear atrial radiofrequency lesions have been used effectively for the treatment of atrial fibrillation. In most cases an endocardial approach has been suggested. A method for epicardial placement of lesions would reduce the complexity of these procedures. We compared lesions created in ovine hearts in vivo using irrigated bipolar or unipolar handheld radiofrequency ablation devices. Radiofrequency lesions were produced around a left pulmonary vein, around the left atrial appendage and in the free wall of the right ventricle in ovine hearts. All lesions were created in the beating heart. A bipolar clamping device (n = 7) or a handheld unipolar device (n = 6) was used. Measurements of local electrograms and pacing thresholds were performed before and after ablation at each site to assess the electrical integrity of lesions. Tetrazolium and digital image analysis were used to assess lesion geometry. In atrial tissue continuous transmural lesions were achieved more often with the bipolar than with the unipolar device (92.3 vs. 33.3%, P < 0.02). In atrial tissue the reduction in signal amplitude caused by the lesions was significantly larger with the bipolar than the unipolar device (87.6+/-9.4% vs. 60.6+/-23.7% reduction, P < 0.01). There was a significant relationship between loss of pacing capture and lesion transmurality (P < 0.05). The bipolar device created narrower lesions than the unipolar device (4.1+/-0.9 mm vs. 5.9+/-2.1 mm, P < 0.001). The bipolar clamping device produces narrower lesions which are more likely to be transmural and lead to electrical isolation of ablated tissue than those produced by the unipolar device. However, both devices failed to consistently produce transmural lesions using the epicardial beating heart technique studied, particularly in thicker tissues. High output pacing within the ablated tissue partially predicts lesion transmurality and be a guide to the need for further ablation. However, endocardial ablation or transmural bipolar ablation are likely to remain the techniques of choice for linear radiofrequency ablation in the atria until improved techniques are developed.

Recently there has been much interest in combining the fields of organic electronics and spintronics. This has been motivated by the fact that low atomic mass of organic materials are predicted to have long spin lifetimes. Also, spintronic devices could benefit from the chemical tunability, ease of fabrication, and mechanical flexibility of organic semiconductors. The nascent field of organic spintronics has already presented many new phenomena which must be explained with novel physics, here we explore one of these phenomena, organic magnetoresistance (OMAR). OMAR is a room temperature spintronic effect in organic devices without any magnetic materials. OMAR is a large change in resistance (up to 25%) at low magnetic fields ( 20mT). OMAR represents a scientific puzzle since no traditional magnetoresistance mechanisms can explain the combination of properties listed above. Another one of the remarkable properties of OMAR is that the sign of the MR can change based operating conditions of the device, like temperature and voltage. In this dissertation we focused in particular on resolving the origin of the sign change since understanding this unique property should be a major step in unraveling the microscopic origin of OMAR. We have explored the properties of the sign change experimentally with bipolar semiconducting small molecule and polymer devices, in which we observed sign changes as functions of voltage and temperature. These devices showed a strong correlation between the sign change and the onset of minority charge carrier injection and we could describe the lineshape and MR(V) behavior as a superposition of two MR effects of opposite sign. From this work we concluded the separate MR effects were from the mobilities of holes and electrons having different responses to magnetic fields, which is best described by the bipolaron model for OMAR. To test this conclusion, we employed analytical and numerical device models assigning separate magnetomobilities to holes and electrons. The models show, counter-intuitively, that in the case when the minority charge carrier contact is injection limited, a decrease in minority charge carrier mobility increases the current. This is a result of the minority carrier contact acting like a constant current source, and of the compensation of the majority carrier space charge by the oppositely charged minority carriers. We show that these models describe the observed MR(V) behavior very well, and if one assumes the magnetic field acts to reduce the mobility of electrons and holes, we observe that our models can reproduce all the sign changes observed in literature. The device model also predicts how different device parameters affect the observed MR, to test its predictions we performed experiments in which we increased the charge recombination by dye doping the organic active layer, we also observed how changing the charge injection by altering the organic semiconductor/ metal contacts experimentally compared with the device model. The fact that the current can increase when the minority carrier mobility decreases may explain the fact that in experiments the magnitude of the negative MR features has been much larger than the positive MR features, even though, microscopically, the bipolaron model predicts the opposite. Therefore, the presence of both signs of magnetoresistance may be related only to the device physics and not to the microscopic mechanism which causes OMAR.

The wide bandgap (WBG) semiconductor SiC has been identified as a potential candidate for use in high power and high frequency devices due to its superior material parameters. SiC has a breakdown field almost six times higher than that of Si or GaAs resulting in a higher attainable power density. SiC has a larger thermal conductivity, 3 times that of Si and 6 times that of GaAs, translating into a smaller chip size and, therefore, more efficient systems. To date, most research on SiC RF devices has focused on majority carrier devices such as SITs and MESFETs. There are only a few reports on SiC bipolar transistors and these studies have concentrated on SiC power switching transistors. Bipolar transistors have potentially superior linearity as well as higher current handling and power density capabilities compared to the aforementioned RF devices making BJTs ideal for use in power amplifiers.

Background:Hemiarthroplasty is currently the most common treatment for displaced femoral neck fractures in the elderly. While bipolar hemiarthroplasty was developed to reduce the risk of acetabular erosion that is associated with traditional unipolar hemiarthroplasty, meta-analyses have reported similar outcomes for bipolar and unipolar hemiarthroplasty devices. The primary objective of this study was to evaluate the risks of aseptic revision and periprosthetic fracture following bipolar versus unipolar hemiarthroplasty in a large integrated health-care system in the United States.Methods:We conducted a retrospective cohort study using data from the hip fracture registry of an integrated health-care system. Patients aged ≥60 years who underwent hemiarthroplasty for hip fracture between 2009 and 2019 were included. The primary outcome measure was aseptic revision, and the secondary outcome measure was revision for periprosthetic fracture. Cause-specific Cox proportional hazards regression was performed, with mortality considered as a competing event. In the multivariable analysis, estimates were adjusted for potential confounders such as age, sex, race/ethnicity, body mass index, American Society of Anesthesiologists classification, femoral fixation, surgeon volume, type of anesthesia, and discharge disposition.Results:The study sample included 13,939 patients who had been treated with hemiarthroplasty by 498 surgeons at 35 hospitals. The mean follow-up time was 3.7 ± 2.9 years. The overall incidence of aseptic revision at 5 years following hemiarthroplasty was 2.8% (386). In the multivariable analysis controlling for potential confounders, bipolar hemiarthroplasty was associated with a lower risk of aseptic revision than unipolar hemiarthroplasty (hazard ratio [HR], 0.74; 95% confidence interval [CI], 0.59 to 0.94; p = 0.012). Rates of revision for periprosthetic fracture were similar between the bipolar and unipolar devices (HR, 0.79; 95% CI, 0.58 to 1.10; p = 0.16).Conclusions:In this study of hemiarthroplasty for hip fracture in elderly patients, bipolar designs were associated with a lower risk of aseptic revision than unipolar designs. In contrast to prior research, we did not find any difference in the risk of periprosthetic fracture between the 2 designs.Level of Evidence:Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.

We have performed a systematic study of carrier injection annealing of the defects introduced by electron irradiation in n-type GaAs in the temperature range 300--355 K and with injected current densities ranging from 0.5 to 2 A${\mathrm{?}}^{\mathrm{\ensuremath{-}}2}$. We first observed that the various defects do not anneal in the same fashion as stated in a previous study. We show that, because the associated activation energies decrease continuously as the injected current increases, the model previously proposed for the enhanced annealing in which the enhancement is induced by the energy which is released when a minority carrier is trapped on the defect site cannot apply. However, this enhanced annealing can be quantitatively understood if the annealing is induced by successive changes of the defect charge state when it traps alternatively minority and majority carriers.

Hospitals are one of the institutions that use considerable energy and contribute the largest amount of waste. To reduce the impact, initiatives have emerged in the form of greening hospitals for effective use of resources. Green Hospital is a hospital concept that is designed by empowering the potential of nature as the main resource so that it is friendly to the environment and saves energy expenditure. The existence of Green Hospital is needed to overcome climate change that can cause human health problems and the environment, because hospitals are one of the contributors to pollution. In Indonesia, Green Hospital is still a concept that emphasizes effective and efficient use of water and electrical energy, as well as environmentally sound waste management. The purpose of this study is to analyze solid medical waste management in Saraswati Dental and Oral Hospital Denpasar in realizing the Green Hospital concept. This research is qualitative. Data were collected by in-depth interviews with1 staff at the Environmental Health Installation and 3 staff of cleaning service, observation, and document review. Data were analyzed by data reduction, data presentation, and conclusion. Data validity used source triangulation and method triangulation. Medical waste management in Saraswati Dental &amp; Oral Hospital Denpasar is generally in has been accordance with the Indonesian health regulation No. 1204/2004, but does not yet have a policy or direction to realise Green Hospital. To achieve Green Hospital, hospitals can start by making policies to implement non-medical waste minimization and increasing the number of personnel in the field of medical and non-medical waste management.

We have analyzed the electric-field-dependent subband population as well as the carrier dynamics in a double-quantum-well GaAs/(Al,Ga)As superlattice using time-resolved photoluminescence (PL) spectroscopy. Applying a rate equation model, the steady-state subband population of the majority carriers in the two quantum wells and the transfer coefficients for the minority carriers can be directly determined from measured time-dependent PL spectra. A comparison with results derived from steady-state PL investigations demonstrates that the dynamics of the minority carriers are essential in order to determine the population of the majority carriers. In the experiments, we used an n–i–n structure, in which electrons are the majority and holes are the minority carriers.

4H-SiC is a promising material for low-loss, high frequency, and high temperature electronic devices. And nowadays, 4H-SiC Schottky diodes, MOSFETs, and its inverters were used in many applications. As well known, a conduction loss and a switching loss shows tread off relationship on these devices. Schottky diode shows high speed switching action and small recovery current based on its unipolar operation, but high series resistance of drift layer. On the other hand, PiN diode shows low series resistance of drift layer due to its conductivity modulation phenomenon based on the bipolar action. However, it causes an adverse reverse recovery current and limited switching speed. To break this tread off relationship, Schottky-PN diode structure so-called SPND or SNPD was proposed with a diamond semiconductor [1]. This diode has a structure with a low doped n- or p- drift layer where is sandwiched and completely depleted with Schottky junction and one side PN junction. Under forward bias condition, barrier height of PN junction is reduced and majority carrier was injected into the drift layer completely depleted with Schottky junction. Injected majority carrier drift to Schottky junction without recombining with minority carrier. It means minority carrier dose not contribute to current flow under forward bias. Then, this SPND structure is expected to show both very low on-resistance and high speed switching action. Under reverse bias condition, blocking voltage is shared throughout the drift layer. However, the diamond semiconductor material is under development and difficult to use production phase. In this study, we demonstrated the possibility of SPND by using 4H-SiC substrate under mass production. In this study, we grew low doped p-layer on n+ 4H-SiC C-face substrate by using hot-wall chemical vapor deposition (CVD) system with H2-SiH4-C3H8 gas system. Trimethylaluminum (TMA) was used as p-type dopant source. The thickness and doing concentration of grown p- layer is 1.1 μm and 5 x 1014 cm-3. Pt was evaporated in vacuum as Schottky electrode on as grown surface. The diameter of Schottky contact is 1mm. Al was used as ohmic back contact. The schematic cross-sectional structure of fabricated 4H-SiC SPND is shown inset of Figure 1. Figure 1 shows typical J-V characteristic of fabricated 4H-SiC SPND at room temperature. Good rectifying characteristic was observed. The blocking voltage of 300 V was obtained (Fig. 1(a)). From this result, we estimated dielectric breakdown field value of 2.5 MV/cm which is almost equal to ideal value of 4H-SiC one. On forward bias characteristic, the current raises at 2.2 V and the current density is reached at 1200 A/cm2 at 2.4 V (Fig. 1(b)). From this value, the differential on-resistance was estimated at 0.18 mΩcm2. In this study, we used 4H-SiC substrate with thickness of 200 μm and resistivity of 20 mΩcm. The calculated series on-resistance of 4H-SiC substrate part was estimated as 0.4 mΩcm2. This result means estimated differential on-resistance of 0.18 mΩcm2 is almost the resistance of substrate and the resistance of drift layer dose not contribute to the series resistance. These results mean 4H-SiC SPND structure was realized and its good performance of low on-resistance with high blocking voltage was confirmed. Details will be shown in this presentation.