Enhanced Reverse Recovery Performance in Superjunction MOSFET with Reduced Hole-Barrier

High reverse recovery charge (QRR) and resultant high switching losses have become the main factors that constrain the performance and application area of superjunction MOSFET (SJ-MOSFET). To reduce QRR, an SJ-MOSFET with reduced hole-barrier is proposed and demonstrated. By introducing a Schottky contact on the bottom of the n-pillar at the drain side, the barrier for the hole carrier is dramatically reduced in the reverse conduction state. As a result, the hole carrier in the drift region is significantly reduced, which results in a low QRR and enhanced reverse recovery performance. Compared with the conventional SJ-MOSFET (Conv-SJ-MOSFET), the proposed device achieves 64.6% lower QRR with almost no sacrifice in other characteristics. The attenuated QRR accounts for a 19.6% ~ 46.8% reduction in total power losses with operation frequency at 5 ~ 200 kHz, demonstrating the great potential of the proposed SJ-MOSFET used in power conversion systems.

The Ionic Selectivity of Lysenin Channels in Open and Sub-Conducting States

The electrochemical gradients established across cell membranes are paramount for the execution of biological functions. Besides ion channels, other transporters, such as exogenous pore-forming toxins, may present ionic selectivity upon reconstitution in natural and artificial lipid membranes and contribute to the electrochemical gradients. In this context, we utilized electrophysiology approaches to assess the ionic selectivity of the pore-forming toxin lysenin reconstituted in planar bilayer lipid membranes. The membrane voltages were determined from the reversal potentials recorded upon channel exposure to asymmetrical ionic conditions, and the permeability ratios were calculated from the fit with the Goldman–Hodgkin–Katz equation. Our work shows that lysenin channels are ion-selective and the determined permeability coefficients are cation and anion-species dependent. We also exploited the unique property of lysenin channels to transition to a stable sub-conducting state upon exposure to calcium ions and assessed their subsequent change in ionic selectivity. The observed loss of selectivity was implemented in an electrical model describing the dependency of reversal potentials on calcium concentration. In conclusion, our work demonstrates that this pore-forming toxin presents ionic selectivity but this is adjusted by the particular conduction state of the channels.

The High-Reliability Optimization of Dead-Time Circuit for Push-Pull DC-DC Converter

Aiming at of the problem of common conduction state in push-pull DC-DC converter’s power transistor, an optimization solution by improving pulse width modulation(PWM) mode and reasonable setting parameters of the transformer and the inductance was proposed. The solution was analyzed in detail and test of the optimized circuit are carried out. The results show that the optimized circuit can effectively eliminate common conduction state, make the converter have a suitable dead-time and greatly improve the reliability of the converter.

4H-SiC Double-Trench MOSFET with Side Wall Heterojunction Diode for Enhanced Reverse Recovery Performance

In this study, a novel 4H-SiC double-trench metal-oxide semiconductor field-effect transistor (MOSFET) with a side wall heterojunction diode is proposed and investigated by conducting numerical technology computer-aided design simulations. The junction between P+ polysilicon and the N-drift layer forming a heterojunction diode on the side wall of the source trench region suppresses the operation of the PiN body diode during the reverse conduction state. Therefore, the injected minority carriers are completely suppressed, reducing the reverse recovery current by 73%, compared to the PiN body diodes. The switching characteristics of the proposed MOSFET using the heterojunction diode as a freewheeling diode was compared to the power module with a conventional MOSFET and an external diode as a freewheeling diode. It is shown that the switching performance of the proposed structure exhibits equivalent characteristics compared to the power module, enabling the elimination of an external freewheeling diode in the power system.

SiC MOSFET with a Self-Aligned Channel Defined by Shallow Source-JFET Implantation: A Simulation Study

A new concept for SiC MOSFET with a self-aligned channel is presented. The channel is defined by shallow source-JFET implantation into a counter-doped layer. This concept is verified by TCAD device simulations. It is shown that the method is applicable to fabrication of functional devices. The most critical parameter of the process is misalignment between channel and p-shield. Almost no change of electrical current in forward conduction state and the leakage current and electric field in the gate oxide in blocking state is observed for the misalignments below 400 nm.

Significance of Prandtl Number on the Heat Transport and Flow Structure in Rotating Rayleigh–Bénard Convection

A direct numerical simulation of rotating Rayleigh–Bénard convection (RBC) for different fluids (Pr=0.015,0.7,1,7,20, and 100) in a cylindrical cell of aspect ratio Γ=0.5 is carried out in this work. The effect of rotation on the heat transfer rate, flow structures, their associated dynamics, and influence on the boundary layers are investigated. The Rayleigh number is fixed to Ra=106 and the rotation rates are varied for a wide range, starting from no rotation (Ro→∞) to high rotation rates (Ro≈0.01). For all the Prandtl numbers (Pr=0.015–100), a reduction in heat transfer with increase in rotation is observed. However, for Pr=7 and 20, a marginal increase of the Nusselt number for low rotation rates is obtained, which is attributed to the change in the flow structure from quadrupolar to dipolar state. The change in flow structure is associated with the statistical behavior of the boundary layers. As the flow makes a transition from quadrupolar to dipolar state, a reduction in the thermal boundary layer thickness is observed. At higher rotation rates, the thermal boundary layer thickness shows a power law variation with the rotation rate. The power law exponent is close to unity for moderate Pr, while it reduces for both lower and higher Pr. At extremely high rotation rates, the flow makes a transition to the conduction state. The critical rotation rate (1/Roc) for which transition to the conduction state is observed depends on the Prandtl number according to 1/Roc∝Pr0.5.

Three-dimensional doubly diffusive convectons: instability and transition to complex dynamics

Three-dimensional doubly diffusive convection in a closed vertically extended container driven by competing horizontal temperature and concentration gradients is studied by a combination of direct numerical simulation and linear stability analysis. No-slip boundary conditions are imposed on all six container walls. The buoyancy number $N$ is taken to be $-1$ to ensure the presence of a conduction state. The primary instability is subcritical and generates two families of spatially localized steady states known as convectons. The convectons bifurcate directly from the conduction state and are organized in a pair of primary branches that snake within a well-defined range of Rayleigh numbers as the convectons grow in length. Secondary instabilities generating twist result in secondary snaking branches of twisted convectons. These destabilize the primary convectons and are responsible for the absence of stable steady states, localized or otherwise, in the subcritical regime. Thus all initial conditions in this regime collapse to the conduction state. As a result, once the Rayleigh number for the primary instability of the conduction state is exceeded, the system exhibits an abrupt transition to large-amplitude relaxation oscillations resembling bursts with no hysteresis. These numerical results are confirmed here by determining the stability properties of both convecton types as well as the domain-filling states. The number of unstable modes of both primary and secondary convectons of different lengths follows a pattern that allows the prediction of their stability properties based on their length alone. The instability of the convectons also results in a dramatic change in the dynamics of the system outside the snaking region that arises when the twist instability operates on a time scale faster than the time scale on which new rolls are nucleated. The results obtained are expected to be applicable in various pattern-forming systems exhibiting localized structures, including convection and shear flows.

Critical torsional modes of convection in rotating fluid spheres at high Taylor numbers

A numerical study of the onset of convection in rotating internally heated self-gravitating fluid spheres is presented. The exploration of the stability of the conduction state versus the Taylor and Prandtl numbers supplies a detailed idea of the laws that fulfil the four types of solutions obtained at low Prandtl numbers. The main result found is that axisymmetric (torsional) modes of convection are preferred at high Taylor numbers in the zero-Prandtl-number limit. This instability appears at low Rayleigh numbers and gives rise to an oscillating single vortex of very high frequency.

Comparison of the Planar-JBS against the Trench-MOS Rectifier-Design Based on 4H-SiC for 3.3 kV Applications

We compare the static electronic performance of the state-of-the-art Junction-Barrier-Schottky (JBS) rectifier (manufactured by ion-implantation) against the Trench-MOS Barrier Schottky (TMBS) rectifier (manufactured by trench-etching and subsequent oxidation). In our 2D numerical simulations, we have chosen identical specifications for the epitaxial drift-layer (3.3 kV application voltage, e.g. traction) and back-side device-design, while investigating the impact of the top-cell active area design on both rectifier IV-characteristics. To enable a meaningful comparison, we designed the depth d of the shields for the electric field E equally deep (p+ peak-plateau dJBS equals trench-depth dt), such that the peak E-field (close to avalanche breakdown) inside the drift-layer of the device is located at a comparable depth near the anode-side of the rectifier (see Fig. 2). By studying systematically the ratio between shield-to Schottky contact-length (w/s ratio), we found that the unipolar conduction state of the TMBS design is basically unaffected by enlarging the Schottky-contact length s, which is enterly different for the JBS-case.