Impacts of hydrogen annealing on the carrier lifetimes in p-type 4H-SiC after thermal oxidation

Thermal oxidation and hydrogen annealing were applied on a 100μm thick Al-doped p-type 4H-Si C epitaxial wafer to modulate the minority carrier lifetime,which was investigated by microwave photoconductive decay(μ-PCD).The minority carrier lifetime decreased after each thermal oxidation.On the contrary,with the hydrogen annealing time increasing to3 hours,the minority carrier lifetime increased from 1.1μs(as-grown)to 3.14μs and then saturated after the annealing time reached 4 hours.The increase of surface roughness from 0.236 nm to 0.316 nm may also be one of the reasons for limiting the further improvement of the minority carrier lifetimes.Moreover,the whole wafer mappings of minority carrier lifetimes before and after hydrogen annealing were measured and discussed.The average minority carrier lifetime was up to 1.94μs and non-uniformity of carrier lifetime reached 38%after 4-hour hydrogen annealing.The increasing minority carrier lifetimes could be attributed to the double mechanisms of excess carbon atoms diffusion caused by selective etching of Si atoms and passivation of deep-level defects by hydrogen atoms.

A 4H-SiC semi-super-junction shielded trench MOSFET: p-pillar is grounded to optimize the electric field characteristics

A 4H-SiC trench gate metal-oxide-semiconductor field-effect transistor(UMOSFET)with semi-super-junction shiel-ded structure(SS-UMOS)is proposed and compared with conventional trench MOSFET(CT-UMOS)in this work.The advantage of the proposed structure is given by comprehensive study of the mechanism of the local semi-super-junction structure at the bottom of the trench MOSFET.In particular,the influence of the bias condition of the p-pillar at the bottom of the trench on the static and dynamic performances of the device is compared and revealed.The on-resistance of SS-UMOS with grounded(G)and ungrounded(NG)p-pillar is reduced by 52%(G)and 71%(NG)compared to CT-UMOS,respectively.Additionally,gate ox-ide in the GSS-UMOS is fully protected by the p-shield layer as well as semi-super-junction structure under the trench and p-base regions.Thus,a reduced electric-field of 2 MV/cm can be achieved at the corner of the p-shield layer.However,the quasi-intrinsic protective layer cannot be formed in NGSS-UMOS due to the charge storage effect in the floating p-pillar,resulting in a large electric field of 2.7 MV/cm at the gate oxide layer.Moreover,the total switching loss of GSS-UMOS is 1.95 mJ/cm2 and is reduced by 18%compared with CT-UMOS.On the contrary,the NGSS-UMOS has the slowest overall switching speed due to the weakened shielding effect of the p-pillar and the largest gate-to-drain capacitance among the three.The proposed GSS-UMOS plays an important role in high-voltage and high-frequency applications,and will provide a valuable idea for device design and circuit applications.

Ship collision impact on the structural load of an offshore wind turbine

When a maintenance and operations ship is berthing,there is a chance the ship may collide into the wind turbine.When these ships collide into wind turbine structures,this can result in significant changes to the foundation and structure of the wind turbine.In this paper,the structural load of a 4 MW offshore wind turbine was analyzed during a collision with an operations and maintenance ship.The variations in the wind speeds on hub height,waves,and the sea currents were measured.The dynamic simulation of the wind turbine was carried out using the test data as the input parameters.As a result,the load condition of the turbine without a collision was obtained.Finally,the measured turbine load was compared with the simulation results.This study shows that the collision of the operation and the maintenance ship increases the bending moments at the tower’s bottom and the blade’s roots.

A Digital Controller Design Method to Improve Performance of PFC Converter

With the rapid development of digital signal processing chip in recent years, DSP began to be used in more switching power supply. The advantages of digital control of their own, making the digital control PFC become a hotspot research. However, compared with the simulation system, the digital control technology still has many problems. In this paper, the problem of digital PI compensator as a voltage compensator is discussed, and the Single-pole voltage loop compensator is used in digital control PFC circuit. Because current loop bandwidth is narrow, a method to expand current loop bandwidth is put forward. Output power 300 W of prototype is made, and experimental results verify the correctness of the theory.

Robust photoluminescence energy of MoS_2/graphene heterostructure against electron irradiation

Two-dimensional （2D） materials have attracted growing attention since the discovery of graphene [1]. Transition metal dichalcogenide （TMD） semiconductors, such as MoS2 and WS2, became popular materials in recent years, because they usually have intrinsic bandgaps and an in- direct-to-direct bandgap transition from bulk to mono- layer limit [2-6]. Although graphene and TMDs are promising materials in field-effect devices [7-9], their heterostructures are more advanced in charge-splitting functions for the applications in optoelectronic devices [10-15].