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In this paper, the ohmic contact mechanism and gate electrostatic control of a deep-recessed ohmic contact structure for multi-channel Al0.3Ga0.7N/GaN high electron mobility transistors (HEMTs) is investigated. A transmission electron microscope and an energy dispersive spectrum are utilized to investigate the ohmic contact interface mechanism. Due to a reduction of source/drain parasitic resistances and simultaneous connection of five channels by using deep-recessed ohmic contact, a large maximum drain current density and a distinct five hump feature of ultra-wider trans-conductance are achieved. More importantly, it is revealed that the downward expansion of the gate potential for the deep-recessed ohmic contact structure is much deeper than that for conventional devices. This characteristic leads to a remarkable reduction in subthreshold swing (SS) and off-state leakage, indicating an ultra-wide and high trans-conductance profile. The fabricated devices show a lower off-state drain leakage, a lower SS, and a wider gate voltage swing (40 V). Due to an enhancement of gate electrostatic control, the current collapse and electrical reliability characteristics of multi-channel Al0.3Ga0.7N/GaN HEMTs with deep-recessed ohmic contact also improve. The results presented here indicate that the multi-channel device has great potential for high current and wide bandwidth applications. [ABSTRACT FROM AUTHOR]

Based on the acoustic radiation of point source, a phase-coded approach is proposed as a general technology to generate controllable acoustical vortices. For N sources, acoustical vortices can be generated with the phase difference of 2πm/N for the source topological charge m. It is proved that more circular pressure distributions of acoustical vortices with higher pressure peak amplitude can be generated for more sources. The number and spiral direction of phase twists are demonstrated to be determined by m and the maximum topological charge of acoustical vortices is |L|=Fix[(N-1)/2], where Fix(x) rounds the element x toward zero. To produce acoustical vortices with a maximum topological charge L, the minimum source number of Nmin=max(2|L|+1,4) should be employed with the phase difference of 2πL/Nmin and the phase difference resolution is also demonstrated to be π. The phase-coded approach has been verified by a 6-source experiment. The measured distributions of pressure and phase as well as the topological charge of acoustical vortices agree well with theoretical results, which suggest the feasibility of the phase-coded approach for controllable generation of acoustical vortices. [ABSTRACT FROM AUTHOR]