We report the existence and stability of gap solitons in parity-time (PT) complex periodic optical lattices with the real part of superlattices. These solitons can stably exist in the semi-infinite gap. We have studied the effects of different relative strengths of the superlattices and different amplitudes of the imaginary part on soliton propagation. It was found that the relative strength of the superlattices and the amplitude of the imaginary part significantly affect the PT symmetry and the stability of solitons in the PT complex periodic optical lattices.

Three different insulator layers SiN_x, SiON, and SiO₂ were used as a gate dielectric and passivation layer in AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMT). The SiN_x, SiON, and SiO₂ were deposited by a plasma-enhanced chemical vapor deposition (PECVD) system. Great differences in the gate leakage current, breakdown voltage, interface traps, and current collapse were observed. The SiON MIS-HEMT exhibited the highest breakdown voltage and I_on/I_off ratio. The SiN_x MIS-HEMT performed well in current collapse but exhibited the highest gate leakage current density. The SiO₂ MIS-HEMT possessed the lowest gate leakage current density but suffered from the early breakdown of the metal–insulator–semiconductor (MIS) diode. As for interface traps, the SiN_x MIS-HEMT has the largest shallow trap density and the lowest deep trap density. The SiO₂ MIS-HEMT has the largest deep trap density. The factors causing current collapse were confirmed by Photoluminescence (PL) spectra. Based on the direct current (DC) characteristics, SiN_x and SiON both have advantages and disadvantages.

A 2-D simulation of off-state breakdown voltage (V_BD) for AlGaN/GaN high electron mobility transistors (HEMTs) with multi field-plates (FPs) is presented in this paper. The effect of geometrical variables of FP and insulator layer on electric field distribution and V_BD are investigated systematically. The FPs can modulate the potential lines and distribution of an electric field, and the insulator layer would influence the modulation effect of FPs. In addition, we designed a structure of HEMT which simultaneously contains gate FP, source FP and drain FP. It is found that the V_BD of AlGaN/GaN HEMTs can be improved greatly with the corporation of gate FP, source FP and drain FP. We achieved the highest V_BD in the HEMT contained with three FPs by optimizing the structural parameters including length of FPs, thickness of FPs, and insulator layer. For HEMT with three FPs, FP-S alleviates the concentration of the electric field more effectively. When the length of the source FP is 24 μm and the insulator thickness between the FP-S and the AlGaN surface is 1950 nm, corresponding to the average electric field of about 3 MV/cm at the channel, V_BD reaches 2200 V. More importantly, the 2D simulation model is based on a real HMET device and will provide guidance for the design of a practical device.

We propose two kinds of stacked materials that effectively yield low drain off-state leakage current, high off-state breakdown voltage and low current collapse simultaneously in AlGaN/GaN HEMT. The two kinds of stacked materials, which is NiOX/SiNX and Al2O3/SiNX respectively, act as gate dielectric layer and passivation layer for the devices. The NiOX and Al2O3 are prepared through depositing Ni and Al thin film by electron beam evaporation and then annealing in O2 ambient by rapid thermal processing. The SiNX is deposited by plasma-enhanced chemical vapor deposition. Compared to the single SiNX layer, the NiOX/SiNX stack layer and Al2O3/ SiNX stack layer can distinctly reduce the drain off-state leakage current of devices. Meanwhile, high off-state breakdown voltage is also achieved. The off-state breakdown voltage in the devices with NiOX/SiNX stack layer and Al2O3/SiNX stack layer are 52% and 36% higher than that of the devices with single SiNX layer respectively. Especially, the current collapse under 200 V stress bias is reduced by 90% and 58% in the devices with NiOX/SiNX stack layer and Al2O3/SiNX stack layer compare to the devices with single SiNX layer respectively.

We report on the existence and stability of the two-dimensional multipeak gap solitons in a parity-time-symmetric periodic potential with defocusing Kerr nonlinearity. We investigate the multipeak solitons with all the peaks of the real parts in-phase. It is found that these solitons can be stable in the first gap. The system can support not only the stable solitons with even number peaks, but also the stable solitons with odd number peaks. The transverse energy flow vector of these solitons is also studied.