We present a numerical analysis on the electric and thermal behaviors of GaN n + n - nn + Gunn oscillations based on Energy Balance (EB) model and Non-isothermal Energy Balance (NEB) models. We can achieve the influence of lattice heating on the Gunn oscillations based on NEB model, as it is based on the coupling of an energy-balance model with a thermal model for the description of the lattice temperature everywhere in the device, which makes it possible to optimize the device.
A theoretical analysis of high-efficiency punch-through operation GaN-based terahertz IMPATT diodes has been carried out in this paper. It is shown that the negative differential mobility (NDM) characteristics of GaN coupled with the space charge effect acting as a self-feedback system can markedly increase the drift velocity of injection carriers, and thereby enhance diode performance under appropriate external RF voltage. The behavior of traveling electrons in the transit zone is investigated in detail. It is found that the IMPATT diode with a punch-through structure operating in the NDM mode exhibits superior characteristics compared with the equivalent diode operating in the Si-like constant mobility mode. In particular, the NDM-mode diode can tolerate a larger RF voltage swing than that operating in constant mobility mode. Numerical simulation results reveal that the highest efficiency of 26.6% and maximum RF power of 2.29 W can be achieved for the NDM-mode diode at a frequency of 225 GHz. A highest efficiency of 19.0% and maximum RF power of 1.58 W are obtained for the diode with constant mobility.
ABSTRACT In this paper, we consider axisymmetric solutions of inhomogeneous incompressible Navier–Stokes system under Dirichlet boundary conditions in the exterior of a cylinder. We establish the global well‐posedness of strong solutions and obtain uniform estimates for the gradient of the density as well as algebraic decay rates for spatial gradients of the velocity. Note that the initial density can contain vacuum states, and the initial data do not need to satisfy compatibility condition.
This paper proposes a new breakdown-enhanced GaN MISFET with the architecture of double channel and P-buried layer, i.e. DCP-MISFET. The lower barrier and lower channel are connected with the drain, the lower two dimensional electron gas can improve the device electric field distribution between gate and drain, achieving an enhanced breakdown voltage (BV). At the same time, the P buried layer below the gate field plate can reduce the peak electric field around the gate field plate. The proposed simulated device with LGD=15 μm presents an excellent breakdown voltage of 2373 V. In addition, the ON-resistance (RON) of 15.07 Ω•mm and Baliga's figure of merit of 3.736 GW•cm-2 are achieved in the optimized DCP-MISFET. Compared with breakdown voltage 1547 V of the optimized field plate conventional GaN MISFET (FPC-MSIFET), the proposed device increases the breakdown voltage by 53.39% and the Baliga's figure of merit is enhanced by 133.89%.
Photoionization spectrum measurement method was designed based on constant photocurrent control by PID technology. Combined with photocurrent and hall effect measurements, this method can provide exact photoionization cross section in GaN epilayers. The measurement results of GaN epilayers show that, the responsiveness of photoelectric detector is the dominating factor affecting test accuracy. The test error increases with the incident photon energy. An 8% test error can be produced under incident photon with 3.2 eV photon energy. Deep level trap in GaN epilayers can still absorb photons with incident energy less than the energy difference between deep level trap and conductor band (2.85 eV), which implies that the lattice relaxation associated with deep level trap takes places in GaN epilayers.
In this paper, we propose a new enhanced GaN MISFET with embedded pn junction, i.e. , EJ-MISFET, to enhance the breakdown voltage. The embedded pn junction is used to improve the simulated device electric field distribution between gate and drain, thus achieving an enhanced breakdown voltage (BV). The proposed simulated device with L GD = 15μm presents an excellent breakdown voltage of 2050 V, which is attributed to the improvement of the device electric field distribution between gate and drain. In addition, the ON-resistance ( R ON ) of 15.37 Ω ⋅mm and Baliga’s figure of merit of 2.734 GW⋅cm −2 are achieved in the optimized EJ-MISFET. Compared with the field plate conventional GaN MISFET (FPC-MISFET) without embedded pn junction structure, the proposed simulated device increases the BV by 32.54% and the Baliga’s figure of merit is enhanced by 71.3%.
Quantum spins, referred to the spin operator preserved by full SU(2) symmetry in the absence of the magnetic anistropy, have been proposed to host exotic interactions with superconductivity4. However, spin orbit coupling and crystal field splitting normally cause a significant magnetic anisotropy for d/f-shell spins on surfaces6,9, breaking SU(2) symmetry and fabricating the spins with Ising properties10. Recently, magnetic nanographenes have been proven to host intrinsic quantum magnetism due to their negligible spin orbital coupling and crystal field splitting. Here, we fabricate three atomically precise nanographenes with the same magnetic ground state of spin S=1/2 on Pb(111) through engineering sublattice imbalance in graphene honeycomb lattice. Scanning tunneling spectroscopy reveals the coexistence of magnetic bound states and Kondo screening in such hybridized system. Through engineering the magnetic exchange strength between the unpaired spin in nanographenes and cooper pairs, quantum phase transition from the singlet to the doublet state has been observed, in consistent with quantum models of spins on superconductors. Our work demonstrates delocalized graphene magnetism host highly tunable magnetic bound states with cooper pairs, which can be further developed to study the Majorana bound states and other rich quantum physics of low-dimensional quantum spins on superconductors.
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