GaN based high electron mobility transistors (HEMTs) have demonstrated extraordinary features in the applications of high power and high frequency devices. In this paper, we review recent progress in AlGaN/GaN HEMTs, including the following sections. First, challenges in device fabrication and optimizations will be discussed. Then, the latest progress in device fabrication technologies will be presented. Finally, some promising device structures from simulation studies will be discussed.

Ultra-wide bandgap beta-gallium oxide (β-Ga2O3) has been attracting considerable attention as a promising semiconductor material for next-generation power electronics. It possesses excellent material properties such as a wide bandgap of 4.6–4.9 eV, a high breakdown electric field of 8 MV/cm, and exceptional Baliga’s figure of merit (BFOM), along with superior chemical and thermal stability. These features suggest its great potential for future applications in power and optoelectronic devices. However, the critical issue of contacts between metal and Ga2O3 limits the performance of β-Ga2O3 devices. In this work, we have reviewed the advances on contacts of β-Ga2O3 MOSFETs. For improving contact properties, four main approaches are summarized and analyzed in details, including pre-treatment, post-treatment, multilayer metal electrode, and introducing an interlayer. By comparison, the latter two methods are being studied intensively and more favorable than the pre-treatment which would inevitably generate uncontrollable damages. Finally, conclusions and future perspectives for improving Ohmic contacts further are presented.

The contact resistance between graphene and metal electrodes is crucial for the achievement of high-performance graphene devices. In this study, we review our recent study on the graphene–metal contact characteristics from the following viewpoints: (1) metal preparation method; (2) asymmetric conductance; (3) annealing effect; (4) interfaces impact.

Transport measurements of both the dc and the low-frequency ac are performed on Pt/HfO2$-${}x/TiN resistive-switching memory cells at various temperatures. The conductance of the pristine cells has a power law $\omega${}STN relationship with temperature and frequency. To account for the much larger conductance of both the high resistance states (HRSs) and the low resistance states (LRSs), an additional conductance term associated with oxygen vacancy filaments is added, which is independent of the cross-sectional area of the memory cell. This additional component of conductance in a HRS is frequency independent but temperature dependent, showing the small polaron originated transport, with an activation energy of 50 (2.1) meV at temperatures above (below) half of the Debye temperature, which agrees with the analysis of the electric field dependence data. The frequency- and temperature-dependent conduction of HRSs indicate the existence of polarization centers which facilitate the transport and make HfO2$-${}x highly polarizable. However, the additional conductance term associated with filaments in LRS, of an order of $\sim${}105 S\hskip0.25emm$-${}1, exhibits a weak metallic behavior in temperature-dependent measurements. Properties of aligned oxygen vacancy chains on the (1\ifmmode\bar\else\textasciimacron\fi{}11) surface are calculated by first-principles simulation. Through analysis of the partial density of states and spatial distribution of the wave function of impurity states generated by oxygen vacancies, this weak metallic behavior is attributed to the delocalization of the impurity band associated with aligned oxygen vacancies.