Terahertz (THz) radiation has received much attention during the past few decades for its potential applications in various fields, such as spectroscopy, imaging, and wireless communications. To use terahertz waves for data transmission in different application systems, the efficient and rapid modulation of terahertz waves is required and has become an in-depth research topic. Since the turn of the century, research on metasurfaces has rapidly developed, and the scope of novel functions and operating frequency ranges has been substantially expanded, especially in the terahertz range. The combination of metasurfaces and semiconductors has facilitated both new opportunities for the development of dynamic THz functional devices and significant achievements in THz modulators. This paper provides an overview of THz modulators based on different kinds of dynamic tunable metasurfaces combined with semiconductors, two-dimensional electron gas heterostructures, superconductors, phase-transition materials, graphene, and other 2D material. Based on the overview, a brief discussion with perspectives will be presented. We hope that this review will help more researchers learn about the recent developments and challenges of THz modulators and contribute to this field.

In this work, we report on demonstrating the firstvertical ${\rm{\beta}-Ga_{2}O_{3}}$ junction barrier Schottky (JBS) diode with theimplementation of thermally oxidized p-type NiO to compensatethe dilemma of the forfeit of the p-type ${\rm{\beta}-Ga_{2}O_{3}}$ . With this widebandgap p-type NiOx, ${\rm{\beta}-Ga_{2}O_{3}}$ JBS diodes with an area of 100×100 ${\mu m^{2}}$ achieves a breakdown voltage (BV) and specific onresistance(Ron,sp) of 1715 V and 3.45 ${m\Omega\cdot cm^{2}}$ , respectively,yielding a Baliga'figure of merit (FOM) of ${BV^{2}/R_{on,sp}= 0.85 \rm{GW/cm^{2}}}$ , which is the highest direct-current (DC) FOM valueamong all ${\rm{\beta}-Ga_{2}O_{3}}$ diodes. Meanwhile, a large size JBS diodewith area of 1×1 ${mm^{2}}$ shows a forward current (IF) and BV of 5A/700 V, which is also the best IF and BV combinations (FOM =64 MW/ ${cm^{2}}$ ) among all published results about large area Ga2O3diodes. Dynamic switching characteristics reveal that the diodesuffers from negligible current collapse phenomenon even at a -600 V and 103 s stress, showing the great promise ofimplementing p-NiO in the future ${\rm{\beta}-Ga_{2}O_{3}}$ power electronicdevices.

In this letter, high-performance p-NiO/ β -Ga ₂ O ₃ heterojunction diodes (HJDs) with composite terminal structures, a p-NiO junction termination extension (JTE), and a small-angle beveled field plate (BFP) are demonstrated. By implementing a p-NiO JTE structure, the optimal breakdown voltage ( V _br ) of β -Ga ₂ O ₃ HJD increases from 955 to 1945 V, and the integration of the small-angle BFP further boosts the breakdown voltage up to 2410 V. An 80-nm thin p-NiO layer is adopted in the heterojunction to reduce the specific on -resistance ( R _on,sp ), while the composite terminal structures have little effect on R _on,sp , due to the super-large lateral spread resistance. The β -Ga ₂ O ₃ HJD with composite terminal structures achieves a low R _on,sp of 1.12 mΩ·cm ² , yielding the highest direct-current Baliga's figure-of-merit (FOM = V _br ² / R _on,sp ) among all reported β-Ga ₂ O ₃ diodes with a value of 5.18 GW/cm ² _, which is about 15% of the theoretical value. These results suggest that the electrical field engineering with a composite terminal structure is a viable and effective technological strategy to enable the realization of β -Ga ₂ O ₃ bipolar power rectifiers.

A GaN Schottky barrier diode (SBD) on SiC for frequency doubler applications was fabricated with a N-/N+ GaN stack of 200 nm/1500 nm in thickness and 5 × 10 ¹⁷ cm ^-3 /8 × 10 ¹⁸ cm ^-3 in doping densities, respectively. A cut-off frequency of 459 GHz at zero bias and reverse breakdown voltage of 15.4 V were obtained. A quartz glass circuit with flip-chip-mounted GaN SBDs was inserted between split-waveguide blocks to form a balanced frequency doubler. When driven with 2 W input power in pulsed mode, measured output power was 200-244 mW from 177-183 GHz with efficiency 9.5-11.8%.

A high-power 300 GHz solid-state source chain based on GaN doublers is proposed. The two doublers working at 150 GHz and 300 GHz are designed and fabricated. The pulsed output power exceeds 1000 mW at 148 GHz to 152.5 GHz with an efficiency of 21 % to 29 %. At 151 GHz, a peak pulsed output power of 1322 mW with 27 % efficiency is derived, while a peak efficiency of 33.1 % is determined at 150 GHz with an input power of 2.3 W. The fabricated 300 GHz doubler delivers pulsed output power that is greater than 110 mW from 292 GHz to 305 GHz with an efficiency of 11.3 % to 14.7 %. A peak output power of 183.4 mW with 13.9 % efficiency is obtained at 302 GHz.