Before the emergence of ultra-wideband (UWB) radios, widely used wireless communications were based on sinusoidal carriers, and impulse technologies were employed only in specific applications (e.g. radar). In 2002, the Federal Communication Commission (FCC) allowed unlicensed operation between 3.1–10.6 GHz for UWB communication, using a wideband signal format with a low EIRP level (−41.3dBm/MHz). UWB communication systems then emerged as an alternative to narrowband systems and significant effort in this area has been invested at the regulatory, commercial, and research levels.

IEEE Transactions on Microwave Theory and Techniques and Antennas and Propagation Announce a Joint Special Issue on Ultra-Wideband (UWB) Technology

With the exponential growth of the data traffic in wireless communication systems, terahertz (THz) frequency band is envisioned as a promising candidate to support ultra-broadband for future beyond fifth generation (5G), bridging the gap between millimeter wave (mmWave) and optical frequency ranges. The purpose of this paper is to provide a comprehensive literature review on the development towards THz communications and presents some key technologies faced in THz wireless communication systems. Firstly, despite the substantial hardware problems that have to be developed in terms of the THz solid state superheterodyne receiver, high speed THz modulators and THz antennas, the practical THz channel model and the efficient THz beamforming are also described to compensate for the severe path attenuation. Moreover, two different kinds of lab-level THz communication systems are introduced minutely, named a solid state THz communication system and a spatial direct modulation THz communication system, respectively. The solid state THz system converts intermediate frequency (IF) modulated signal to THz frequency while the direct modulation THz system allows the high power THz sources to input for approving the relatively long distance communications. Finally, we discuss several potential application scenarios as well as some vital technical challenges that will be encountered in the future THz communications.

A survey on terahertz communications

III-N transistors with recessed gates. An epitaxial stack includes a doped III-N source/drain layer and a III-N etch stop layer disposed between a the source/drain layer and a III-N channel layer. An etch process, e.g., utilizing photochemical oxidation, selectively etches the source/drain layer over the etch stop layer. A gate electrode is disposed over the etch stop layer to form a recessed-gate III-N HEMT. At least a portion of the etch stop layer may be oxidized with a gate electrode over the oxidized etch stop layer for a recessed gate III-N MOS-HEMT including a III-N oxide. A high-k dielectric may be formed over the oxidized etch stop layer with a gate electrode over the high-k dielectric to form a recessed gate III-N MOS-HEMT having a composite gate dielectric stack.

III-N material structure for gate-recessed transistors

A GaN Schottky barrier diode (SBD) on SiC for frequency doubler applications was fabricated with a $\text{N}-/\text{N}+$ GaN stack of 200 nm/1500 nm in thickness and $5\times 10^{17} cm^{-3} / 8\times 10^{18} cm^{-3}$ 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 177–183 GHz High-Power GaN-Based Frequency Doubler With Over 200 mW Output Power

A novel miniaturized and light weight W-band bandpass filter (BPF) is proposed based on the gap waveguide (GWG) concept using the micro-electromechanical system (MEMS) technique. The BPF is achieved by cascading four rectangular GWG resonant cavities with designed rectangular and cross-shaped coupling structure. Two standard WR-10 waveguide coupling windows are designed as the input/output couplings, which contributes to the flexible application in various packaging configurations. By utilizing the classical coupling matrix theory, the proposed MEMS GWG BPF is designed to work from 89.8 to 97.4 GHz, which is fabricated by the deep reactive ion etching process using the silicon wafers, and the fabricated silicon wafers are also electroplated and bonded together as a whole structure. The measured insertion loss is within 1.6 dB, and the return losses are larger than 15 dB.

Miniaturized W-Band Gap Waveguide Bandpass Filter Using the MEMS Technique for Both Waveguide and Surface Mounted Packaging

GaN planar Schottky barrier diode (SBD) with an n−/n+ structure was grown and fabricated on sapphire substrate. An n+ GaN epitaxial layer with doping concentration of 8 × 1018 cm−3 was employed to reduce the parasitic resistance. An air-bridge structure and 50 μm substrate thinning-down technique were adopted in order to reduce the parasitic capacitance. A record cut-off frequency (f c) of 902 GHz was achieved for GaN planar SBD with 2 μm anode diameter.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/el.2016.1937

GaN planar Schottky barrier diode with cut-off frequency of 902 GHz

The invention discloses a polarization doping-based GaN Schottky diode, and belongs to the field of semiconductor devices. The diode comprises a semi-insulated substrate layer used for supporting the overall GaN Schottky diode, a highly doped N+ type GaN layer grown on the substrate layer, and an N- type AlxGa1-xN (x is greater than 0 and less than or equal to 1) layer which is grown in the N+ type GaN layer by adopting polarization doping; and the component Al of the N- type Al[x]Ga[1-x]N (x is greater than 0 and less than equal to 1) layer is distributed non-uniformly from the interface of the N+ type GaN layer. An ohmic contact electrode and a Schottky contact electrode are also arranged on the diode. According to the polarization doping-based GaN Schottky diode, the N- type AlxGa1-xN (x is greater than 0 and less than or equal to 1) layer is grown on the N+ type GaN layer in a polarization doping mode, so that the mobility of a GaN material is improved, the serial resistance of the Schottky diode is reduced, and the working frequency of the Schottky diode is improved; therefore, the working frequency and the output power of a frequency multiplier circuit in the range of millimeter wave and terahertz are improved. In the polarization doping mode, the capacitance variation ratio of the Schottky diode can be controlled effectively and the Q value of a device can be improved.

Polarization doping-based GaN Schottky diode

The invention discloses a novel hybrid integrated circuit for terahertz mixers, and relates to the technical field of transmission devices for electric communication The novel hybrid integrated circuit comprises quartz substrate circuits, a GaAs substrate circuit, a radio frequency waveguide and a local oscillation waveguide The GaAs substrate circuit comprises a GaAs substrate and a GaAs terahertz Schottky diode disposed in the GaAs substrate The front of the GaAs terahertz Schottky diode faces upward The two ends of a GaAs terahertz Schottky diode pad are respectively integrated with a section of GaAs-based micro strip transmission line or suspended micro strip transmission line, and the GaAs-based micro strip transmission lines or the suspended micro strip transmission lines are respectively and electrically connected with the quartz substrate circuits on the two sides The integrated circuit reduces the difficulty in device micro assembly Compared with the current commonly-used hybrid integrated circuit form, the diode assembly precision is improved, the frequency conversion loss of mixers can be reduced, and the test result of mixers is enabled to be closer to the designed result

Novel hybrid integrated circuit for terahertz mixer

In this letter, we report GaN planar Schottky barrier diodes (SBDs) with the DC and RF performance. The air-bridge structure is adopted to reduce the parasitic parameters. The cut-off frequency (ƒ c ) of the diode with an anode diameter of 5 µm is calculated to be 655 GHz at zero bias. By using four anodes series GaN SBDs chip, a frequency tripler with a peak output power of 2.1 mW at 103.5 GHz is demonstrated for the first time.

High-frequency multiplier based on GaN planar Schottky barrier diodes

The invention discloses a terahertz modulation amplifier based on a graphene waveguide structure and relates to the technical field of modulation amplifiers. The terahertz modulation amplifier comprises a substrate, a waveguide core layer, two ohmic contact electrodes, graphene layers and insulating medium layers. The waveguide core layer is located in the middle of the upper surface of the substrate, more than two graphene layers are arranged on top of the waveguide core layer and on the left side and the right side of the waveguide core layer, every two adjacent graphene layers are electrically isolated through the insulating medium layers, the two ohmic contact electrodes are located on the substrate on the left side and the right side of the waveguide core layer, and the uppermost graphene layer and the lowermost graphene layer are connected with positive voltage and negative voltage respectively through the ohmic contact electrodes. Modulation of terahertz (THz) waves can be achieved through the modulation amplifier, and amplification of the THz waves is achieved simultaneously.

Xing Dong

Papers

GaN planar Schottky barrier diode with cut-off frequency of 902 GHz

Polarization doping-based GaN Schottky diode

Novel hybrid integrated circuit for terahertz mixer

High-frequency multiplier based on GaN planar Schottky barrier diodes

Terahertz modulation amplifier based on graphene waveguide structure