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Hexagonal semiconductors such as 4H SiC have important high-frequency, high-power, and high-temperature applications. The applications require accurate knowledge of both ordinary and extraordinary relative permittivities, ε and ε||, perpendicular and parallel, respectively, to the c axis of these semiconductors. However, due to challenges for suitable test setups and precision high-frequency measurements, little reliable data exists for these semiconductors especially at millimeter-wave frequencies. Recently, we reported ε|| of 4H SiC from 110 to 170 GHz. This paper expands on the previous report to include both ε and ε|| of the same material from 55 to 330 GHz, as well as their temperature and humidity dependence enabled by improving the measurement precision to two decimal points. For example, at room temperature, real ε and ε|| are constant at 9.77 ± 0.01 and 10.20 ± 0.05, respectively. By contrast, the ordinary loss tangent increases linearly with the frequency f in the form of (4.9 ± 0.1)  10−16 f. The loss tangent, less than 1  10−4 over most millimeter-wave frequencies, is significantly lower than that of sapphire, our previous low-loss standard. Finally, both ε and ε|| have weak temperature coefficients on the order of 10−4 /°C. The knowledge reported here is especially critical to millimeter-wave applications of 4H SiC, not only for solid-state devices and circuits, but also as windows for high-power vacuum electronics. 

In this paper, an inverted scanning microwave microscope (iSMM) is used to characterize the channel of a gateless GaN/AlN high-electron-mobility transistor (HEMT). Unlike conventional SMM, iSMM allows for 2-port measurements. Unlike conventional iSMM, the present iSMM probe is connected to Port 1 of a vector network analyzer with the HEMT drain and source remain on Port 2. Under different DC biases VGS (applied through the iSMM probe) and VDS (kept constant at 1 V), changes in both reflection coefficient S11 and transmission coefficient S21 are monitored as the iSMM probe scans along the width of the channel, revealing significant nonuniformity. Additionally, changes in S11 and S21 are significant when VGS ≥ −4 V, but insignificant when VGS = −8 V, consistent with the measured threshold voltage at −6 V for a gated HEMT. These results confirm that iSMM can be used to locally modulate the channel conduction of a HEMT while monitoring its RF response, before the actual gate is added. In turn, the nonuniformity measured by the iSMM can be used to diagnose and improve HEMT materials and processes. 

Hexagonal semiconductors such as GaN and SiC have important power applications at radio and millimeter-wave (mmW) frequencies. They are characterized by both ordinary and extraordinary permittivities, parallel and perpendicular to the densest packed c plane, respectively. However, due to the challenges of high-frequency measurements, little reliable data exist for these permittivities especially at mmW frequencies. Recently, for the first time, we reported the extraordinary permittivity of 4H SiC at mmW frequencies using substrateintegrated waveguides. We now report the ordinary permittivity of the same material using several Fabry-Perot resonators to cover most mmW frequencies. The resulted relative ordinary permittivity of 9.76 ± 0.01 exhibits little dispersion and is significantly lower than the previously reported extraordinary permittivity of 10.2 ± 0.1. This confirms that both ordinary and extraordinary permittivities are needed for accurate design and model of devices fabricated on 4H SiC. By contrast, the measured loss tangent increases linearly from 3  10−5 to 1.6  10−4 from 55 GHz to 330 GHz and can be fitted with (4.9 ± 0.1)  10−16 f, where f is the frequency in Hz. In fact, 4H SiC is the lowest-loss solid we have ever measured. The present approaches for permittivity characterization can be extended to other solids. 

This paper demonstrates the monolithic integration of a substrate-integrated waveguide bandpass filter (BPF) and a low-noise amplifier (LNA) at F-band, fabricated in a 70-nm GaN-on-SiC technology. The three-stage LNA alone achieves a state-of-the-art average noise figure of 3.6 dB over 87–115 GHz. The LNA + BPF exhibits a peak gain of 13.6 dB over a 3 dB bandwidth of 17 GHz from 104 to 121 GHz. The average noise figure is 4.9 dB over 87–115 GHz. The OP1 dB and saturated output power are 17.6dBm and >20 dBm, respectively. 

This paper demonstrates a novel approach to the design of D-band power dividers, capitalizing on the benefits of Substrate Integrated Waveguide (SIW) technology in 100-μm thick SiC substrate. By leveraging the unique characteristics of SIW and utilizing silicon carbide as the substrate material, an average insertion loss as low as 0.26 dB, and average return loss of up to 24 dB has been achieved in simulation in D-band. Although D-band dividers employing coplanar waveguides and microstrip lines have been reported, to the best of our acknowledge, this is the first work on D-band SIW power dividers. The SIW technology is compatible with GaN-on-SiC MMIC fabrication process flows, and provides a novel platform for the integration of low-loss millimeter-wave combiners with III-N based electronics. 

This is the first report of a distributed amplifier (DA) realized through monolithic integration of transistors with a substrate-integrated waveguide (SIW). The DA uses a stepped-impedance microstrip line as the input divider like in conventional DAs, but uses a low-loss, high-power-capacity SIW as the output combiner. The input signal is distributed to four GaN high-electron mobility transistors (HEMTs) evenly in magnitude but with the phase successively delayed by 90° at the fundamental frequency. The HEMTs are separated by a half wavelength at the second harmonic frequency in the SIW, so that their outputs are combined coherently at the SIW output. To overcome the limited speed of the GaN HEMTs, they are driven nonlinearly to generate second harmonics, and their fundamental outputs are suppressed with the SIW acting as a high-pass filter. The measured characteristics of the DA agree with that simulated at the small-signal level, but exceeds that simulated at the large-signal level. For example, under an input of 68 GHz and 10 dBm, the output at 136 GHz is 24-dB above the fundamental. Under an input of 68 GHz and 20 dBm, the output at 136 GHz is 14 dBm, with a conversion loss of 6 dB and a power consumption of 882 mW. This proof-of-principle demonstration opens the path to improving the gain, power and efficiency of DAs with higher-performance transistors and drive circuits. Although the demonstration is through monolithic integration, the approach is applicable to heterogeneous integration with the SIW and transistors fabricated on separate chips. 

F-band substrate-integrated waveguides (SIWs) are designed, fabricated, and characterized on a SiC wafer, along with SIW-based filters, impedance standards, and transitions to grounded coplanar waveguides (GCPWs). The GCPW-SIW transitions not only facilitate wafer probing, but also double as resonators to form a 3-pole band-pass filter together with an SIW resonator. The resulted filter exhibits a 1.5-dB insertion loss at 115 GHz with a 34-dB return loss and a 19-GHz (16%) 3-dB bandwidth. The size of the filter is only 63% of previous filters comprising three SIW resonators. These results show the feasibility for monolithic integration of highquality filters with high-efficiency antennas and amplifiers in a single-chip RF frontend above 110 GHz, which is particularly advantageous for 6G wireless communications and nextgeneration automobile radars. 

As 6G wireless communications push the operation frequency above 110 GHz, it is critical to have low-loss interconnects that can be accurately tested. To this end, D-band (110 GHz to 170 GHz) substrate-integrated waveguides (SIWs) are designed on a 100-μm-thick SiC substrate. The fabricated SIWs are probed on-wafer in a single sweep from 70 kHz to 220 GHz with their input/output transitioned to grounded coplanar waveguides (GCPWs). From CPW-probed scattering parameters, two-tier calibration is used to de-embed the SIW-GCPW transitions and to extract the intrinsic SIW characteristics. In general, the record low loss measured agrees with that obtained from finite-element full-wave electromagnetic simulation. For example, across the D band, the average insertion loss is approximately 0.2 dB/mm, which is several times better than that of coplanar or microstrip transmission lines fabricated on the same substrate. A 3-pole filter exhibits a 1-dB insertion loss at 135 GHz with 20-dB selectivity and 11% bandwidth, which is order-of-magnitude better than typical on-chip filters. These results underscore the potential of using SIWs to interconnect transistors, filters, antennas, and other circuit elements on the same monolithically integrated chip.