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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. 

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. 

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. 

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. 

At-temperature calibration is not only inconvenient, but also complicated by the temperature dependence of impedance standards. This paper examines the validity of a room-temperature calibration for on-wafer measurements from 70 kHz to 220 GHz, from 25 °C to 125 °C, and up to 48 h. The results indicate that the room-temperature calibration is applicable up to 125 °C provided errors up to 0.5 dB in magnitude and 5° in phase are tolerable. Consistent with previous reports up to 110 GHz, the present errors are mainly caused by the time-dependent system drift instead of the temperature dependence of impedance standards. For unknown reasons, the system proven to be stable at room temperature drifts significantly at elevated temperatures. This makes elevated-temperature measurements challenging because presently it takes approximately three hours for the system to stabilize at a new temperature. Therefore, in the near future, efforts should be concentrated on stabilizing the system faster rather than correcting for the temperature dependence of impedance standards. 

A D-band (110‒170 GHz) SiC substrate-integrated waveguide (SIW) is characterized on-wafer by two different vector network analyzers (VNAs): a 220-GHz single-sweep VNA and an 110-GHz VNA with WR8 (90‒140 GHz) and WR5 (140‒220 GHz) frequency extenders. To facilitate probing, the SIW input and output are transitioned to grounded coplanar waveguides (GCPWs). Two-tier calibration is used to de-embed the SIWGCPW transitions as well as to extract the intrinsic SIW characteristics. In general, the two VNAs are in agreement and both result in an ultra-low insertion loss of approximately 0.2 dB/mm for the same SIW, despite stitching errors at band edges.