We report a solid-state traveling-wave amplifier (TWA) realized through monolithic integration of transistors with a SiC substrate-integrated waveguide (SIW). The TWA uses a stepped-impedance microstrip line as the input divider, but 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 90°successive phase delays at the fundamental frequency. The HEMTs are distributed in the SIW in a period of a half wavelength at the second harmonic frequency, so that their outputs are combined coherently at the SIW output. To overcome the limited speed of the 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 TWA agree with that simulated at the small-signal level, but exceeds that simulated at the large-signal level. For example, under an input of 15 dBm at 70 GHz, the output at 140 GHz is 38-dB above that at 70 GHz. Under an input around 70 GHz and 20 dBm, the output around 140 GHz is 14 dBm with a 3-dB bandwidth of 6%. This is not only the first D-band frequency multiplier based on the GaN HEMT technology, but also one with the highest output power and the lowest fundamental leakage among all D-band multipliers of different transistor technologies. This proof-of-principle demonstration opens the path to improve the power, gain and efficiency of sub-terahertz TWAs 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.
