A D-band Traveling-Wave Amplifier by Embedding GaN HEMTs as Current Probes in a SiC SIW

Conventional power combiners based on coplanar or microstrip transmission lines suffer from high loss at D band or higher frequencies. By contrast, power combiners based on substrate-integrated waveguides (SIWs) can have low loss, high power capacity, and minimum crosstalk. Recently, a D-band traveling-wave amplifier (TWA) was demonstrated by embedding transistors in the middle of an SIW as voltage probes to radiate power into the SIW. The voltage probes were realized through hot through-substrate vias (TSVs), which were not supported by all semiconductor foundries. As an alternative, this work embeds transistors along the sidewalls of an SIW as current probes through standard TSVs that are grounded to the backside metal layer. This approach combines twice as many transistors per unit length of the SIW without either hot TSVs or cross-SIW interconnects. This results in at least 5-dB-higher output power compared to the previously demonstrated TWA with voltage probes. The measured output power of the present TWA at 140 GHz is at least 19 dBm, limited by the available input power. The measured output power agrees with that simulated and simulation indicates an output power of 23 dBm under 1-dB gain compression. In this proof of concept, the demonstration is based on monolithic integration of unit-cell transistors and an SIW. In the future, larger transistors in a multi-stage amplifier can be used to drive each current probe for a TWA of higher gain, output power, and efficiency. The present concept of efficient power combining is equally applicable to heterogeneous integration and is material- and technology-agnostic. For example, transistors can be fabricated in a Si chiplet before integration with an SIW fabricated in a Si interposer.