With the bandwidths of 6G wireless communications and next-generation automotive radars extending above 110 GHz, high-frequency, high-power transmitters are needed. However, conventional microwave monolithic integrated circuits (MMICs) are based on coplanar or microstrip transmission lines, which, at frequencies above 110 GHz, suffer from high loss, significant crosstalk, and limited power capacity. By contrast, substrate-integrated waveguides (SIWs) have low loss, minimum crosstalk, and high power capacity. However, because the size of SIWs is on the order of the guided wavelength λSIW, SIWs are usually implemented as interconnects and transitions, filters, antennas, or power combiners at the board level for hybrid integration with active devices. Monolithic integration becomes feasible when the operation frequency exceeds 110 GHz, so that λSIW < 1 mm in a typical semiconductor such as Si, GaAs, GaN or SiC. This work explores the possibilities of implementing SIWs as interconnects and power combiners for sub-THz wireless communications, including D-band SIWs with 0.2-dB/mm insertion loss and SIW power amplifiers with 14-dBm output power, both of record performance. We demonstrated that in the sub-THz frequency range, SIWs are small enough from the integration perspective of view to replace the coplanar or microstrip transmission lines in certain cases. Using power amplifier as an example, this dissertation starts with the introduction and motivation of the SIW as a novel power combiner on-chip. Following the introduction, the design, fabrication, and characterization of SIW interconnects on a SiC substrate are discussed. The main section of the dissertation focuses on the discussion of the novel monolithic SIW power combiner, which is also the first report of a solid-state distributed power amplifier realized through monolithic integration of transistors with a SIW, as well as the first D-band frequency multiplier based on gallium nitride (GaN). Although the SiC SIW provides an easy entry because of its mechanical strength, the present power-combining concept can be implemented in Si where the SIW is fabricated in an interposer before integration with complementary metal-oxide-semiconductor chiplets.