The THz spectrum (0.1-10 THz) has been recently explored for unique applications, such as Tbit/s wireless communications and high-resolution imaging in farfield. However, improving the technology readiness level for moving the THz systems from the laboratory level to real-world applications still requires significant improvements in the output power of chip-scaled THz sources, the sensitivity of THz receivers at room temperature, and the development of low-loss THz packages. Recent advances in photonics have led the photodiodes to become key devices for generating ultra-broadband and low-noise THz signals, leveraging the optical heterodyning of two laser lines in a photodiode chip. Furthermore, the optical heterodyning technique enables a wide range of tunability, facilitating the development of coherent THz continuous-wave photonic systems by exploiting the photodiodes as local oscillators for driving subharmonic THz mixers.

This thesis presents high-power InP-based THz MUTC-PDs for pumping subharmonic THz SBD mixers. It also introduces an efficient packaging approach of THz MUTC-PDs with standard WR outputs using monolithically integrated GCPW and CPW-to-WR E-plane transitions. Furthermore, it provides a novel multilayered 2×1 WR-power combiner designed for integration with THz MUTC-PDs to enhance the output power of THz MUTC-PD modules.

InP-based THz MUTC-PDs with a world-record RF output power of +3.4 dBm at a frequency of 150 GHz were realized and employed as a local oscillator for pumping a subharmonic THz SBD mixer at room temperature within the upper range of the WR3-band (270-320 GHz). An average conversion loss of 16.8 dB was achieved, which is comparable to the ~12 dB conversion loss of electrically pumped subharmonic SBD mixers. Moreover, the frequency-dependent complex impedance of the developed THz MUTC-PDs was determined up to 2 THz to facilitate impedance matching with GCPW and CPW-to-WR transitions.

InP-based GCPW-to-WR E-plane transitions with low-pass filters were designed for monolithic integration with THz MUTC-PDs in the WR6-band (110-170 GHz) and WR3-band (245-320 GHz) and experimentally characterized. The average measured insertion loss was 3.9 dB within the WR6-band and 4 dB within the WR3-band. Additionally, two packaging approaches of THz MUTC-PDs with a standard WR3 output, including split-block modules and chip-on-carrier packages, were discussed using a monolithically integrated GCPW-to-WR3 E-plane transition.

Furthermore, a frequency-scalable InP-based CPW-to-WR E-plane transition was developed to extend the monolithic integration approach for developing low-loss THz MUTC-PD packages across all the THz WR-bands from WR3 to WR0.51 (0.22-2.2 THz). This design employed a Rohacell 71 HF carrier with a dielectric constant approximating air (ε_r ≈ 1) to mechanically support ultra-thin InP substrates without compromising electrical performance. The simulation results of the CPW-to-WR E-plane transition revealed an insertion loss below 1.4 dB and a return loss better than 10 dB across the entire frequency range. As proof of concept, monolithically integrated CPW-to-WR3 E-plane transitions with THz MUTC-PDs were fabricated on a 95 µm-thick InP substrate. The measured insertion loss within the WR3-band (220-320 GHz) exhibited an average value of 8.6 dB, indicating higher losses, which can be traced back to the measurement conditions such as chip misalignment and the use of a soda-lime glass carrier for mechanical stability.

Finally, a novel multilayer packaging technology that allows for the integration of two-dimensional arrays of THz MUTC-PDs with THz WR-power combiners was developed. The proposed technology is based on the thermocompression bonding of gold-metallized glass-reinforced epoxy FR4 laminates to form the desired WR structure. First, a multilayered standard WR3 was fabricated using 57 FR4 unit-cells with a total length of 5.472 mm. The average measured transmission loss was found to be 0.29 dB/mm. Furthermore, a multilayered 2×1 WR3-power combiner that consists of 41 FR4 unit-cells with a total length of 3.9 mm was manufactured and employed for combining the RF output power of two THz MUTC-PDs. The measured total RF output power showed an average power improvement of about 1.3 dB over the RF output power of a single THz MUTC-PD within the WR3-band (240-320 GHz). The achieved power enhancement was limited by the insertion loss of the multilayered 2×1 WR3-power combiner, which was 1.8 dB.