## 1 Introduction  
Signals in the frequency range of 30-300 GHz are best suited for ultra-broadband wireless communications. Unfortunately, most of the solid-state sources produce insufficient power (limited to few milliwatts) at the upper end of this spectrum to support high data-rate communication over useful distances (>100 m). To respond to this challenge, novel baseband processing technology, identified as the enabling technology for the future beyond 5G networking, is capable of focusing energy into finer-grained areas and provide significant increase in both transmission energy efficiency and spectrum efficiency. For some years, in the world of solid-state high-frequency microelectronics, III-V double heterojunction bipolar transistors (DHBTs) on InP substrate have led the frequency race for the electronic amplification of >100 GHz signals and amplifier circuits offering high output power density [1]. In the parallel world of photonics, the uni-traveling carrier photodiode (UTC-PD) has been widely studied for ultrafast optoelectronic applications such as fiber-optic receivers and THz signal generation. InP UTC photodiodes, invented by Nippon Telegraph and Telephone Corporation (NTT) in 1997 [2; 3], rely only on electron transport which significantly improves high power capability and the maximum achievable bandwidth of UTC photodiodes compared to that of the PIN photodiodes. Therefore, UTC photodiodes have become a key component for electronic/photonic technology convergence in the mm-wave band [4; 5]. Nevertheless, interfacing photodiodes with amplifier circuits is required to obtain sufficient radio-frequency (RF) power and to perform signal processing. Combining these two worlds will thus benefit from the key features of each: bandwidth, tunability, stability and fiber compatibility for the photodiode, as well as the power handling capability of the transistors. To that end, the overall objective would be to develop high-performance terahertz-wave transmitter modules based on a disruptive monolithic opto-electronic integrated circuit (OEIC), which can be installed in radio base stations to directly convert the optical signals coming from midhaul/fronthaul networks into radio signals. This OEIC technology will pave the way for beyond 5G communication technologies, by delivering compact and efficient modules at room temperature. Such an innovative and ambitious objective requires integrating the photonic and electronic technologies monolithically on InP substrate, in order to boost the power of the single photodiode within the frequency range of 220-325 GHz. One of such most sought-after examples demonstrates the integration of UTC-PDs with InP DHBT amplifiers (either a transimpedance amplifier (TIA) or a power amplifier (PA)). Indeed, the optical generation of mm and THz waves represents a promising approach for next generation radio communication to overcome the difficulty in obtaining high-performance (and low-cost) electronic oscillators and modulators. Significant progress has been made on this subject [6] and optical generation appears to be the most suitable solution for frequencies above 100 GHz [7]. Most importantly, at present, the convergence between optical and radio networks offers a timely solution for developing communication technologies beyond 5G communication systems [8]. To realize such a monolithically integrated electronic-photonic technology (sketched in Figure 1), one should ideally rely on the stacked epitaxial growth of the two device structures and the subsequent fabrication process steps of the monolithic OEIC. This approach must be chosen to (i) optimize the epitaxial design of each device independently; and (ii) minimize the distance and the electronic propagation losses between them at very high operating frequencies while (iii) managing the mutual heat coupling. In order to take into account parasitic contributions accurately, passive components required for the integrated circuits should also be developed and optimized.