## 8 Conclusions  
In this review, we addressed several of the most important aspects of monolithic integrated circuit design for future applications in beyond-5G communication systems. For the design of such novel technology, InP-based bipolar transistors and UTC photodiodes integrated on the same substrate are viewed as the most promising solution, owing to the respective merits of these two technologies. However, aspects such as the absence of a multiphysics simulation tool, requirement for interconnect electromagnetic modeling at  
Figure 23: Methodology flow illustrating the reliability improvement loop. high-frequency operation and tedious thermal management are some of the roadblocks for the current state-of-the art. Most importantly, reliability and thermal management framework at the system level is a prerequisite in the electronic-photonic co-simulation infrastructure. In this paper, design methodologies and modeling approaches as well as integrated reliability analysis modules are discussed in detail in order to converge towards a multiphysics design approach equipped to tackle these emerging concerns. Extensive characterization, including DC, RF (up to 300 GHz) and optical measurements, have been realized on InP DHBTs and UTC photodiodes, based on the same InP substrate, both technologies developed at III-V Lab. Compared to our previous study [34] where the previous generation of the DHBT technology was investigated up to 40 GHz, the characterization and parameter extraction have been extended to 300 GHz for the new generation of the DHBTs. To accommodate the needs of both time and cost-effective design as well as accuracy of the simulations, computationally efficient as well as physics-based compact models have been developed. While the industry-standard HiCuM model was used to extract the parameters of this new generation of DHBT technology, the UTC-PD compact model developed in [13] has been further improved, including more accurate description of the dark and photo currents, and utilized on the UTC-PD technology from the III-V lab. From an industrial design viewpoint, the models are made compatible with existing electronic design framework. In addition, thermal management and reliability analysis modules are integrated within the models. Particularly, inferences are drawn on the similar values of thermal resistances in the two technologies, owing to their similar epitaxial composition. We have noted, however, a higher rise in junction temperature in the DHBTs compared to that of the UTC-PDs owing to their larger device geometry and consequently more efficient heat evacuation. Nevertheless, further investigations are required in order to quantify the heat dissipation and mutual thermal coupling at more severe operating conditions of the UTC-PDs, which were not addressed in this work. Furthermore, in order to study the expected performances of this technology, model projections are compared with state-of-the-art technologies and their predictive capabilities are demonstrated. Coupled TCAD-QT simulations are leveraged to better understand the underlying physics in order to improve the formulation of analytic models. Finally, an elaborate design methodology was detailed that assembles all the preceding modules under one roof, taking into account compact models, thermal management and reliability as well as failure mechanisms, in order to address the system level design of the monolithic optoelectronic integrated circuits. Among the aspects discussed earlier, electromagnetic (EM) simulations have proven to be quite essential as the frequency of operation of the active devices fast approaches the sub-terahertz range. As illustrated in [28], as the frequency increases beyond 110 GHz, the electromagnetic coupling between the measurement probes and the substrate can have significant as well as detrimental impacts on the correct assessment of device performance.