## 1 Introduction
Indeed, in mm-wave frequencies, the connection between a photodiode and an amplifier is critical for limiting the parasitic influence. Therefore, accurate models of UTC-PDs and DHBTs are required to optimize the bias points and the interconnections between the photonic and the electronic parts. Using the same design methodology as the silicon industry, the compact modeling of the photodiodes and the InP DHBTs is the most promising way to achieve the best performance of the monolithically integrated optoelectronic circuits. As a consequence, accurate compact models and the associated parameter values for the UTC-PDs, the DHBTs as well as the passive elements should be jointly elaborated in order to guide the development of the OEIC at the early stage of its design. These compact models should be included in a single process design kit (PDK) that could be incorporated into the OEIC design tools. Featuring the ensemble of the building blocks, the PDK could thus allow the design and simulation of UTC-TIA-PA. The aim of this article was to provide a comprehensive evaluation of technological advances both in terms of sheer technological performances (photonic, electrical and thermal) as well as the software tools available for accomplishing this innovative design. In terms of design tools, in particular, electronic and photonic communities have very different simulation tools available, owing to the current status of their respective state of the arts. Integrated circuit design would thus necessitate multiphysics tools for facilitating the development of this emerging technology. Despite the differences in the design methods of the two communities, electronic design automation (EDA) tools have evolved and matured through years of industrial endeavors, leading to an abundance of SPICE compatible compact models and circuit design environments. While the photonic community is still lacking a key circuit design environment, the best possible solution would be to ensure the compatibility of photonic device simulation with existing EDA environments. Therefore, in the current work, we propose not only an up-to-date DHBT compact model dedicated to devices on InP substrate but we also developed and enhanced a compact model specifically for UTC photodiodes with the goal of making way for a common design framework. Both compact models are fully compatible with the EDA circuit libraries. Moreover, to ensure that common key physical quantities be handled, DHBT and UTC-PD compact models compute either electronic or photonic quantities along with their thermal parameters, which can, in the long run, pave the way for multiphysics tools. In order to pursue the aforementioned ideas further, in this paper, we present efficient compact models that are compatible with UTC photodiode and DHBT technologies developed by III-V Lab, which may be considered as the first step towards the efficient monolithic co-integration of electronic and photonic parts for this unique OEIC technology of the future. The rest of the paper is organized as follows. Section 2 is dedicated to the detailed technologies under investigation, UTC-PD and InP DHBT technologies, both from III-V Lab. Section 3 presents the electrical compact models associated with the two devices. Section 4 shows the DC characterization of UTC-PDs and InP DHBTs for the purpose of compact model validation. Section 5 provides insights into the thermal characterization and associated compact model sub-circuits that allow the management of self-heating as well as the mutual thermal coupling. Section 6 deals with the high frequency characterization and modeling of both devices. Section 7 discusses additional effects to be taken into consideration through the technology computer-aided design (TCAD) simulation, the evaluation of key performance metrics though model extrapolation in comparison with the state of the art as well as the methodology for assessing OEIC circuit reliability. Finally, Section 8 provides the conclusion and future perspectives.