Modelling and optimization of Ge/GaAs uni-travelling carrier photodiodes

Uni-Travelling-Carrier Photodiodes (UTC-PDs) are pivotal for the advancement of high-speed optical communication systems. Current UTC-PDs have a trade-off between high performance and low production costs. The performance of conventional Ge/Si UTC-PDs is limited by the materials, primarily due to the relatively low electron mobility of Si compared with that of GaAs and InP. The UTC-PDs based on high-mobility materials such as InP have high production and fabrication costs, which limits their effectiveness in next-generation applications. Germanium (Ge) and Gallium Arsenide (GaAs), with their high carrier mobility and lattice-matching capabilities, are strong candidates for UTC-PDs. Their compatibility with GaAs and Si substrates allows for large-scale production and integration into silicon photonics. The Ge/GaAs UTC-PDs are poised to meet the increasing performance demands while keeping production at relatively low costs. This study provides an in-depth analysis and simulation of Ge/GaAs-based UTC-PDs operating at 1550 nm and demonstrates an innovative front-illuminated design. Simulations indicate a 3-dB bandwidth from 30 GHz of up to 54 GHz, responsivity ranging from 0.5 A/W to 0.7 A/W at -2 V, and device diameter between 5 μm and 8 μm. This research opens avenues for a new generation of UTC-PD designs, highlighting the feasibility and potential of the Ge/GaAs material system and providing new options for the development of future photodetectors.