Monolithic optoelectronic integrated circuits (OEICs) are seen as key enabling technologies for future terahertz communications and upcoming beyond-5G networks. Co-design of monolithic OEICs requires computationally efficient compact models compatible with existing methodology/tool/design flow for timely and cost-effective realization of OEICs. This paper, inherently multidisciplinary, describes the first compact model for Uni-Traveling Carrier photodiodes (UTC PD), a key component for OEICs. Its equations capture the electronic transport and frequency response along with the photocurrent of the UTC PD under various applied optical power. It also dynamically computes the device junction temperature, accounting for self-heating effect. Excellent agreement between model and measurements as well as model scalability have been demonstrated.