The operation and performance of the InP-based uni-traveling carrier photodiode (UTC-PD) has been studied using a two dimensional, drift-diffusion approach utilizing a commercial numerical device simulator. The UTC-PD has been proposed as a replacement for the InGaAs PIN photodiode for long wavelength optical communications since it retains its performance at high levels of optical injection and exhibits a very high frequency capability. In this work, the results of an investigation of the effects of the device's structure on the operational performance are reported as well as the effects of device biasing and the optical injection level. In particular, the origins of the degradation in device performance at high optical injection levels have been investigated. The results are discussed in relation to reports of the device's experimentally observed performance. (C) 2003 Elsevier Ltd. All rights reserved.