We report an atypical device structure for a silicon-on -insulator (SOI) avalanche photodiode (APD) that uses vertically oriented surface plasmons to enable high device performance including high bandwidth and high responsivity. This device structure would allow for high bandwidth (> 10 GHz) read out integrated circuitry to be fabricated alongside the photodetector. A detailed numerical simulation of the device is performed that includes the use of the surface impedance boundary condition (SIBC) algorithm to calculate the optical resonance modes of the structure. The SIBC algorithm is integrated with a Poisson's equation solver and a Monte Carlo algorithm to model many aspects of the Sol APD device including bandwidth and responsivity as a function of applied bias. A brief discussion of surface plasmon modes and other optical modes in the APD and similar structures is also given. (c) 2005 Elsevier Ltd. All rights reserved.