The successful application of finite element analysis to ultrafast optoelectronic devices is demonstrated. Finite element models have been developed for both an alloyed- and surface-contact metal-semiconductor-metal photodetectors. The simulation results agree with previously reported experimental data. The alloyed device, despite having a somewhat larger capacitance, has a non-illuminated region of lower resistance with a more-uniform and deeper-penetrating electric field and carrier transport current. The latter explains, in terms of the equivalent lumped parameters, the experimentally observed faster response of the alloyed device. The model is further used to predict improved responsivity, based on electrode spacing and antireflective coating. We project that increasing the depth of the alloyed contact beyond approximately half of the optical penetration depth will not yield significantly improved responsivity. (C) 2009 Elsevier Ltd. All rights reserved.