Betavoltaic devices are suitable for delivering low-power over periods of years. Typically, their power density is on the order of nano to micro-Watts per cubic centimeter. In this work we evaluate the potential for using high-aspect ratio three-dimensional semiconductor structures to enhance the power and efficiency of these devices. The Monte Carlo transport code MCNP6 is used to provide realistic estimates of the theoretical levels of charge generation, which is in turn used to make predictions about the power output from three-dimensional betavoltaics. The focus of this work is on silicon and promethium-147, but other semiconductors and radioisotopes are considered as well. In the case of silicon diodes with three-dimensional features that are comparable to what is commercially available we estimate that power densities in the range of 20-25 mW/cm3 can be achieved at efficiencies of 2.9-5.8% when coupled with promethium-147 oxide.
