In this work, we present a demonstration of a high-aspect ratio, three-dimensionally structured betavoltaic device. High-aspect ratio silicon PIN diodes were used as the semiconductor absorber and ¹⁴⁷PmCl₃ was used as the beta emitter. Three devices were fabricated with ¹⁴⁷Pm activities of 2.4 mCi, 7.4 mCi, and 29.5 mCi. The device with the highest activity produced an initial power output of 200 nW and was monitored over a period of 8 months to observe the current–voltage behavior over time, during which time the output current decreased in accordance with the radioactive half-life of ¹⁴⁷Pm. Small deviations in the output current of a few percent during the long-term measurement were found to be attributable to the humidity in the room where the experiment was done. The output current generated from the devices was 68–77% of the theoretical maximum, indicating significant infiltration of the radioisotope into the ridged structures.