Three-dimensionally integrated semiconductor systems include a photoactive device operationally coupled with a current/voltage converter on a semiconductor-on-insulator (SeOI) substrate. An optical interconnect is operatively coupled to the photoactive device. A semiconductor device is bonded over the SeOI substrate, and an electrical pathway extends between the current/voltage converter and the semiconductor device bonded over the SeOI substrate. Methods of forming such systems include forming a photoactive device on an SeOI substrate, and operatively coupling a waveguide with the photoactive device. A current/voltage converter may be formed over the SeOI substrate, and the photoactive device and the current/voltage converter may be operatively coupled with one another. A semiconductor device may be bonded over the SeOI substrate and operatively coupled with the current/voltage converter.