The reverse-biased performance of a molecular-beam-epitaxy-grown high-power optothyristor has been systematically characterized for pulsed power-switching applications. The device has a P+N-SI-PN+ thyristor-like structure with the bipolar junctions formed by AlGaAs. The semi-insulating (SI) GaAs used is liquid-encapsulated-Czochralski grown, undoped, and 650 μm in thickness. It is found that the reverse-biased optothyristor can be triggered by a light-emitting diode operated at 10-5 W, and miniature semiconductor lasers can trigger the switch with 132 A current using only a 1-mm-diam optical aperture. The reverse switching di/dt and the maximum peak current are reported as a function of blocking voltage. The effects of bipolar junctions on both sides of the SI-GaAs are also reported by comparing the bulk photoconductive current with the optothyristor switched current. It is shown that a laser beam of 0.05 μJ can be used to trigger on and switch about the same current as a 0.3 μJ laser beam, suggesting the possibility of integrating miniature semiconductor lasers and the optothyristors on the same chip to form a portable, compact, high-power solid-state pulser.