Photodetectors operable to achieve multiplication of photogenerated carriers at ultralow voltages. Embodiments include a first p-i-n semiconductor junction combined with a second p-i-n semiconductor junction to form a monolithic photodetector having at least three terminals. The two p-i-n structures may share either the p-type region or the n-type region as a first terminal. Regions of the two p-i-n structures doped complementary to that of the shared terminal form second and third terminals so that the first and second p-i-n structures are operable in parallel. A multiplication region of the first p-i-n structure is to multiply charge carriers photogenerated within an absorption region of the second p-i-n structure with voltage drops between the shared first terminal and each of the second and third terminals being noncumulative.