Semiconductor-graphene-semiconductor (SGS) transistors have been proposed for potential THz transistors for future ultra-high-speed communication, however, the current gain is restricted by the hetero-interface quality and the quantum reflection effect. Here, new SGS transistors are fabricated using an epitaxial graphene on a germanium substrate to reduce the interface scattering, leading to a record-high 30% common-base current gain approaching the theoretical limit. A common-emitter current gain over 1000 is further achieved based on a barristor mechanism. Our results pave the way for the application of the SGS transistors in the high-frequency area.