Two-dimensional layered materials, such as molybdenum disulfide, are emerging as an exciting material system for future electronics due to their unique electronic properties and atomically thin geometry. Here we report a systematic investigation of MoS₂ transistors with optimized contact and device geometry, to achieve self-aligned devices with performance including an intrinsic gain over 30, an intrinsic cut-off frequency f_T up to 42 GHz and a maximum oscillation frequency f_MAX up to 50 GHz, exceeding the reported values for MoS₂ transistors to date (f_T~0.9 GHz, f_MAX~1 GHz). Our results show that logic inverters or radio frequency amplifiers can be formed by integrating multiple MoS₂ transistors on quartz or flexible substrates with voltage gain in the gigahertz regime. This study demonstrates the potential of two-dimensional layered semiconductors for high-speed flexible electronics.