Multigigahertz flexible electronics are attractive and have broad applications. A gate‐after‐source/drain fabrication process using preselectively doped single‐crystal silicon nanomembranes (SiNM) is an effective approach to realizing high device speed. However, further downscaling this approach has become difficult in lithography alignment. In this full paper, a local alignment scheme in combination with more accurate SiNM transfer measures for minimizing alignment errors is reported. By realizing 1 μm channel alignment for the SiNMs on a soft plastic substrate, thin‐film transistors with a record speed of 12 GHz maximum oscillation frequency are demonstrated. These results indicate the great potential of properly processed SiNMs for high‐performance flexible electronics.