High-speed fully-depleted thin-film transistors (TFTs) are required for next-generation electronics, such as three-dimensional large-scale integrated circuits and advanced system-in-displays. For this purpose, a novel growth technique, i.e., interface-modulated solid-phase crystallization, of Sn-doped Ge has been developed. This achieves high carrier mobility (~100cm²/Vs) of ultrathin films (20nm) on insulators.