We report a series of our experiments using organic single crystals to reach the maximum performance intrinsic to the materials. A consequence of the experiments is that a prescription for realizing high-mobility devices is to induce carriers in inner crystals to avoid scattering at the surfaces. Intrinsic-semiconductor character of the high-purity organic crystals favors thermal diffusion of the carriers into the crystals in the presence of weak gate-electric fields. Furthermore, it is demonstrated that the high-mobility transport of the in-crystal carriers are highlighted in double-gate single-crystal transistors with the two gate electric field balanced with each other.