We investigated the electron transport properties of n-GaN under an Ohmic-metal. Hall measurement results were compared for n-GaN (A) before Ti-based metal deposition, (B) after Ti-based metal deposition but before annealing, (C) after Ohmic annealing, and (D) after Ohmicmetal removal, where multi-probe-Hall device measurements are required for (C), while the others, (A), (B), and (D), can be characterized by conventional Hall device measurements. The multi-probe-Hall device measurements for (C) elucidated that, under the Ohmic-metal, the electron concentration is increased and the electron mobility is enhanced in comparison with those for the other cases, (A), (B), and (D). The increased electron concentration indicates that high-density doping takes place in the n-GaN by the Ohmic annealing. However, the highdensity doping is not observed after the Ohmic-metal removal. Moreover, the electron mobility enhancement under the Ohmic-metal cannot be explained by donor doping with ionized impurity scattering. These suggest that, under the Ohmic-metal, high-density donors are not formed, and high-density polarization doping owing to strain from the Ohmic-metal takes place. From theoretical calculations, we clarified that the increase in the electron density by polarization doping without donors leads to the suppression of ionized impurity scattering and consequently the electron mobility enhancement. (c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).