(a) The schematic diagram of GaN samples sliced from a GaN boule grown by HVPE, (b) the dislocation density and Si concentration for each sample, (c) the relationship between mobility and temperature for each sample, (d) the relationship between mobility and temperature for sample S1. μ_disl, μ_ac, μ_i indicate the mobility due to dislocations, acoustic phonons, and ionized impurities. μ_Exp and μ_T indicate the experimental results and the calculated values according to the model.