Carrier concentration and mobility of materials are key factors affecting device performance. Hall tests at different temperatures demonstrate that the carrier concentration and mobility in impurity-doped AlGaN decrease with temperature decreasing. However, carrier concentration and mobility obtained by polarization-induced doping are independent of temperature. Using quasi-insulating GaN as substrate, the electron concentration obtained in the linearly graded AlGaN film through impurity-doping is only 10-17 cm-3 or less. In this study, using unintentional impurity doped (n-type, 10-16 cm-3) GaN template, graded AlGaN film is grown by molecular beam epitaxial, in which polarization induced ultra-high electron concentration is up to 1020 cm-3 in graded AlGaN film without using any dopant. Using quasi-insulating GaN as substrate, only the surface of the free electrons serves as polarization dopant, while unintentionally doped GaN template is used as a substrate, in addition to free electrons on surface/interface, it is also reasonable to surmise more negative charges attracted by polarization electric field to be the source of polarization doping, in the unintentional doped GaN template, thereby achieving an ultra-high carrier concentration via polarization induced n-type doping.