Highly conductive Si-doped n-type Al 0.7Ga 0.3N alloys were grown by metalorganic chemical vapor deposition on sapphire substrates. Variable temperature Hall-effect measurements have been employed to study the electrical properties for samples with nominal Si dopant concentration (N si) from 2.6 to 6.8 × 10 19 cm -3. For the sample with N si=6.0 × 10 19 cm -3, we have achieved n-type resistivity of 0.0075 Ω cm with an electron concentration of 3.3 × 10 19 cm -3 and mobility of 25 cm 2/V s at room temperature. For the same sample, the effective donor (Si) activation energy E 0 was determined to be as low as 10 meV. E 0 increases to 25 meV as N si is reduced to 2.6 × 10 19 cm -3, which can be explained by the bandgap renormalization effect. This implies that heavy doping is necessary in high-Al-content AlGaN alloys to bring down the donor activation energy, therefore a higher conductivity.