As the building blocks of deep UV light emitting diode (LED) technology and high‐power electronic devices, AlGaN alloys have attracted considerable attention. In this study, AlGaN films with varying compositions doped with Si were deposited on homoepitaxial AlN layers grown on AlN single crystal substrates. The room temperature resistivity of AlGaN alloys of different compositions grown on AlN and sapphire substrates with a constant Si doping level of 6x1018 cm‐3 was compared. AlGaN films grown on AlN substrates consistently exhibited a lower n‐type resistivity than those grown on sapphire. An n‐type resistivity of 0.1 Ω cm was obtained for an AlGaN film with 80% Al content and a sheet resistance of 235 Ω/sq. for an AlGaN film with 70% Al content. The carrier activation energy as a function of Al content in AlGaN for these n‐type films was measured. For compositions below 80% Al, the activation energy was around 15 meV due to impurity potential screening. For higher Al compositions, the carrier concentration was limited by a high compensation ratio, except for AlN, which has activation energy of 250 meV. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)