In this work, a set of Al_xGa_1−xN/GaN heterostructures with increased aluminum mole fraction up to 0.30–0.45 was grown using metalorganic vapor-phase epitaxy on silicon substrates. It was shown that the dependence of sheet  resistance on the aluminum mole fraction had a minimum at x ~ 0.37. The lowest experimentally obtained value was 236 Ω/sq., which, to the best of our knowledge, is the lowest reported value for AlGaN/GaN structures grown on Si  substrates.