The influence of n-type doping of the AlGaN barrier layer in AlGaN/AlN/GaN single- and triple-channel heterostructures on their electrical properties was studied. It was found that the optimal thickness of i-AlGaN spacer is 3 nm, and  the Si concentration in n-AlGaN is 7·1018 cm-3. The lowest predicted sheet resistance at room temperature for the triple-channel structure of the optimal design is ~ 90 Ω sq-1, three times lower than that of the single-channel  structure.