The growth features of n-doped ultrashort-period monolayer superlattices GaN/AlN:Si superlattices - digital alloy with a minimum AlN (x) content of up to 85 %, which can replace conventional Al_xGa_1-xN:Si compounds with a similar x, using plasma-activated molecular beam epitaxy are described. It is shown that the original technology of pulsed plasma-activated molecular beam epitaxy in metal-rich conditions allows growing GaN_m/AlN_n digital alloys with layer thicknesses in monolayers (1 ML = 0.25 nm) 0.5<m<2 ML, 2<n<6 ML and a total thickness of ~500 nm. Digital alloys exhibit a planar, atomically smooth surface morphology with a root-mean-square roughness of <0.39 nm over all scan areas up to (10x10) μm². In contrast, AlGaN layers exhibit a grainy surface morphology with an average grain diameter of ~300 nm, resulting in an increase in root-mean-square surface roughness of >2 nm over scan areas ≥(3x3) μm². This difference is attributed to the different growth mechanisms of digital alloys and layers: while the former grows according to a two-dimensional nucleation mechanism, the latter follows a spiral growth mechanism. X-ray diffraction measurements revealed submonolayer (~ 0.3 monolayer) control accuracy for the thickness of GaN and AlN layers in digital solid solutions, ensuring x was specified with an accuracy of 3-7 %. Comparative studies of n-doping of GaN/AlN digital alloys with silicon demonstrated a maximum electron concentration of up to ~10¹⁹ cm^-3 for digital solid solutions with x=0.7 and the possibility of obtaining conductive layers of digital solid solutions up to x=0.85, while in Al_xGa_1-xN:Si layers with the same x, the electron concentrations were an order of magnitude lower and at x>0.8, a semi-insulating nature of conductivity was observed. These results indicate the possibility of using n-doped digital alloys (GaN/AlN):Si with high x (up to 85 %) in device heterostructures. Keywords: plasma-activated molecular beam epitaxy, digital alloys, wide bandgap semiconductors, monolayer superlattices, surface morphology, doping, AlGaN.