Differential Hall-effect measurements are used to obtain profiles of the mobility, 1, and carrier concentration, n, in a 6-1m-thick GaN layer grown on Al2O3 by hydride vapor phase epitaxy (HVPE). In the top 1-1m region (surface), 1 1000 cm2/V-s and n 3 x 1016 cm-3, whereas in the bottom 0.75-im region (interface), i 50 cm2/V-s and n 2 x 1019 cm-3. Throughout the layer, the carrier concentration correlates well with the O and Si concentrations, with [Si] dominant near the surface, and [O] dominant near the interface, proving the shallow-donor nature of O. The average mobility and carrier concentration in the top 5 1m, i.e., the "bulk" region, are close to the values deduced by a much simpler analysis, introduced previously.