We report the fabrication and performance of a highly efficient, monolithic distributed-Bragg-reflector channel waveguide laser in ytterbium-doped aluminum oxide. The $1\mathrm{cm}$ long device was fabricated on a standard thermally oxidized silicon substrate and was optically pumped with a $976\mathrm{nm}$ laser diode. Single-longitudinal-mode and single-polarization operation was achieved at a wavelength of $1021.2\mathrm{nm}$ . Continuous-wave output powers of up to $47\mathrm{mW}$ and a launched pump power threshold of $10\mathrm{mW}$ resulted in a slope efficiency of 67%.