We report on a study of buffer layer leakage currents in AlGaN/GaN HEMT structures with AlN buffer layers grown on 6H-SiC substrates by RF-plasma molecular beam epitaxy. We find that the use of a thin Be:GaN layer dramatically reduces the leakage currents in AlGaN/GaN HEMTs. Four structures were grown under the same conditions, except for the Be concentration, using quarters of the same n(+) SiC substrate. Device isolation is improved by a factor of 10(3) and the transistor pinch-off characteristics are improved. Details of the growth conditions and measurements are presented. Published by Elsevier Science B.V.