a concentration of approximately 2???1017 cm-3 ; a GaAs base layer 610 having a thickness of approximately 0.08 ??m, and doped with carbon for example to a concentration of approximately 3???1019 cm-3 ; a GaAs collector layer 612 having a thickness of approximately 1.0 ??m, and doped with silicon for example to a concentration of approximately 2???1016 cm-3 ; and a GaAs collector contact layer 614