Measurement of the Electron Ionization Coefficient At Low Electric-fields In Heterojunction Bipolar-transistors

New data for the electron impact-ionization coefficient alpha(n) in GaAs and In0.53Ga0.47As are derived from measurements of the multiplication coefficient M-1 in Heterojunction Bipolar Transistors (HBT). We show that the intrinsic limiting factors of the M-1 measurement at low electric fields are the Early effect and the uncertainty in the electric field value in the collector depletion region. By adopting HBT's characterized by lightly doped collectors and heavily doped base regions we were able to extend previously available data on the impact-ionization coefficient of GaAs and In0.53Ga0.47As by two orders of magnitude, down to alpha(n) = 1 cm(-1). Our data on In0.53Ga0.47As also confirm the theoretical results on the weak field dependence of alpha(n) at low electric fields.