MEASUREMENT OF THE ELECTRON IONIZATION COEFFICIENT AT LOW ELECTRIC-FIELDS IN GAAS-BASED HETEROJUNCTION BIPOLAR-TRANSISTORS

Values of the electron ionization coefficient an in (100) GaAs extending the previously available data by two orders of magnitude, down to 1 cm-1, are presented. The data are directly extracted from the multiplication factor, M-1, measured in lightly doped collector n-p-n AlGaAs/GaAs Heterojunction Bipolar Transistors (HBT's). It is shown that the sensitivity of the technique is limited by the Early effect, whose influence can be reduced by driving the device at constant emitter-base bias and by using heavily doped base regions. HBT's can provide simultaneously high base doping and current gain, and represent therefore an excellent tool for these measurements.