Measurement of the electron ionization coefficient at low electric fields in GaAs-based heterojunction bipolar transistors

Values of the electron ionization coefficient /spl alpha//sub n/ in <100> GaAs extending the previously available data by two orders of magnitude, down to 1 cm/sup -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.<>