Charge-collection mechanisms of AlGaAs/GaAs HBTs

AlGaAs/GaAs heterojunction bipolar transistors are investigated via time-resolved and time-integrated laser induced charge-collection measurements and by two-dimensional computer simulation. Both experiment and simulation results suggest that charge collection occurs by a simple model in which carriers created by the ionizing event are collected at the device terminals, with no evidence for charge enhancement processes. The charge-collection efficiency in all cases is less than unity, with the collected charge exhibiting a linear dependence on the deposited charge for all laser wavelengths used (below the saturation limit). The simulation results reveal that the fast and slow contributions to the charge collection transient arise from electron and hole collection, respectively, with the lower mobility of the holes giving rise to their slower collection dynamics