A comparison of AlGaAs/GaAs and GaInP/GaAs heterojunction bipolar transistors (HBTs) of similar geometry is presented. First-order measurements show the importance of recombination currents in AlGaAs/GaAs transistors. Passivation improves the d.c. performance of GaInP/GaAs devices only. Low-frequency noise measurements are then performed. Spectra show 1/f noise and g - r components. White noise is never reached. An analysis of each excess noise component is performed versus bias current. The 1/f noise level and bias dependence is quite similar in all transistors. Generation - recombination noise component levels are more important in AlGaAs/GaAs HBTs. This is attributed to characteristic traps. For both types of HBT no appreciable influence from the passivation is observed and noise sources are found to be located principally in the emitter - base junction.