Avalanche breakdown in abrupt p-n AlGaAs/GaAs heterojunctions is invesigated, and the breakdown voltage, the maximum electric field, and the depletion layer width are calculated as functions of the doping densities, the temperature, and the AlAs mole fraction in AlGaAs. The model employed is an extension of Hauser's model of homojunction breakdown [Appl. Phys. Lett. 33, 351 (1978)], and it includes the effects of the band offsets at the interface.