The intrinsic response time &tgr;iof semiconductor avalanches has been determined from microwave measurements on Si, Ge, and GaAs IMPATT diodes of various structures (n+‐p, p+‐n, Schottky barrier). A theory has been developed to calculate &tgr;ifor these diodes. It was found that the effect of carrier diffusion has to be taken into account to remove a systematic discrepancy between experimental and theoretical values of &tgr;i. For Si the value of the average high‐field diffusion constantD¯=80 cm2/sec forE=400 kV/cm, taken from the literature, could be used satisfactorily. For Ge and GaAs, where no information onD¯is available,D¯=100 cm2/sec forE=200 kV/cm andD¯=250 cm2/sec forE=400 kV/cm, respectively, had to be chosen to explain the experimental values of &tgr;i.