The intrinsic response time τi of 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 τi for 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 τi. For Si the value of the average high‐field diffusion constant D=80 cm2∕sec for E=400 kV∕cm, taken from the literature, could be used satisfactorily. For Ge and GaAs, where no information on D is available, D=100 cm2∕sec for E=200 kV∕cm and D=250 cm2∕sec for E=400 kV∕cm, respectively, had to be chosen to explain the experimental values of τi.