Small-signal microwave transistor characteristics are used to construct and fit a comprehensive model of their dynamic behaviour. The model includes thermal effects and trap-related effects, which influences such a large range of frequencies that they are not well characterized by large-signal or pulse measurements alone. Correlation of the model with smallsignal characteristics demonstrates the region of influence of specific dynamic effects. The model extrapolates beyond the measurement space to quantify the very significant impact that transistor dispersion has on microwave circuit performance. The results question the adequacy of conventional transistor characterization techniques for accurate circuit design.