The critical terrace width λ for 2D island nucleation and growth (2DNG) on large-scale atomically flat terraces of a step-bunched Si(111)-(7×7) surface has been studied by in situ ultrahigh vacuum reflection electron microscopy as a function of the substrate temperature T and Si deposition rate R. The dependence of λ(2)(R) is characterized by a power law with scaling exponent χ=1.36-1.46, validating an attachment limited (AL) growth kinetics up to 720 °C. At this temperature, the Arrhenius dependencies lnλ(2)(1/T) change their slope, so that the effective 2DNG activation energy E(2D) drops from 2.4 eV down to 0.5 eV at T>720 °C. We first show that the E(2D) change is caused by a transition between AL and DL (diffusion limited) growth kinetics accompanied by a step shape transformation. The AL growth mode is characterized by kinetic length d(-)~10(5)a and the preferential step-down attachment of atoms to steps limited by an energy barrier E(ES)(-)≈0.9 eV.