We study theoretically how the stable structure of Td impurities in covalent semiconductors is changed by two types of electronic excitation: intra-photoexcitation and carrier capture. The Td T-U-S model is used, which is tetrahedrally constructed from four sites representing four antibonding (or bonding) orbitals for a substitutional donor (acceptor) impurity in the diamond and zincblende structures. It is shown that the character of the stable structure is either T-d, C-3v, Q(2v), C-s or C depending on the ratio of parameters (T, U, S), the electron occupation number n (hence on the charge state), the total spin and the total electronic state. Structural changes and the relaxation dynamics of DX-center in AlxGa1-xAs and EL2-center in GaAs are discussed with the obtained adiabatic potential surfaces and the phase diagrams. (C) 2003 Elsevier B.V. All rights reserved.