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We report the study of ultrafast carrier dynamics of low temperature grown GaAs over wide spectral range (2.75 eV–0.88 eV) with above and below bandgap (E g) optical excitations using ultrafast transient absorption spectroscopy. The presence of defects and their subsequent effect on the pathway and lifetime of carrier relaxation with probing at different energy levels is presented. The carrier relaxation pathways and their corresponding models are predicted. It is observed that for pumping (2.58 eV) and probing above E g , the transient behavior is dominated by band filling while renormalization is in prominence when probed near to E g. For below E g pumping (0.88 eV) the optical behavior is mostly governed by defects. The calculated trap time is in the range of ~128 fs to ~807 fs while overall recombination time shows a wide variation from ~1.38 ps to ~1.25 ns depending on the position of traps/defects in the forbidden band. • Comprehensive analysis of transient behaviour of LT-GaAs is presented and models are proposed for the possible carrier dynamics as per rate equation model. • Below bandgap pumping gives a behaviour of trap/defect states. • For above band gap pumping, band filling effect dominates but near to band edge bandgap renormalisation overpowers bandfilling. • In LT-GaAs there are basically two trap bands. • Decay time is governed by position of trap state in forbidden gap. [ABSTRACT FROM AUTHOR]