We report on differential transmission measurements on low temperature grown (LT)-GaAs with and without applied electrical fields at different wavelengths. Electrical fields up to 100 kV/cm can be applied via an interdigitated contact structure to our LT GaAs samples which have been removed from the substrate by epitaxial lift off. In the presence of an electric field, both, the absorption bleaching due to phase space filling and field induced absorption changes due to the Franz–Keldysh effect contribute to the transmission changes. We observe an extended carrier lifetime with applied field. The response time of a biased metal–semiconductor–metal detector, therefore, exceeds the carrier life time of the substrate material. ©1997 American Institute of Physics.