In order to investigate the feasibility of Si-doped Ga0.51In0.49P for modulation-doped field effect transistor applications, single Ga0.51In0.49P llayers and Ga0.51In0.49P/InxGa1-xAs/GaAs (x = 0, 0.15 and 0.25) modulation doped heterostructures grown by gas source molecular beam epitaxy were characterized through deep level transient spectroscopy and Hall-effect measurements. Electrical characterization of the undoped and moderately Si-doped (N-D = 3 x 10(17) cm(-3)) GaInP layers yielded an electron trap with an activation energy of 0.75 eV and a temperature dependent capture cross section with a capture barrier of 0.593 eV. The density of this trap increased, and an anomalous decrease in the free carrier concentration of GaInP was observed after the samples were annealed at temperatures typically used in device processing. While, this trap showed characteristics similar to DX centers, it was not detected in highly Si doped (N-D approximate to 4 x 10(18) cm(-3)) as grown layers suggesting that the trap is a defect complex including a residual impurity. While very high two-dimensional electron gas density (2.6 x 10(12) cm(-2) at 30 K) was achieved in the lattice matched (x = 0) structures, the strained structures were found to be very sensitive to heat treatment, although the InGaAs layers thicknesses were below the theoretical critical thickness. Persistent photoconductivity and a significant reduction in the interface sheet electron density were observed after annealing. The anomalous behavior can be attributed to the decrease in the carrier concentration of the doped GaInP barrier layer and to the strain relaxation at the hetero-interface after annealing. While other explanations may be possible, the decrease in the GaInP electron concentration can be attributed to Si atoms moving from donor to acceptor sites.