The mechanism of slow positron annihilation in Si-doped GaAs has been discussed in terms of the diffusion trapping model (DTM). The trapping of positrons has been considered in SiAs acceptors i.e. shallow defects and in VGa-SiGa vacancy complexes. The model has been used to obtain the Doppler broadening line shape parameter (S-parameter) and average positron lifetime in Si-doped GaAs, for a temperature range 20K to 290K and for different doping concentrations. Observations are made regarding the effect of doping on the nature and concentration of point defects. The change in point defect concentration due to Si- doping has been found to be proportional to the doping concentration. The effect of detrapping from the shallow defects has been found to be important at higher temperatures.