We have investigated the distribution of the electric field in p-i-n type and separate absorption and multiplication (SAM) type GaN avalanche photodiodes under different reverse bias values. We have also analyzed the influences of the parameters of each layer, including width and concentration, on the distribution of the electric field, especially on the breakdown voltage. It is found that a relatively high concentration of p-GaN (higher than 1×10¹⁸ cm⁻³) and low carrier concentration of i-GaN (lower than 5×10¹⁶ cm⁻³) are helpful to restrict the electric field and reduce the breakdown voltage. In a SAM (p-i-n-i-n) structure, a suitable choice should be made for the concentration and thickness of the intermediate n-GaN layer in order to decrease breakdown voltage and prevent the device from degenerating into a p-i-n structure. Finally, the optimized material parameters of each layer are proposed.