We report on the design of an attached guard ring (AGR) and a floating guard ring (FGR) in a planar separate absorption, grading, charge, and multiplication In_0.53Ga_0.47As/InP avalanche photodiode to prevent premature edge breakdowns. The depths of the two Zn diffusions were utilized to manipulate the guard ring structures. Results from TCAD simulation indicate that the optimal AGR diffusion depth is right at the turning point where the breakdown current shifts from the edge of active region to the AGR region. The devices with optimal AGR depth contain significantly higher breakdown voltages than those of devices either with shallower—or without any— AGR. For the FGR design, a series of devices with different spacings between AGR and FGR and different FGR opening widths for diffusion were fabricated and characterized. We show that when the spacing is longer than the critical value, the breakdown voltage can increase ~1.5 V higher than those of the APD devices without FGR. In addition, the wider the FGR opening width, the higher the breakdown voltage. TCAD simulations were also performed to study the effect of FGR, but showed less pronounced improvements, which could be due the discrepancy between the calculated and experimental diffusion profile.