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We obtained a series of experimental results showing the effects of floating guard rings (FGRs) in InGaAs-InGaAsP-InP separate absorption, grading, charge, and multiplication avalanche photodiodes. It was confirmed from the scanned photocurrent curves that the essential role of FGRs is to disperse the curved equipotential lines at the lateral junction periphery and to give a low field route for a carrier beneath the FGRs. FGR effect mainly depends on guard ring spacing and it also depends on the magnitude of the applied bias. In our optimum guard ring condition, the current gain at the active planar region found to be 1.4 times larger than that at the curved edge.