This paper discusses design optimization for silicon avalanche photodetectors (APDs) fabricated in standard complementary metal-oxide-semiconductor (CMOS) technology in order to achieve the highest possible performance. Such factors as PN junctions, guard ring structures, active areas, and back-end structures are considered for the optimization. CMOS-APDs reflecting varying aspects of these factors are fabricated and their performances are characterized. In addition, their characteristics are analyzed with technology computer-aided-design simulations and equivalent circuit models. From these investigations, dominant factors that influence the CMOS-APD performance are identified. Furthermore, three different techniques enabling further performance improvements of CMOS-APDs are investigated, which are spatial-modulation, carrier-acceleration, and multijunction techniques. The state-of-the-art CMOS-APDs' structures and performances are presented and compared, and the best optimized CMOS-APD is proposed. These results should be extremely useful for realizing optimal silicon APDs in standard CMOS technology for various applications.