Two advantages (freedom from ground loops and enhanced performance of diffused guard rings) of P⁺/P⁻ epitaxial versus homogeneous crystal substrates in 64K DRAM are presented. Reduced refresh time and pattern sensitivities of the early TMS 4164, built on homogeneous substrate, are chronicled. To better understand the significant improvements with epitaxy, models are investigated for substrate capacitive/drift current coupling and for guard rings. Substrate-coupling modeling primarily used an instantaneous view of potential distributions via solution of Laplace's equation, followed by time-domain analysis. A comparison is made with a discrete resistor mesh. Guard ring study was empirical. Sets of biased diffused annular rings, surrounding an n⁺ dot used for minority carrier injection, were fabricated on both types of substrate. Results of the theoretical and experimental studies are compared with device performance.