Hardware electronic synapse and neuro-inspired computing system based on phase change random access memory (PCRAM) have attracted an extensive investigation. However, due to the intrinsic asymmetric reversible phase transition, the defective weight update of PCRAM synapses in aspects of tuning range, linearity and continuity has long required a system-level complexity of circuits and algorithms. The cell-level improvements to a great extent may slim the system thus achieving efficient computing. We report in this work the great enhancement of Ge2Sb2Te5 (GST) based PCRAM synapses by combining materials engineering and pulse programming. It is found that carbon doping in GST retards the rate of phase changing thus increasing the controllability of the conductance, while non-linear programmable pulse excitations can eventually lead to a reliable synaptic potentiation and depression. A set of improved programmable pulse schemes for spike-timing dependent plasticity was then demonstrated, suggesting its potential superiority in flexible programming and reliable data collection. Our methods and results are of great significance for implementing PCRAM electronic synapses and high-performance neuro-inspired computing.