In this paper we present the engineering of highly Sb-rich Ge-Sb-Te phase-change materials integrated in state-of-the-art Phase-Change Memory devices in 4Kb arrays. Thanks to an innovative composition called “delta” or δ-GST, high speed performance and high material stability under cycling is achieved in arrays and demonstrated by both physicochemical analysis and electrical characterization. Finally, the origin of the outstanding high speed in our innovative compound is revealed.