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Energy efficiency is the major optimization criterion for systems-on-chip (SoCs) for mobile devices (smartphones and tablets). Through silicon via (TSV) technology enables 3-D integration of dies and the heterogeneous stacking of multiple memory or logic layers, allowing increased bandwidth and lower energy consumption of the memory interface compared to traditional approaches. In this paper, we explore the 3-D-DRAM architecture design space. The result is an optimized 2 Gb 3-D-DRAM, which shows a 83% lower energy/bit than a 2 Gb device. Furthermore, we propose a highly energy-efficient DRAM subsystem for next-generation 3-D-integrated SoCs, consisting of a SDR/DDR 3-D-DRAM controller and an attached 3-D-DRAM cube with fine-grained access and a flexible (WIDE-IO) interface. We assess the energy efficiency using a synthesizable model of the SDR/DDR 3-D-DRAM channel controller (CC) as well as functional models of the 3-D-stacked DRAM, including an accurate power estimation engine. We also investigate different DRAM families (WIDE IO SDR/DDR, LPDDR, and LPDDR2) and densities from 256 Mb to 4 Gb per channel. The implementation results of the proposed 3-D-DRAM subsystem show that energy optimized accesses to the 3-D-DRAM enable up to 50% energy savings compared to standard accesses. To the best of our knowledge this is the first design space exploration for 3-D-stacked DRAM considering different technologies based on real-world physical data and the first design of a 3-D-DRAM CC and 3-D-DRAM model featuring co-optimization of memory and controller architecture.