Brain-like associative learning using a nanoscale non-volatile phase change synaptic device array

Recent advances in neuroscience together with nanoscale electronic device technology have resulted in huge interests in realizing brain-like computing hardwares using emerging nanoscale memory devices as synaptic elements. Although there has been experimental work that demonstrated the operation of nanoscale synaptic element at the single device level, network level studies have been limited to simulations. In this work, we demonstrate, using experiments, array level associative learning using phase change synaptic devices connected in a grid like configuration similar to the organization of the biological brain. Implementing Hebbian learning with phase change memory cells, the synaptic grid was able to store presented patterns and recall missing patterns in an associative brain-like fashion. We found that the system is robust to device variations, and large variations in cell resistance states can be accommodated by increasing the number of training epochs. We illustrated the tradeoff between variation tolerance of the network and the overall energy consumption, and found that energy consumption is decreased significantly for lower variation tolerance.Comment: Original article can be found here: http://journal.frontiersin.org/Journal/10.3389/fnins.2014.00205/abstrac

Adaptive Keeper Design for Dynamic Logic Circuits Using Rate Sensing Technique

The increasing variability in device leakage has made the design of keepers for wide OR structures a challenging task. The conventional feedback keepers (CONV) can no longer improve the performance of wide dynamic gates for the future technologies. In this paper, we propose an adaptive keeper technique called rate sensing keeper (RSK) that enables faster switching and tracks the variation across different process corners. It can switch upto 1.9x faster (for 20 legs) than CONV and can scale upto 32 legs as against 20 legs for CONV in a 130-nm 1.2-V process. The delay tracking is within 8% across the different process corners. We demonstrate the circuit operation of RSK using a 32 x 8 register file implemented in an industrial 130-nm 1.2-V CMOS process. The performance of individual dynamic logic gates are also evaluated on chip for various keeper techniques. We show that the RSK technique gives superior performance compared to the other alternatives such as Conditional Keeper (CKP) and current mirror-based keeper (LCR)

Microthermal Stage for Electrothermal Characterization of Phase-Change Memory

Abstract-This letter describes a novel experimental structure that captures the impact of rapid temperature transients and repetitive cycling on the thermal and electrical properties of Ge 2 Sb 2 Te 5 (GST). The microthermal stage dramatically improves the temporal resolution for heating and enables simultaneous thermal and electrical characterizations. Thermal conductivity measurements show phase transitions of GST accompanied by abrupt changes in electrical resistance. Repetitive cycling with durations down to 100 ns produces melt-quenched amorphous GST with the thermal conductivity 40% lower than that of crystalline GST. Recrystallization increases conductivity but not up to the value achieved by long-timescale bulk annealing. This is potentially because the rapidly recrystallized GST contains more disorder near the interface. Index Terms-Ge 2 Sb 2 Te 5 (GST), microthermal stage (MTS), phase-change memory (PCM), thermal conductivity

Ultrafast terahertz-induced response of GeSbTe phase-change materials

The time-resolved ultrafast electric field-driven response of crystalline and amorphous GeSbTe films has been measured all-optically, pumping with single-cycle terahertz pulses as a means of biasing phase-change materials on a sub-picosecond time-scale. Utilizing the near-band-gap transmission as a probe of the electronic and structural response below the switching threshold, we observe a field-induced heating of the carrier system and resolve the picosecond-time-scale energy relaxation processes and their dependence on the sample annealing condition in the crystalline phase. In the amorphous phase, an instantaneous electroabsorption response is observed, quadratic in the terahertz field, followed by field-driven lattice heating, with Ohmic behavior up to 200 kV/cm. Phase change materials (PCMs) have been studied for use in nonvolatile memory devices due to their fast electrically-induced switching between a high-resistance amorphous phase and a low-resistance crystalline phase. 1,2 The field-driven heating of the material is essential to the switching process, although alternative switching mechanisms have also been proposed recently. The initial steps in the field-driven response are closely associated with the nature of electrical transport in the materials. Numerous models attempt to explain the conduction mechanisms in amorphous PCMs. 7-13 For example, Ielmini and Lacaita have measured and modeled the field dependent conduction in PCMs for steady state conditions, with results signifying both a Poole-Frenkel and phonon-assisted tunneling mechanism for conduction in the amorphous phase. 14 Siegrist et al. have investigated electrical transport in the crystalline phase, using static electrical measurements to characterize the progression of the electrical properties with increasing annealing temperature. 15 Conduction mechanisms were associated with localized charge carriers in the low annealing temperature, highly disordered crystalline state, transitioning to delocalized carriers at higher annealing temperatures, thereby undergoing a disorder-driven insulatormetal transition. Breznay et al. have measured disorderdominated transport even in the metallic phase. 16 GeSbTe (GST) alloys are widely used and of technological importance for phase-change applications. We have employed a pump-probe measurement scheme to determine the time-resolved response of GST films to THz electric fields

Ultrafast Characterization of Phase-Change Material Crystallization Properties in the Melt-Quenched Amorphous Phase

Phase change materials are widely considered for application in nonvolatile memories because of their ability to achieve phase transformation in the nanosecond time scale. However, the knowledge of fast crystallization dynamics in these materials is limited because of the lack of fast and accurate temperature control methods. In this work, we have developed an experimental methodology that enables ultrafast characterization of phase-change dynamics on a more technologically relevant melt-quenched amorphous phase using practical device structures. We have extracted the crystallization growth velocity (<i>U</i>) in a functional capped phase change memory (PCM) device over 8 orders of magnitude (10^–10 < <i>U</i> < 10^–1 m/s) spanning a wide temperature range (415 < <i>T</i> < 580 K). We also observed direct evidence of non-Arrhenius crystallization behavior in programmed PCM devices at very high heating rates (>10⁸ K/s), which reveals the extreme fragility of Ge₂Sb₂Te₅ in its supercooled liquid phase. Furthermore, these crystallization properties were studied as a function of device programming cycles, and the results show degradation in the cell retention properties due to elemental segregation. The above experiments are enabled by the use of an on-chip fast heater and thermometer called as microthermal stage (MTS) integrated with a vertical phase change memory (PCM) cell. The temperature at the PCM layer can be controlled up to 600 K using MTS and with a thermal time constant of 800 ns, leading to heating rates ∼10⁸ K/s that are close to the typical device operating conditions during PCM programming. The MTS allows us to independently control the electrical and thermal aspects of phase transformation (inseparable in a conventional PCM cell) and extract the temperature dependence of key material properties in real PCM devices