Simulation and Design of a Silicon Nanowire based Phase Change Memory Cell
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Simulation and Design of a Silicon Nanowire based Phase Change Memory Cell Ramin Banan Sadeghian 1 , Yusuf Leblebici 2, and Ali Shakouri 1,3 1
Baskin School of Engineering, University of California at Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, U.S.A. 2
Microelectronic Systems Laboratory, Swiss Federal Institute of Technology (EPFL), ELD 340, Station 11,CH-1015 Lausanne, Switzerland. 3
School of Electrical and Computer Engineering, Purdue University, 465 Northwestern Ave., West Lafayette, IN 47907, U.S.A. ABSTRACT In this work we present preliminary calculations and simulations to demonstrate feasibility of programming a nanoscale Phase Change Random Access Memory (PCRAM) cell by means of a silicon nanowire ballistic transistor (SNWBT). Memory cells based on ballistic transistors bear the advantage of having a small size and high-speed operation with low power requirements. A one-dimensional MOSFET model (FETToy) was used to estimate the output current of the nanowire as a function of its diameter. The gate oxide thickness was 1.5 nm, and the Fermi level at source was set to -0.32 eV. For the case of VDS = VGS = 1 V, when the nanowire diameter was increased from 1 to 60 nm, the output power density dropped from 109 to 106 W cm-2 , while the current increased from 20 to 90 A. Finite element electro-thermal analysis were carried out on a segmented cylindrical phase-change memory cell made of Ge2Sb2Te5 (GST) chalcogenide, connected in series to the SNWBT. The diameter of the combined device, d, and the aspect ratio of the GST region were selected so as to achieve optimum heating of the GST. With the assumption that the bulk thermal conductivity of GST does not change significantly at the nanoscale, it was shown that for d = 24 nm, a ‘reset’ programming current of ID = 80 A can heat the GST up to its melting point. The results presented herein can help in the design of low cost, high speed, and radiation tolerant nanoscale PCRAM devices. INTRODUCTION Phase Change Random Access Memories (PCRAMs) are nonvolatile memories that employ the large electrical conductivity contrast between the amorphous and the crystalline phases of so-called phase-change materials (PCMs) such as Ge2Sb2Te5 (GST) and GeTe chalcogenide glasses. Phase transition is induced by proper heating of the PCM through controlled electrical pulses. Such a transition is reversible, rapid, and radiation resistant. The reset operation entails heating the PCM above its melting temperature using a relatively intense but short electrical pulse, followed by rapid quenching, resulting a highly resistive amorphous phase. The set (recrystallization) operation is accomplished by applying an electrical pulse with a smaller amplitude but longer duration to heat up the material to a temperature between the glass transition and melting points. The state of the memory can then be read by measuring its electrical resistance using nondestructive currents/voltages. Reducing power consumption has been a key challenge, especially during the reset operation where hig
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