Analysis of the Use of Molecular Resonant Tunneling Diodes for Local Refresh of Dynamic Random Access Memory Cells
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Analysis of the Use of Molecular Resonant Tunneling Diodes for Local Refresh of Dynamic Random Access Memory Cells Jonas Berg, Stefan Bengtsson and Per Lundgren Chalmers University of Technology, Microtechnology Centre at Chalmers and Solid State Electronics Laboratory, Department of Microelectronics, SE-412 96 Göteborg, Sweden ABSTRACT Simulations have been made to analyze the use of molecular resonant tunneling diodes for local refresh of DRAM (Dynamic Random Access Memory) cells. Local refresh can be provided by a latch consisting of a pair of resonant tunneling diodes connected to the storage capacitor of the cell. Such a solution would significantly reduce the standby power consumption of the DRAM cell. We have compared the requirements on the resonant tunneling diodes for proper refresh operation with the electrical properties of published molecules with resonant IV-curves. The simulations show that no molecules with resonant electrical properties published so far in the literature have properties making them useful for this particular application. This is true also for low temperature operation. The issues of maximum tolerable series resistance and of maximum tolerable fluctuations in the number of attached molecules have also been addressed. Our results show that the focus for development of molecules with resonant electrical properties should be to find molecules with resonance for lower applied voltages and lower current levels than the molecules published so far. If the synthesis of new molecules with attractive properties is successful the merging of silicon technology and molecular electronics, for instance for new generations of DRAM cells, is a realistic future path of microelectronics. INTRODUCTION Molecules have many properties that make them attractive for electronic applications. One probable development is that molecular electronics is used together with standard state-of-the-art CMOS technology. The molecular electronic will in such a case add benefits to the existing silicon technology. For such a combined technology of molecular electronics and CMOS to be viable, it must show better performance and/or lower cost than standard silicon-only technology. As one example of an application using both nanoscale elements and silicon technology, we have been studying the improvement of standard computer memory cells (DRAM) by using molecular resonant tunneling diodes. This could reduce the standby power of DRAM cells significantly, and is especially interesting for use in handheld applications. In this particular case, the advantages given by adding molecular electronics are hard to realize in silicon-only solutions. From the properties of future DRAM cells, design rules for the electrical properties of resonant molecules can be found. In a DRAM the information is stored as charge in capacitors. Applying a voltage to the corresponding capacitor constitutes writing to a cell, and reading is done by connecting the capacitor to a sense amplifier via a bitline. Due to the large parasitic capacitance of the
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