Memory cell design of optically coupled three‐dimensional common memory

An optically coupled three‐dimensional common memory has been proposed for more functional and faster parallel processing of information with several CPUs. A detailed design of the memory cell was carried out. First, the effect of stray light which is a significant problem in the design of the optically coupled three‐dimensional common memory was studied quantitatively by deriving the optical coupling efficiency between the light‐emitting and detecting devices. Next, by means of the static analysis taking into account the fluctuations of the device dimensions, alignment error between the layers and the variation of the power supply voltage, the safe operating region of the memory cell was evaluated. As a result, an optimum condition was found on the transverse pitch between the light‐emitting and detecting devices and the supply current to the emitting device for a stable operation of the memory without being affected by the stray light. Further, with this optimum condition, a dynamic analysis was carried out for the memory cell circuit to which the bit‐line peripheral circuit is attached. In the analysis, the reduction of the load resistance of the memory cell by the optical writing was described with an equivalent circuit consisting of resistors and MOSFETs. The analysis was carried out with the driving circuit for the light‐emitting device characteristic to the optically coupled three‐dimensional common memory taken into account. It was confirmed that the memory cell designed by means of this dynamic analysis operates correctly.