New devices for sequential access memory

Abstract

Two classes of devices are seriously challenging the electromechanical disk file for bulk storage applications---magnetic bubble and charge-transfer. These devices can be used for sequential access memory and feature solid-state reliability, small size and potential low costs.Magnetic bubble devices consist of a thin layer of uniaxial magnetic material imbedded in a bias magnetic field. The easy direction of magnetization is perpendicular to the surface of the thin layer and is in the same direction as the bias field. There is a range of bias fields for which regions of magnetization exist in the uniaxial material with a magnetic polarity opposite to that of the bias field. These regions of reverse magnetization have the form of right circular cylinders and are known as magnetic bubble domains or simply bubbles. The bubble is a stable entity that can be propagated by use of current carrying conductors on the surface of the thin magnetic layer or by the application of an in-plane rotating magnetic field which is used to magnetize small permalloy features on the surface of the magnetic layer (field access).Rare earth garnet magnetic materials are now available in very thin layers (2 to 5 μm) with the necessary anisotropy and magnetization to produce extremely small bubble domains (4 to 10 μm diameter), and with very few defects to impede domain motion (~2 cm-2). These materials should make possible storage densities of the order of several million bits per square inch. In this talk, the design of bubble circuits using single conductor current propagation is discussed with particular application to a multi-megabit bubble file.Two semiconductor devices are available which serve as dynamic-shift registers. One is of a functional nature and is called charge-coupled device (CCD). This device operates on the principle of charge transfer along the interface between a semiconductor and an insulating layer. The charges are moved by appropriately controlling the potential of metal electrodes placed on the surface of the insulator. The other semiconductor device is an integrated version of the IGFET bucket-brigade shift register. Recent developments in both of these technologies are discussed with application to large sequential access memories.