Conductor-insulator-junction (cij) optical memory device and a memory system dependent thereon

Abstract

Abstract An optically activated (write and read) semiconductor memory employing a conductor-insulator-junction (CIJ) structure is disclosed. The layered memory structure comprises a front transparent conductive layer; insulator means containing a storage interface capable of trapping charges thereat, said insulator means being susceptible to tunneling between the storage interface and a back surface of the insulator means; semiconductor means having a front surface and a rectifying junction, said semiconductor means having an inversion stop grid means associated with the front surface thereof, and contact means for connecting external voltages to both sides of the rectifying junction for biasing the junction. The insulator means preferably comprises a relatively thick silicon nitride layer and a relatively thin silicon oxide layer, where the storage interface is the interface between the nitride and the oxide. The semiconductor means preferably comprises successive n-type and ptype regions with a pn junction therebetween. The inversion stop grid is preferably a grid of more heavily doped semiconductor of the same conductivity type as the front surface of the semiconductor. The grid divides the memory device into individual memory cells. The contact means preferably comprises an ohmic contact to the edge of the front semiconductor region and a back electrode making ohmic contact with the other semiconductor region. To write information into the memory device, a write voltage on the order of eighty volts is established between the transparent conductor and the front semiconductor region. This establishes individual inversion regions along the front surface of the semiconductor, the individual regions being isolated by the high conductivity grid. A light beam for which h 3/4 >Egap of the semiconductor is focused on the individual regions in which a charge (binary one) is to be stored. The light beam generates holeelectron pairs within the front semiconductor region. Because of the inversion region, the carriers which would be minority carriers in the first semiconductor region drift to the semiconductorinsulator interface. These charges cause a tunneling electric field to be established between the storage interface and the insulator back surface. This field induces a tunneling current which neutralizes part of the collected charge and establishes trapped storage charges at the storage interface. No significant tunneling occurs where the writing beam does not impinge. Information is read out by back biasing the pn-junction and establishing a voltage on the order of 3 volts between the transparent conductor and the front semiconductor region. The resulting electric field establishes individual inversion regions in the front semiconductor region behind those storage sites (memory cells) which do not have any significant charge stored at the storage interface. No inversion regions are established under the storage sites which store significant charge. A reading light beam having h 3/4 >Egap is scanned across the structure to read the stored data. The minority carriers generated by the light beam drift in opposite directions in accordance with the existence or nonexistence of an inversion region. Where there is an inversion region, the minority carriers diffuse to the semiconductor-insulator interface and there is a minimum pnjunction current and a minimum output current from the back electrode. Where there is no inversion region the minority carriers drift to the pn-junction and produce a maximum pnjunction current and a maximum output current from the back electrode. This system provides long term storage of data and nondestructive readout. Data is erased by applying an erase voltage of opposite polarity to the write voltage across the insulation means. The opposite polarity of the voltage causes electrons to collect at the semiconductor insulator interface and tunnel to the storage interface so that the charge stored at the storage interface is neutralized, thus erasing the stored data. This process produces block erasing. If it is desired to preserve some of the data within a memory block which is to be erased, then that data must be read out, stored, and rewritten after the memory block has been erased. In the memory system employing the CIJ device the writing beam is digitally controlled to write desired data into the memory structure. Light emitting diodes (LED''s) are the preferred light source, with Ga1-xAlxAs the preferred diode material because of its high energy output. The memory structure is preferably mounted on a rotatable disc so that a limited number of light sources can be used for writing into and reading out of a much greater number of storage cells. A 20 inch diameter disc, having memory units over 350* can store 9.24 X 1010 bits of data using the CIJ memory device.

Inventors

• 👤 R Gudmundsen

Bibliographic Details