NOR is a non-volatile flash memory that met Masuoka's goal, albeit only partially. His research continued and brought an even more significant discovery in the form of NAND flash memory in 1987. You can read more about NAND here. NOR is a memory type based on so-called floating-gate transistors, which retain their value without power thanks to an insulating layer. However, the architecture of NOR memory predetermines this type of memory for use in environments where smaller data volumes are primarily intended for reading. Writing and erasing are more challenging and slower with NOR. Since its introduction to the market, NOR has been used for BIOS storage in personal computers, ROMs in hard drives, various embedded systems, and mobile phones in the past. The first NOR memory introduced to the market had a capacity of 32 KB (256 kilobits). Given the primary use of NOR, capacities are still relatively low today, ranging from 64 kB to 32 MB.
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NOR flash memory architecture:
NOR flash memory consists of memory cells arranged in a matrix, with each memory cell containing one floating-gate transistor. The architecture of NOR flash memory allows direct access to each memory cell due to the parallel connection of rows (wordlines) and columns (bitlines). This architecture enables fast data reading access but slower writing and erasing.
Data Reading:
NOR flash memory reads data by directly accessing each memory cell. During reading, voltage is applied to the row (wordline) and column (bitline) parts, opening the transistor and allowing current flow. The presence or absence of charge in the floating gate of the transistor determines the bit value (1 or 0).
Data Writing:
The process of writing data to NOR flash memory involves injecting electrons into the floating gate of the transistor. This process is called Fowler-Nordheim tunneling and requires higher voltage than data reading. High voltage is applied to the row part (wordline), while the column part is grounded. Electrons overcome the insulating layer between the floating gate and the substrate and are stored in the floating gate, changing the bit value.
Data Erasing:
Erasing data from NOR flash memory requires removing electrons from the floating gate of the transistor. This is done using Fowler-Nordheim tunneling in the opposite direction. For erasing, high voltage is applied to the column part (bitline) and the row part (wordline) is grounded. Electrons leave the floating gate and return to the substrate, restoring the original bit value. NOR flash memory erasing occurs in blocks, meaning that an entire memory block is erased at once, even if only a few bits need to be removed. This erasing process is slower than with NAND flash memory.
