Introduction to Flash Memory - NAND vs. NOR

Flash memory is the type of non-volatile memory that can be electrically erased and reprogrammed. The concept of flash memory came in the year 1967. Because of its small size, erase capability of a large block at a time and reliability, it is the today one of the most important component of portable electronic gadgets. Prime examples are Digital Camera, USB Pendrive etc. These days some of the popular embedded system development boards like Raspberry Pi, Intel Galileo also support flash memory. Flash memory is a kind of EEPROM only, but there is an important differentiation that needs to be understood. EEPROM is very small in size (around few hundred of Kbytes) and is expensive to manufacture. The architecture is little complex as it allows user to write to any byte of memory. On the other hand flash allows user to erase/rewrite data in blocks so it has relative simple structure and therefore is inexpensive. EEPROM finds application where data needs to be re-written quite frequently and it supports more number of write cycles when compared to Flash.

Flash memory is made up of an array of memory cells that are nothing but floating gate metal oxide semiconductor field effect transistor (floating gate MOSFET or FGMOS for short). Each memory cell stores one BIT of information. Based on the memory cells (or FGMOS) configuration we classify flash memories in two categories - NOR Flash & NAND Flash. In NAND flash, memory cells are connected in series with one extreme memory cell is connected to BIT line and other extreme is connected to the ground. The reason it is called NAND flash is because it operates like a NAND gate. It means the BIT line is pulled low or 0 only when gate voltage of all memory cells is pulled high or 1 (above threshold voltage). Figure 1 show the schematic & silicon structure of NAND Flash.

Figure 1 - NAND Flash

In NOR flash each memory cell has ground connection on one end and BIT line connection on the other end. This memory works like a NOR gate. Figure 2 shows the schematic & silicon structure of NOR Flash.

Figure 2 - NOR Flash

There is a series connection of memory cells to BIT line in NAND flash technology so it takes less space as compared to NOR flash. However, parallel connection of memory cells in NOR flash helps to attain random access. NAND flash technology is used for making high storage data devices due to its series connection and NOR flash is preferred for code-storage applications. NOR flash gives great read performance and even allows program to run directly from the flash instead of RAM. But on the other hand NAND flash offers high storage capacity, extremely good write/erase performance and is available at lower cost. NAND flash was introduced by Toshiba in 1989 just one year after the introduction of NOR flash by Intel.

By: Ms. Shiveta Bhat, ME Scholar (ECE), Chitkara University

References

1. https://en.wikipedia.org/wiki/Flash_memory#NAND_flash
2. J. G. Lisoni, A. Arreghini, G. Congedo, M. Toledano-Luque, I. Toqué-Tresonne, K. Huet, E. Capogreco, L. Liu,C.-L. Tan, R. Degraeve, SG. Van den bosch, J. Van Houdt "Laser Thermal Anneal of polysilicon channel to boost 3D memory performance" 2014 Symposium on VLSI Technology Digest of Technical Papers.pp. 1-2