What is Flash Memory?

- overview and tutorial about the basics of what is semiconductor Flash memory, how it works, its performance, variants, applications & types of flash memory.

Flash memory of a form of semiconductor memory is widely used for many electronics data storage applications.

Although first developed in the 1980s, the use of flash memory has grown rapidly in recent years as forms the basis of many memory products.

Flash memory can be seen in many forms today including flash memory USB memory sticks, digital camera memory cards in the form of compact flash or secure digital, SD memory. In addition to this flash memory storage is used in many other items from MP3 players to mobile phones, and in many other applications

There are also different flash memory types and these different types are each suited to their own applications.

What Is Flash memory?

Flash memory storage is a form of non-volatile memory that was born out of a combination of the traditional EPROM and E2PROM.

In essence it uses the same method of programming as the standard EPROM and the erasure method of the E2PROM.

One of the main advantages that flash memory has when compared to EPROM is its ability to be erased electrically. However it is not possible to erase each cell in a flash memory individually unless a large amount of additional circuitry is added into the chip. This would add significantly to the cost and accordingly most manufacturers dropped this approach in favour of a system whereby the whole chip, or a large part of it is block or flash erased - hence the name.

One of the major uses of Flash memory is within memory sticks. This image shows a typical memory stick.

Today most flash memory chips have selective erasure, allowing parts or sectors of the flash memory to be erased. However any erasure still means that a significant section of the chip has to be erased.

Flash memory history

Flash memory dates back to around 1980 when the concept was developed at Toshiba by Dr. Fujio Masuoka. It was then later presented at the 1984 IEEE International Electron Devices Meeting, IEDM held in San Francisco, California.

The basic laboratory concept took a few years to develop into a product that could be launched commercially. Intel introduced the first commercial chips onto the market in 1988 - these were NOR based types.

The NOR flash memories had relatively long erase and write times. Toshiba again developed their technology further and were able to announce NAND technology at the 1987 IEDM. This technology, though, still needed further development to take it through to commercial launch.

The advantage of NAND flash was that it had reduced erase and write times and it had a greater storage density.

Flash memory advantages & disadvantages

As with any technology there are various advantages and disadvantages. It is necessary to consider all of these when determining the optimum type of memory to be used.

Flash Memory
Flash Memory
  • Non-volatile memory
  • Easily portable (e.g. USB memory sticks)
  • Mechanically robust
  • Higher cost per bit than hard drives
  • Slower than other forms of memory
  • Limited number of write / erase cycles
  • Data must be erased before new data can be written
  • Data typically erased and written in blocks

Flash memory types

There are two basic types of Flash memory. Although they use the same basic technology, the way they are addressed for reading and writing is slightly different. They two flash memory types are:

  1. NAND Flash memory:   NAND Flash memories have a different structure to NOR memories. This type of flash memory is accessed much like block devices such as hard disks. When NAND Flash memories are to be read, the contents must first be paged into memory-mapped RAM. This makes the presence of a memory management unit essential.

  2. NOR Flash memory: NOR Flash memory is able to read individual flash memory cells, and as such it behaves like a traditional ROM in this mode. For the erase and write functions, commands are written to the first page of the mapped memory, as defined in "common flash interface" created by Intel.

NAND / NOR tradeoff:   NAND Flash memories and NOR Flash memories can be used for different applications. However some systems will use a combination of both types of Flash memory. The NOR memory type is used as ROM and the NAND memory is partitioned with a file system and used as a random access storage area.

Flash memory is a particularly important form of semiconductor memory. It is now widely used and is possibly one of the most important forms of medium term storage. As mentioned earlier Flash memory can be seen in a variety of forms and uses ranging from Flash memory USB sticks to Compact Flash cards used for cameras. In addition to this many other items of electronics can be seen using Flash memory ranging from mobile phones to MP3 players and many more. In view of its current importance, Flash memory will be seen in widespread use for many years to come.

By Ian Poole

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