Understanding Flash: What Is NAND Flash?

In the early 1980s, before we ever had such wondrous things as cell phones, tablets or digital cameras, a scientist named Dr Fujio Masuoka was working for Toshiba in Japan on the limitations of EPROM and EEPROM chips. An EPROM (Erasable Programmable Read Only Memory) is a type of memory chip that, unlike RAM for example, does not lose its data when the power supply is lost – in the technical jargon it is non-volatile. It does this by storing data in “cells” comprising of floating-gate transistors. I could start talking about Fowler-Nordheim tunnelling and hot-carrier injection at this point, but I’m going to stop here in case one of us loses the will to live. (But if you are the sort of person who wants to know more though, I can highly recommend this page accompanied by some strong coffee.)

Anyway, EPROMs could have data loaded into them (known as programming), but this data could also be erased through the use of ultra-violet light so that new data could be written. This cycle of programming and erasing is known as the program erase cycle (or PE Cycle) and is important because it can only happen a limited number of times per device… but that’s a topic for another post. However, while the reprogrammable nature of EPROMS was useful in laboratories, it was not a solution for packaging into consumer electronics – after all, including an ultra-violet light source into a device would make it cumbersome and commercially non-viable.

US Patent US4531203: Semiconductor memory device and method for manufacturing the same

A subsequent development, known as the EEPROM, could be erased through the application of an electric field, rather than through the use of light, which was clearly advantageous as this could now easily take place inside a packaged product. Unlike EPROMs, EEPROMs could also erase and program individual bytes rather than the entire chip. However, the EEPROMs came with a disadvantage too: every cell required at least two transistors instead of the single transistor required in an EPROM. In other words, they stored less data: they had lower density.

The Arrival of Flash

So EPROMs had better density while EEPROMs had the ability to electrically reprogram cells. What if a new method could be found to incorporate both benefits without their associated weaknesses? Dr Masuoka’s idea, submitted as US patent 4612212 in 1981 and granted four years later, did exactly that. It used only one transistor per cell (increasing density, i.e. the amount of data it could store) and still allowed for electrical reprogramming.

If you made it this far, here’s the important bit. The new design achieved this goal by only allowing multiple cells to be erased and programmed instead of individual cells. This not only gives the density benefits of EPROM and the electrically-reprogrammable benefits of EEPROM, it also results in faster access times: it takes less time to issue a single command for programming or erasing a large number of cells than it does to issue one per cell.

However, the number of cells that are affected by a single erase operation is different – and much larger – than the number of cells affected by a single program operation. And it is this fact that, above all else, that results in the behaviour we see from devices built on flash memory. In the next post we will look at exactly what happens when program and erase operations take place, before moving on to look at the types of flash available (SLC, MLC etc) and their behaviour.

NAND and NOR

To try and keep this post manageable I’ve chosen to completely bypass the whole topic of NOR flash and just tell you that from this moment on we are talking about NAND flash, which is what you will find in SSDs, flash cards and arrays. It’s a cop out, I know – but if you really want to understand the difference then other people can describe it better than me.

In the meantime, we all have our good friend Dr Masuoka to thank for the flash memory that allows us to carry around the phones and tablets in our pockets and the SD cards in our digital cameras. Incidentally, popular legend has it that the name “flash” came from one of Dr Masuoka’s colleagues because the process of erasing data reminded him of the flash of a camera. Presumably it was an analogue camera because digital cameras only became popular in the 1990s after the commoditisation of a new, solid-state storage technology called …