NAND flash memory and SSDs keep data even when the power is off. NAND flash memory is the kind of memory chip, and an SSD, or solid-state drive, is a drive built from those chips. Flash holds the photos, apps and files in phones, memory cards and pen drives, and inside SSDs in laptops and servers. It is the "128 GB storage" printed on a phone's box. Flash is slower than DRAM, the working memory, but far faster than a hard disk drive, because it has no moving parts.
Each flash cell is a transistor, a microscopic electrical switch, with an extra layer that can trap electrons. Electrons trapped in that layer stay there for years without power. The number of trapped electrons sets the cell to one of several levels. Most cells today store 3 bits by telling 8 levels apart, and some store 4 bits with 16 levels. To fit more cells on a chip, makers stack them in more than 200 layers, one above the other. This is called 3D NAND. The name NAND comes from the way the cells are wired together in a chain, which resembles a NAND circuit in digital logic.
Flash has three limits. It is written in small pages, about 16 KB each, but it can be erased only in much larger blocks. Each erase wears the cell a little, and a cell storing 3 bits lasts about 1,000 to 3,000 erases. And cells make occasional errors, more often as they wear. So every SSD and phone storage chip has a controller, a small processor that spreads writing evenly and fixes errors. The page on the SSD controller explains how. The controller runs firmware, a program stored on the device itself.
Take, for example, a wedding photographer in Madurai. Over one wedding day, she takes 2,000 photos of about 25 MB each, which is 50 GB in all, on two 64 GB memory cards. In the evening, she copies the photos to the 1 TB SSD in her laptop. The memory card reads at about 300 MB per second, so the copy takes about three minutes. The laptop's SSD can read at a few gigabytes per second. So when she opens the photos in her editing software, each one appears almost at once. Her older weddings are kept on an external hard drive, which is cheaper for each terabyte but takes longer to open each file.
For comparison, a phone's storage chip reads at up to about 4 GB per second, a laptop's SSD at 3 to 7 GB per second, and a hard drive at about 0.25 GB per second.
Only a handful of companies make flash chips. As with other memory, prices rise and fall sharply with supply and demand, which is explained on memory chip cycle. In data centres, enterprise storage systems built entirely of flash now hold the data that is read most often. Laptops and desktops sold to Indian offices have used SSDs instead of hard drives for years. The controllers and firmware for some of the world's flash are designed in Bengaluru.