Everyone talks about flash, but not too many users actually know how it works. Flash is a transistor-based silicon memory technology that can store information permanently by trapping electrons into so-called floating-gate transistors. Depending on the threshold voltage of the flash cell, the transistor will either remain insulated or become conductive. SLC flash (single-level cell) has only one voltage level, while MLC (multi-level cell) flash memory is capable of storing multiple bits per cell. Flash transistors become worn out with use and typically offer anywhere between 10,000 and a few million write cycles. Many flash products incorporate wear-leveling algorithms, which ensure all cells are worn evenly to maximize the lifespan of the product. The downside can be somewhat erratic performance.
There are two flash memory technologies: NOR and NAND, where NOR represents a "joint denial." This means that high (1) output only results if the two inputs, called the control gate and the floating gate, are low (0). If either one or both are high (1), no current will flow. With SLC flash, the stored data is reconstructed by detecting whether or not a current flows through the transistor. In case of MLC, the amount of current is used to determine the precise charge level of the floating gate. NOR flash can be fully addressed via an external bus for read operations, but shows rather slow write and erase times, because writing and erasing have to be performed block-wise. Also, NOR does not have any bad block management - this has to be taken care of by the host system. NOR is the ideal flash technology for non-volatile, long-term storage such as for firmware or BIOS applications.
Unlike NOR, NAND allows for a current flow if one of the inputs are high (1). NAND flash cannot be addressed cell by cell, but has to be read or written very much like hard drives; erasure only works block by block. Hence, a controller is required to access NAND flash properly, which typically also takes care of bad block management. NAND is used for memory cards and consumer devices and thanks to the controller, which is required in any case, manufacturers can easily optimize their production output for consumer products by marking existing bad blocks and by designating a large number of spare blocks. As a result, a 4 GB flash memory device will typically have at least several hundred megabytes of spare memory to equate bad blocks that may turn up over time. NAND's operation is thus more efficient, as increased data densities allow for better utilization of the production output at existing capacity points. This is because more cells can be used as spares. Even if a device has bad blocks, the user will never know.
Flash-based memory and storage products can be found in almost all market segments. While flash memory was first used to store firmware or BIOS information, it is increasingly used for such applications as flexible storage (think of portable USB flash sticks), temporary storage in such devices as a USB flash memory unit to boost the main memory of Windows Vista (ReadyBoost) and even for permanent storage. On the one hand, flash memory is starting to become physically integrated with hard drives, turning them into so-called Hybrid Hard Drives (H-HDDs). Hybrids are conventional hard drives with rotating platters, but they also have between 128 MB and a few gigabytes of flash memory to enable the operating system to utilize it both as permanent storage and as cache memory. This allows the drive's spindle motor to come to a halt. Finally, flash is making inroads in the traditional hard drive space as well. For example, 1.8" and 2.5" flash-only hard drives have been around for several months, but low capacities or high costs have prevented it from penetrating anything except for the very high-end market (Bitmicro has a significant share in this sector). As the dollar-per-bit ratio continues to decrease, the future for flash SSDs is bright, indeed. Almost all memory vendors are getting ready to offer devices with 6- and 32 GB capacities, and it is only a matter of time until 64 GB and 128 GB devices become affordable. However, expect to pay roughly $400 for a 32 GB flash SSD.