RAM storage cells are tiny, fast, and don’t degrade over time. Each cell can store an electrical charge that distinguishes a 0 from a 1. However they forget their contents whenever they’re being read or if they’re not topped up occasionally. RAM needs to be refreshed by writing back contents after a cell is read, and making sure that periodically every cell in the chip is read and written back. Without constant power to refresh, RAM forgets its contents.
Flash is similar to RAM because it also stores data as a charge. To make the contents last without power and survive being read, each cell is tied to the gate of a MOSFET transistor. The MOSFET is like an amplifier in that it takes the small charge at the input and controls a larger signal without depleting the input charge. Think of the cell as a light switch: the switch stays in the same position no matter how much electricity passes through. The part that stores the charge and makes the flash MOSFET different from a normal MOSFET used in other electronics is called the floating gate. This is made of layers just a few atoms thick of insulators and conductors.
The catch is in programming and erasing the cell. To make the flash cells last, you need to eliminate any possible conductive path in or out of the charge storing part of the cell. Programming a flash cell to a 0 (flash is a “1” in it’s erased state, the N in NOR and NAND flash stands for “not”, i.e. negative logic) requires pushing a high voltage onto the MOSFET, causing it to conduct just enough to push some electrons into the MOSFET’s floating gate. Erasing requires an even higher voltage applied in a different way to drain the charge out. Both processes take advantage of normally undesirable features of MOSFETs called breakdown, where a high enough voltage causes it to conduct in ways it wouldn’t in normal operation.
Those high voltages, particularly the erase voltage, cause permanent wear on the floating gate and MOSFET, causing the charge to leak faster than normal. Even flash that’s not written or erased often isn’t perfect and a programmed cell will degrade over many years from a 0 to a 1. There is a whole science to counteracting flash wear and inevitable errors that I won’t go into here. The SSD controller chip is responsible for managing wear and data integrity, which is why you sometimes hear about SSDs that could lose your data if you don’t have a bugfixed firmware.
There are other technologies that are available that could replace flash. MRAM stores magnetically and is immune to wear from writing, not unlike core memory used in computers from the '50s through the '70s. FeRAM is similar to MRAM but less dense. You already have FeRAM in your car’s dash to store the constantly updating mileage, as it’s immune from damage by that constantly updating number or automotive temperature extremes. Phase change memory stores by heating tiny chunks of a crystalline material to get it to change it’s structure. Intel used this in its now-defunct 3D Xpoint memory. Memristor is another that takes advantage of a relatively new kind of electronic component type.
All of these new technologies are better than flash in longevity and many are close to RAM in performance. However none yet can be made as dense as RAM or as cheaply as flash. Memristor and MRAM are both frontrunners for replacing both RAM and flash, but it’s only fairly recently that fabs started offering the processing steps needed to make these in high density devices.
RAM storage cells are tiny, fast, and don’t degrade over time. Each cell can store an electrical charge that distinguishes a 0 from a 1. However they forget their contents whenever they’re being read or if they’re not topped up occasionally. RAM needs to be refreshed by writing back contents after a cell is read, and making sure that periodically every cell in the chip is read and written back. Without constant power to refresh, RAM forgets its contents.
Flash is similar to RAM because it also stores data as a charge. To make the contents last without power and survive being read, each cell is tied to the gate of a MOSFET transistor. The MOSFET is like an amplifier in that it takes the small charge at the input and controls a larger signal without depleting the input charge. Think of the cell as a light switch: the switch stays in the same position no matter how much electricity passes through. The part that stores the charge and makes the flash MOSFET different from a normal MOSFET used in other electronics is called the floating gate. This is made of layers just a few atoms thick of insulators and conductors.
The catch is in programming and erasing the cell. To make the flash cells last, you need to eliminate any possible conductive path in or out of the charge storing part of the cell. Programming a flash cell to a 0 (flash is a “1” in it’s erased state, the N in NOR and NAND flash stands for “not”, i.e. negative logic) requires pushing a high voltage onto the MOSFET, causing it to conduct just enough to push some electrons into the MOSFET’s floating gate. Erasing requires an even higher voltage applied in a different way to drain the charge out. Both processes take advantage of normally undesirable features of MOSFETs called breakdown, where a high enough voltage causes it to conduct in ways it wouldn’t in normal operation.
Those high voltages, particularly the erase voltage, cause permanent wear on the floating gate and MOSFET, causing the charge to leak faster than normal. Even flash that’s not written or erased often isn’t perfect and a programmed cell will degrade over many years from a 0 to a 1. There is a whole science to counteracting flash wear and inevitable errors that I won’t go into here. The SSD controller chip is responsible for managing wear and data integrity, which is why you sometimes hear about SSDs that could lose your data if you don’t have a bugfixed firmware.
There are other technologies that are available that could replace flash. MRAM stores magnetically and is immune to wear from writing, not unlike core memory used in computers from the '50s through the '70s. FeRAM is similar to MRAM but less dense. You already have FeRAM in your car’s dash to store the constantly updating mileage, as it’s immune from damage by that constantly updating number or automotive temperature extremes. Phase change memory stores by heating tiny chunks of a crystalline material to get it to change it’s structure. Intel used this in its now-defunct 3D Xpoint memory. Memristor is another that takes advantage of a relatively new kind of electronic component type.
All of these new technologies are better than flash in longevity and many are close to RAM in performance. However none yet can be made as dense as RAM or as cheaply as flash. Memristor and MRAM are both frontrunners for replacing both RAM and flash, but it’s only fairly recently that fabs started offering the processing steps needed to make these in high density devices.