system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can flash memory retain its data without power? Is it possible for it to lose data over extended period of time?
gpt
Well, the thing about normal memory like the RAM in your computer is that information is stored based on whether electricity is flowing through the transistors within the memory chip. When electricity stops flowing through the transistors, they all reset to a default state and whatever was stored beforehand is lost.

Flash memory on the other hand is a type of electronically erasable programmable read only memory (EEPROM). It's sort of like a combination of RAM and ROM due to its ability to retain information without electricity as well as having the information stored on it be erasable and rewritten. Flash memory utilizes a "dam" system that is able to store residual electrons from electrical current regardless of whether there is electricity running through it or not.

There are 4 main parts to a flash transistor: a source, a drain, two gates called the floating gate and control gate, and an insulating material dividing the other 3 parts. They are in a o8o/inverted T shape, with the lower half being the source and the drain and the upper extension being the gates, with the control gate above the floating gate. The gates are surrounded by oxide layers through which current normally cannot pass through.

Negatively charged electrons are present at the source (the originating point of the electrical current) and the drain (the draining point of the electrical current) due to the type of silicon used to make these parts. However, they cannot flow naturally because of the electron deficient insulating material between the source and the drain. When you apply a positive voltage to the electrical contacts at the drain and the gates (the top and one side of the inverted T), the negatively charged electrons are attracted to the positive charge and get pulled to the drain. Some electrons also get pulled up into the floating gate through the oxide layers through a process known as quantum tunneling. Electrons that have gone into the floating gate remain there. These electrons are unable to escape the oxide layers, and will remain in the floating gate indefinitely. Flash transistors with stored electrons are akin to RAM transistors with current flowing through them. To remove the electrons, a negative voltage is applied to the electrical contact above the floating gate, which causes electrons to be repelled away from the floating gate downwards into the drain, through the same quantum tunneling process. This lack of electrons is then similar to a RAM transistor without current applied.

Flash memory has a certain number of program/erase cycles before it begins to lose its integrity. Degradation mostly occurs within the oxide layers that separate the gates. When the oxide begins to break down with repeated usage, it no longer is able to consistently prevent the flow of electrons in and out of the floating gate. Since the transistor relies on the electrons being unable to escape to store information, this causes the flash memory to become unreliable. It may still work, but may not work consistently or may lose information as a result.