Electrical – Why NAND FLASH is slower than flip-flops

flash-memoriesflipflopnand-flash

I find that both NAND FLASH and flip-flops are made up of NAND gates. The only difference between them is NAND FLASH uses floating gate transistors to store charge.

Your Flash drive is actually made up of NAND FLASH and cache memory which has speed equivalent to CPU is made up of flip-flops.

My question is if both operates at 5V and both are made up of basic cell unit NAND gates then what makes flip-flops so faster than Nand-flash? Or What makes Nand-flash slower than flip-flops (probably due to floating gate transistors?) ?

In other words what makes cache memory so faster than a flash drive? The drift velocity of charge in both the architecture must be same as 5V is used for them.

Best Answer

First: Be careful not mixing up NAND gates (which are logical circuits) and the gate contact of a MOS transistor. These are two completely different things.

The idea having NAND gates makes the internal circuit identical is wrong. Flipflops inside a chip are usually not made from gates but simplified to save space. See e.g. an SRAM cell made from CMOS pass transistor logic.

But indeed, the reason why writing to Flash memory is substantially slower is because of the large amount of charge which has to be put through an isolator (SiO2, so the drift velocity is much lower) onto the floating gate. That takes time, while the transistors which have a normal gate contact can have that one fully charged and de-charged within picoseconds.