Flash Memory

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Flash Memory

Flash Memory is a semiconductor memory device that is electrically erasable and programmable in sections of memory called 'blocks'. In a flash memory, a whole block of memory cells can be erased in a single action, or in a 'flash,' which is how this device got its name. Flash memory is non-volatile, i.e., it can retain its memory contents even if it is powered off.

A basic flash memory cell consists of a MOSFET that was modified to include an isolated inner gate between its external gate and the silicon (see Figure 1). This inner gate is known as a 'floating gate', which is the data-storing element of the memory cell. Flash memory is not the first memory device to use a floating gate to store information. The uv-erasable EPROM, which preceded the Flash memory, is also a 'floating gate' memory device.

Figure 1. A Typical Flash Memory Cell

Data is stored in a flash memory cell in the form of electrical charge accumulated inside the floating gate. The amount of charge stored in the floating gate depends on the voltage applied to the external gate of the memory cell that controls the flow of charge into or out of the floating gate. The data contained in the cell depends on whether the voltage of the stored charge exceeds a specified threshold voltage Vth or not.

Intel has developed flash memory technology wherein memory cells can hold two or more bits of data instead of just one each. The trick is to take advantage of the analog nature of the charge stored in the memory cell and allow it to charge to several different voltage levels. Each voltage range to which the floating gate can charge can then be assigned its own digital code. Thus, a 2-bit cell can distinguish 4 distinct voltage ranges, while a 3-bit one can distinguish 8 of them. Intel calls this technology 'Multi-Level Cell (MLC)" technology.

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See Also: What is a Semiconductor?; EPROMs; SRAMs; DRAMs

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