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.

A typical MLC consists of a single transistor with direct electrical connections to its gate, source, and drain that allow very precise control of the charging of the cell's floating gate. For a multi-level cell to work, it must be able to deposit charge with precision, sense charge with precision, and store charge over time. High-precision charging and charge sensing are the key to a MLC's ability to distinguish several charge levels. Table 1 illustrates how a 2-bit multi-level cell assigns digital codes to 4 different charge voltage levels.

Table 1. 2-Bit Intel MLC Digital Code Assignment

Charge Level	Digital Code
Level 3	00
Level 2	01
Level 1	10
Level 0	11

MLC programming is accomplished by charging the floating gate through a precise process of Channel Hot-Electron (CHE) injection. During programming, the source of the MLC transistor is usually grounded. Column decoding of the MLC provides direct bitline connection to the drain which is pulsed at a constant voltage. Row decoding of the MLC, on the other hand, provides direct wordline connection that causes the MLC transistor gate to be connected to an internally generated supply voltage. This direct and precise control of the drain and gate is critical to the correct charging of the floating gate and, hence, correct storage of information.

Reading the contents of multi-level cells involves highly precise sensing of the amount of charge in the floating gate, measured in terms of cell currents that have an inverse relationship with the Vth. The sensed currents are compared to reference currents, with the comparison results inputted to a logic circuit that encodes them into the corresponding digital data.

Flash memory erasure is achieved by 'discharging' the floating gate through a phenomenon known as Fowler-Nordheim tunneling, wherein electrons from the floating gate pass through the thin dielectric layer and get dissipated at the source of the memory cell transistor.

Flash memory is used in a variety of applications such as: personal and notebook computers, digital cell phones, digital cameras, portable memory devices, LAN switches, embedded controllers, etc.

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

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