Flash Memory
Flash memory is a form of EEPROM
(Electrically Erasable Programmable Read-Only Memory) that allows multiple
memory locations to be erased or written in one programming operation. In
layman's terms, it is a form of rewritable memory chip that, unlike a
Random Access Memory chip, holds its content without the need of a power
supply. It is also an example of a Non-Volatile Read Write Memory (NVRWM).
The memory is commonly used in memory cards, USB flash drives, MP3 players,
digital cameras and mobile phones.
Flash memory is non-volatile, which
means that it does not need power to maintain the information stored in the
chip. In addition, flash memory offers fast read access times (though not
as fast as volatile DRAM memory used for main memory in PCs) and better
shock resistance than hard disks. These characteristics explain the
popularity of flash memory for applications such as storage on
battery-powered devices.
Flash memory stores information in an
array of floating gate transistors, called "cells", each of which
traditionally stores one bit of information. Newer flash memory devices,
sometimes referred to as multi-level cell devices, can store more than 1
bit per cell, by varying the number of electrons placed on the floating
gate of a cell.
In
NOR flash, each cell looks similar to a standard MOSFET, except that it has
two gates instead of just one. One gate is the control gate (CG) like in
other MOS transistors, but the second is a floating gate (FG) that is
insulated all around by an oxide layer. The FG is between the CG and the
substrate. Because the FG is isolated by its insulating oxide layer, any
electrons placed on it get trapped there and thus store the information.
When electrons are on the FG, they modify (partially cancel out) the
electric field coming from the CG, which modifies the threshold voltage (Vt) of the cell. Thus, when the cell is
"read" by placing a specific voltage on the CG, electrical
current will either flow or not flow, depending on the Vt
of the cell, which is controlled by the number of electrons on the FG. This
presence or absence of current is sensed and translated into 1's and 0's,
reproducing the stored data. In a multi-level cell device, which stores
more than 1 bit of information per cell, the amount of current flow will be
sensed, rather than simply detecting presence or absence of current, in
order to determine the number of electrons stored on the FG.
A NOR flash cell is programmed (set to
a specified data value) by starting up electrons flowing from the source to
the drain, then a large voltage placed on the CG provides a strong enough
electric field to suck them up onto the FG, a process called hot-electron
injection. To erase (reset to all 1's, in preparation for reprogramming) a
NOR flash cell, a large voltage differential is placed between the CG and
source, which pulls the electrons off through quantum tunnelling. In
single-voltage devices (virtually all chips available today), this high
voltage is generated by an on-chip charge pump. Most modern NOR flash
memory components are divided into erase segments, usually called either
blocks or sectors. All of the memory cells in a block must be erased at the
same time. NOR programming, however, can generally be performed one byte or
word at a time.
NAND Flash uses tunnel injection for
writing and tunnel release for erasing. NAND flash memory forms the core of
the removable USB interface storage devices known as key drives.
|
