The non-volatile semiconductor devices are categorized into the NAND (Not-AND) type for high degree of device integration and the NOR (Not-OR) type for high speed according to memory cell array structures. The NAND flash memory device has a structure of strings, in which a plurality of memory cells is connected from a single bitline in series. A memory cell of a NAND flash memory has a MOSFET structure in which a floating gate and control gate (word line) are stacked, via an insulating film, on a semiconductor substrate serving as a channel region. A NAND cell is formed by serially connecting a plurality of memory cells while making adjacent memory cells share the source/drain. The source/drain means an impurity region having at least one of the functions of the source and the drain. A NAND flash memory device is comprised of memory cells serially connected between a drain selection transistor and a source selection transistor in the unit of 16 or 32 in number. In the NOR-type memory the individual EEPROM transistors are connected in parallel. The NOR flash memory device is constructed of a plurality of memory cells connected to a single bitline in parallel. The NOR type nonvolatile memory device programs data of eight memory cells at a time in byte units, namely, in parallel due to current restrictions, and thus is disadvantageous in program speed. NOR-type flash memory devices typically use a channel hot electron (CHE) method of programming. In the NOR array architecture, the floating gate memory cells of the memory array are arranged in a matrix. The gates of each floating gate memory cell of the array matrix are coupled by rows to word select lines (word lines) and their drains are coupled to column bit lines. The source of each floating gate memory cell is typically coupled to a common source line. The NOR architecture floating gate memory array is accessed by a row decoder activating a row of floating gate memory cells by selecting the word line coupled to their gates. The row of selected memory cells then place their stored data values on the column bit lines by flowing a differing current if in a programmed state or not programmed state from the coupled source line to the coupled column bit lines. Arrays of NOR memory cells are faster for read/write (program/erase) than the conventional NAND memory cells. This array of NAND memory cells has slower read times than the NOR-type memory, but the NAND circuit consumes much less power and has much higher cell density than the NOR-type memory. NOR cells require higher power but are much larger and require greater area per unit memory cell on the substrate.  Since the NAND type flash memory device requires a small operation current in the data program operation, the NAND type flash memory device is easily provided with the current from a boosting circuit in the chip and easily operated with a single current. The NAND array is higher in its integration comparing to NOR array, because cells in an NAND memory are in series connection. The NAND-type flash memory which attains high integration and has a replaceable hand disk is mostly used to store high integration voices or images. Accordingly, NAND structures are mainly used in high-integrated flash memory devices. NOR-flash memory is preferred for programming applications, but are not desirable for mass storage of data. NAND type flash memory device can perform the data program operation on memory cells connected to a selected word line in page units. NOR memory is more desirable for storing CPU and application programs. NAND memory is preferred for mass storage of data, in which the data requires frequent updates.

A flash memory is a type of EEPROM that can be erased and reprogrammed in blocks instead of one byte at a time. Flash memory is widely used as a type of non-volatile semiconductor memory. Flash memory is a semiconductor memory comprising a cell transistor having a floating gate or trap gate, and the memory core generally comprises a plurality of sectors. The memory cells in a flash memory are organized into small units such as bytes or words, and larger units such as pages or sectors. A typical flash memory comprises a floating gate and a control gate. The floating gate and the control gate are separated by a dielectric layer. Each of the memory cells includes a floating gate field-effect transistor capable of holding a charge. The data in a cell is determined by the presence or absence of the charge in the floating gate. The floating gate and the substrate are separated by a tunnel oxide layer. A layer of high-quality tunnel oxide used as gate oxide separates the transistor channel and the floating gate, and an oxide-nitride-oxide (ONO) dielectric stack separates the floating gate from the control gate. In a flash memory, a nonvolatile memory element having a control gate and a floating gate is used as a memory cell, and the memory cell is constituted by a single transistor. Flash memory cells are often fabricated on the same substrate with logic or linear transistors. Memory cell transistors constituting a flash memory each have a floating gate, a control gate, a source and a drain. A flash memory comprises a memory-cell array having an array of memory cells arranged in rows and columns and grouped into blocks. Memory cells in a flash memory device are typically connected in an array of rows and columns, with the control gates of the cells in a row being connected to a respective word line and the drains of the cells in a column being connected to a respective bit line. The flash memory device performs a write operation to store data into the memory cells. Each of the memory cells includes a floating gate field-effect transistor capable of holding a charge. Each of the cells within a block can be electrically programmed in a random basis by charging the floating gate. Each block of memory cells can store data used during system operations. Flash memory devices can be categorized structurally as p-channel memory devices and n-channel memory devices. Generally, a flash memory device includes a source connection layer that connects the sources of unit cells to form a source line. In general, flash memory devices perform a read operation, a programming operation and an erase operation. The programming operation of the flash memory devices is performed through hot electron injection. The electrons are injected into the floating gate by a hot electron injection method (HEI) and are erased by a Fowler-Nordheim (F-N) tunneling method using an inter-gate insulating layer between the floating gate and the control gate. The erase operation of the flash memory devices is performed through Fowler-Nordheim (FN) tunneling that occurs between a source electrode of a memory cell and a floating gate of the memory cell. Flash memory devices are generally operated by first setting all bits in a block to a common state, and then reprogramming them to a desired new state. A read operation can be used to retrieve data previously written to memory cells selected by a read address applied to the row/column decoders during a read operation in the flash memory device. A cell is typically read by applying a voltage to the word line to which the control gate of the cell is connected, applying a voltage to the bit line to which the drain of the cell is connected, grounding the source, and sensing the bit line current. Reading the state of the flash cell is performed by applying an intermediate voltage such as gate voltage Vg onto the control gate. When the cell is erased, the cell transistor draws current from the bit line to the grounded source, and the drop in bit-line voltage can be sensed. The flash memory devices may achieve a non-volatility by adding a floating gate between a control gate of each memory transistor and a substrate region. Generally, a flash memory cell is implemented using a shallow trench isolation (STI) process as a device isolation process. The erasing process is carried out on one or more pages or sectors at a time, typically by removing electrons from the floating gates of the memory cells; the writing or programming process is carried out one byte or one word at a time, typically by injecting electrons into the floating gates. A cell can be erased by applying a voltage to the P-well (substrate) and a negative voltage to the control gate, while allowing the source/drain to float, by applying a relatively high voltage to the source, grounding the control gate and allowing the drain to float. In yet another arrangement, or by applying a negative voltage to the control gate, applying a voltage to the source and allowing the drain to float.

Compared with other memory devices, flash memory devices have many advantages which include their non-volatility, speed, ease of erasure and reprogramming, small physical size and related factors. A flash memory is a semiconductor memory showing low power consumption and maintaining stored information even when power is off. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Flash memories can program, read and erase data for multiple times, and data stored therein can be maintained even if the power applied thereto is off. Flash memory devices do not require refreshing and can store data indefinitely after the power is removed. Flash memory devices are applicable for multiple operations of data writing, reading and erasing. Flash memory devices have achieved a commercial success in an electronic industry because they are able to store data for a relatively long time even without a power supply. Flash memory has been widely used for high volume data storage in devices such as personal computers, personal digital assistants (PDA's), cellular phones, digital TV, digital camcorder, digital camera, cellular phone, PDA, game machine, MP3 player, and MP3 players. Small flash-memory cards have been designed that have a connector that can plug into a specialized reader, such as for compact-flash, secure-digital, memory stick, or other standardized formats. Flash memory cards are formed in card shapes in which memory chips are sealed in card type outer shells. Flash memory cards are designed in many standards such as an SD (Secure-Digital) card, a multimedia card, a Smartmedia, a compact flash (CF) card, and a memory stick. SD card is an extension of the earlier MultiMediaCard (MMC) format. SD cards are hot-swappable, allowing the user to easily insert and remove SD cards without rebooting or cycling power. Information equipment such as personal computers, personal digital assistants (PDAs), digital cameras, and cellular phones are generally provided with a card slot for mounting a flash memory card. More recently, flash memory cards are being designed that contain a USB connector which is constructed that the flash memory card is insertable and extractable in a direction parallel to a mounting wiring board through the slot port. USB flash-memory drives and devices have been developed to transport data from one host to another, replacing floppy disks. A USB-flash card can have a capacity of more than ten floppy disks in an area not much larger than a large postage stamp.