1. Field of Invention
Various embodiments of the invention relate to a semiconductor memory device.
2. Description of Related Art
Semiconductor memory devices may be classified into volatile memory devices and non-volatile memory devices.
Volatile memory devices such as Static Random Access memory (SRAM), Dynamic RAM (DRAM) and Synchronous DRAM (SDRAM) do not retain data stored therein when power supply to the device is turned off. Non-volatile memory devices operate at relatively lower write and read speeds than volatile memory devices, but they retain the stored data regardless of power on/off conditions. Therefore, non-volatile memory devices are used to store data which need to be maintained even in the absence of power supply. Examples of non-volatile memory include read only memory (ROM), mask ROM (MROM), programmable ROM (PROM), erasable programmable ROM (EPROM), electrically erasable and programmable ROM (EEPROM), flash memory, Phase-change random access memory (PRAM), magnetic RAM (MRAM), resistive RAM (RRAM) and ferroelectric RAM (FRAM). Flash memories are used widely and may be classified into NOR- or NAND-type memories.
Generally, flash memories enjoy the advantages of both RAM and ROM devices. For example, flash memories may be freely programmed and erased similar to a RAM. Also, similar to a ROM, flash memories may retain the stored data even when they are not powered. Flash memories have been widely used as the storage media of portable electronic devices such as mobile phones, digital cameras, personal digital assistants (PDAs), and MP3 players.