Flash-erase-type nonvolatile semiconductor storage device

DETAILED DESCRIPTION An object of the present invention is to provide an efficient redundant architecture for a flash-erase-type nonvolatile semiconductor storage device, and a method capable of improving the efficiency of the entire device.
Another object of the invention is to provide an efficient method of distinguishing defective sectors and avoiding the use of those sectors in an external storage system employing a flash-erase-type nonvolatile semiconductor storage device.
The above-described objects can be attained by taking advantage of the fact that in a semiconductor external storage system the inside of a sector is accessed serially.
That is, memory cells on a word line of a flash-erase-type nonvolatile semiconductor storage device are associated with a sector of a semiconductor external storage system, and all the memory cells on that word line are activated every time the sector is accessed.
The term "sector" includes the areas for storing system data such as ECCs.
In addition to the data area, which is necessary for allowing the semiconductor external storage system to store user data, a tag area including areas for storing defective memory cell information and ECCs is connected to this word line.
The data area and the corresponding tag area are activated at the same time, because they share the same word line.
But they can be erased separately because they are formed in separate wells.
That is, even when a data area is erased, the corresponding tag area is retained and the defective memory cell information and other data are not destroyed.
Information in the tag area is written at the stage of the chip test that is performed after manufacture.
The system refers to this information at every access to the sector, thereby skipping defective memory cells.
When the system detects an unrecoverable error, it writes predetermined data to the tag area to avoid subsequent access to that sector.
According to specific features of the invention, a flash-erase-type nonvolatile semiconductor storage device is constituted so that a memory cell array is physically divided into a data area and a tag area with shared word lines, and so that those two areas can separately be erased en bloc