1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device which allows reusing and a method of reusing same.
2. Description of the Related Art
There is a known nonvolatile semiconductor memory device that has two charge accumulation portions in a memory cell, and has a memory capacity of two bits per memory cell, by storing logic data (“0” or “1”) in each of the two charge accumulation portions (see Japanese Patent Kokai No. H09-91971 (Patent Literature 1) and Japanese Patent Kokai No. 2010-27162 (Patent Literature 2)). The memory cell used in this type of semiconductor memory device has an nMOSFET structure for example, and two charge accumulation portions spaced apart from each other are formed on the drain terminal side and the source terminal side respectively. Storing of data, one bit per charge accumulation portion, two bits per memory cell, can be performed by assigning a state that electric charge is accumulated in the charge accumulation portion to data “0” for example, and assigning a state that electric charge is not accumulated in the charge accumulation portion to data “1”, for example.
Writing of data into or reading of data from the memory cell of this type and erasing of data are performed in the following method, for example.
When performing the writing of data “0” into the charge accumulation portion on the drain terminal side for example, a positive voltage is applied to the drain terminal and the gate terminal, and the source terminal is held at the ground voltage. Consequently, hot electrons are injected into the charge accumulating portion on the drain side and held therein, thereby writing of data “0” is performed.
Next, when reading the data stored in the charge accumulation portion on the drain terminal side, a positive voltage is applied to the source terminal and the gate terminal, and the drain terminal is held at the ground voltage. In this instance, a relatively large read current is obtained if electric charge is not accumulated in the charge accumulation portion on the drain terminal side, that is, data “1” is recorded in the charge accumulation portion on the drain terminal side. Conversely, if electric charge not accumulated in the charge accumulation portion on the drain terminal side, that is, data “0” is recorded in the charge accumulation portion on the drain terminal side, the read current becomes smaller relative to the current obtained when data “1” is recorded, due to the effect of the accumulated electric charge. Thus, a difference in magnitude of the read current appears depending on the presence/absence of electric charge in the charge accumulation portion, so that the reading of data can be performed by judging the largeness/smallness of the read current.
When erasing the data stored in the charge accumulation portion on the drain terminal side, a positive voltage is applied to the drain terminal, zero or a negative voltage is applied to the gate terminal, and the source terminal is held in an open state. Consequently, hot holes produced near the drain area are injected into the charge accumulation portion, and the data is erased by the neutralization of the electric charge accumulated in the charge accumulation portion.
There exists a use of writing a program to this type of semiconductor memory device and mounting the semiconductor memory device in an appliance. In some cases, the program may be repeatedly rewritten by a user as the need arises. In such event, security issues can arise as discussed in Japanese Patent Kokai No. 2009-146008 (Patent Literature 3).
To cope with the security issue, a shipping mode is conceivable in which data, e.g., a program, is written into an EEPROM (Electrically Erasable and Programmable Read Only Memory) of which the capability for writing is then limited. However, if such a memory device in which data is already written has remained unused by a user, the memory device will become just useless for the user.
Conventionally, in order to cope with a situation that such “useless” memory devices having data written beforehand remain unused and are maintained by a user, a business plan has been considered, that includes procedures of buying the unused memory devices from users, erasing data written on them, writing necessary data subsequently, and shipping them to users.