1. Field of the Invention
The present invention relates to a semiconductor memory and an integrated circuit built therefrom, and more specifically, to a technique which uses for a channel formation region a semiconductor thin film such as SOI (Semiconductor On Insulator) or SON (Semiconductor On Nothing). The semiconductor thin film is formed on an insulating substrate (SOI) in some cases, is suspended and held at both ends by substrates in a hollow state (SON) in some other cases, and has a projecting portion which is connected at one end to a substrate in still other cases.
2. Description of the Related Art
H. J. Wann et al. have proposed in 1993 to obtain a dynamic memory that does not use a capacitor by incorporating two complementary transistors in a partially depleted SOIMOS transistor structure (See Non-patent Document 1, for example).
Recently, a memory has been proposed in which carriers are generated utilizing a carrier multiplication phenomenon such as avalanche breakdown in a drain high electric field region of a partially depleted SOIMOS transistor and the obtained carriers are used to charge a neutral body, resulting in a change in current flowing between the drain and the source of the transistor (See Non-patent Document 2, for example).
The term partially depleted SOI, abbreviated as PD SOI, refers to SOI in which a depletion layer spreads only partially in the depth direction of its semiconductor thin film to give it a neutral region. ‘Body’ is a simplified term for the above semiconductor thin film in which a channel is formed.
[Non-Patent Document 1]
H. J. Wann, C. Hu, “A capacitor-less DRAM cell on SOI substrate”, 1993, IEDM (International Electron Device Meeting) Technical Digest, pp. 635–638
[Non-Patent Document 2]
S. Okhonin et al., “A Capacitor-less 1T-DRAM Cell”, IEEE Electron Device letters, Volume 23, Number 2, pp. 85–87, February 2002
On the other hand, fully depleted (FD) SOI is used for low power consumption uses or for advanced miniaturization of SOIMOS transistor, thereby creating the need for SOI memory cells that can be applied to FDSOI. The term FD (fully depleted) SOI refers to SOI having such thickness and impurity concentration that makes the depletion layer cover the entire depth of the semiconductor thin film under some gate bias of a transistor fabricated there.
The method of utilizing carrier multiplication in a drain high electric field portion also causes carrier multiplication of a small degree in a not-selected cell which is connected to a bit line for driving at high voltage a drain of a cell to which a signal is to be written. This leads to erroneous, albeit mild, writing called ‘write disturb’ and therefore makes it difficult to assemble a large array in which a large number of cells are connected to each bit line. Furthermore, the method also needs a relatively large current, which prevents parallel programming of a large number of the cells.