Static information storage and retrieval – Systems using particular element – Ferroelectric
Patent
1998-08-28
2000-08-01
Mai, Son
Static information storage and retrieval
Systems using particular element
Ferroelectric
365203, 365205, G11C 1122, G11C 700
Patent
active
060976233
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a ferroelectric memory device. And more particularly, the invention relates to a ferroelectric memory device having memory cells each of which is made up of a capacitor with an insulating film of ferroelectric material and a MOS transistor.
BACKGROUND ART
As a nonvolatile memory realizing high-speed write operation, much attention has been recently focused on a ferroelectric memory which uses a ferroelectric capacitor. In particular, a memory of such a type that each memory cell is made up of a capacitor with an insulating film of ferroelectric material and a MOS transistor and a constant voltage is applied to one of electrodes of the capacitor called a plate has a possibility of being able to realize a nonvolatile memory with nearly the same operating speed and area as those of a dynamic random access memory (DRAM). An example of a basic arrangement of such a prior art ferroelectric memory is shown in FIG. 36. In the drawing, reference symbol MC denotes a memory cell, which is made up of a ferroelectric capacitor with an insulating film of ferroelectric material such as PZT and an NMOS transistor. The remanent polarization of the ferroelectric capacitor stores information. The ferroelectric capacitor is connected at its one end to the NMOS transistor and at the other end (plate electrode) to 1/2 voltage (VCC/2) corresponding to half of a source voltage VCC. For simplicity, only one memory cell MC is illustrated in the illustrated example, but actually a plurality of such memory cells are connected to each of a pair Dt and Db of data lines and are selected by a word line W for data transfer to the data-line pair Dt or Db. Although omitted for simplicity in the drawing, a dummy cell is actually provided to each of the data-line pair Dt and Db. Reference symbol PC denotes a precharge circuit which precharges the data-line pair Dt and Db to a ground voltage VSS. Reference symbol SA denotes a sense amplifier which detects voltages of the data-line pair Dt and Db and amplifies the voltages differentially. Further, though omitted for simplicity in the drawing, a switch is actually provided to the sense amplifier for signal transfer to or from external.
The operation of the above arrangement will be explained with use of a timing chart shown in FIG. 37. In a standby state, a control signal FPC causes the precharge circuit PC to be put in its ON state, so that the data-line pair Dt and Db are precharged to the ground voltage VSS, that is, are in a so-called VSS precharge state. In operation, the control signal FPC causes the precharge circuit PC to be turned OFF. Thus when the word line W has a selected-word-line voltage VCH, the memory cell MC is selected. This causes an NMOS transistor in the memory cell MC to be turned ON, so that a voltage of VCC/2 corresponding to a difference in voltage between a data line Dt and a plate electrode is applied to the ferroelectric capacitor, whereby the remanent polarization is read out as charge to the data line Dt. This varies the voltage on the data line Dt and then a control signal FSA activates the sense amplifier SA, which in turn amplifies the voltage of the data line D with positive feedback to sense data. Though not illustrated in FIG. 37, when the data sensed by the sense amplifier SA is output externally, read operation is carried out. Further, when the voltage of the data line is used as a write voltage in accordance with externally entered data, write operation is carried out. When the word line W is lowered to turn OFF the NMOS transistor in the memory cell MC, rewrite operation to the memory cell MC is carried out. Thereafter, the control signal FSA stops the operation of the sense amplifier SA, the control signal FPC turns ON a precharging switch, thus returning the current state to the standby mode.
The operation of the ferroelectric capacitor in the standby mode will be explained with use of a hysteresis characteristic shown in FIG. 38. In the drawing, horizontal axis denotes a voltage applied to the ferroe
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Fujisawa Hiroki
Isoda Masanori
Kajigaya Kazuhiko
Kimura Katsutaka
Sakata Takeshi
Hitachi , Ltd.
Hitachi ULSI Systems Co. Ltd.
Mai Son
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