Erase discharge control method of nonvolatile semiconductor memory device

According to an example embodiment, an erase discharge method may include drawing charges accumulated in a floating gate of a floating gate type field effect transistor into a semiconductor substrate to perform an erase operation by applying a first voltage to a word line, a second voltage to an N-well and a P-well, and/or opening a bit line and a ground line. The word line may be grounded, and a discharge transistor connected to the bit line may be turned on. The N-well and the P-well may be grounded to discharge charges accumulated in the N-well and P-well.

PRIORITY STATEMENT

This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2006-175540, filed on Jun. 26, 2006, in the Japanese Patent Office, the entire contents of which are incorporated herein by reference.

BACKGROUND

FIG. 4is a block diagram of an example, conventional NOR flash memory device. As shown, a memory cell array MA may be configured with a plurality of word lines WL, a plurality of bit lines BL, and a plurality of transistors Tr (for example, floating gate field effect transistors) forming memory cells. The plurality of transistors Tr may be disposed at intersections of the word lines WL and the bit lines BL. Each of the transistors Tr may include a control gate, a drain, and a source connected to the word line WL, the bit line BL, and a ground line SL, respectively.

A source106and a drain108of the transistor Tr may be formed in a P-well104provided in an N-well102of a semiconductor substrate100, as illustrated inFIG. 5.

A NOR flash memory may be structured such that the memory cells of MOS transistors having a stack gate structure are connected in parallel in a NOR type configuration.

As illustrated inFIG. 4, a row decoder RD may be provided for selecting word lines WL0through WL1023of the memory cell array MA, and a column decoder CD may be provided for selecting bit lines BL0through BL511. Addresses may be transmitted to the row decoder RD and the column decoder CD as internal row address signals and internal column address signals, respectively, through an address control circuit (not shown). Based on a command decoded by a controller (not shown), sequence control including programming and erasing of data may be performed.

A negative voltage boosting circuit20may be connected to the row decoder RD through a negative voltage switch10. The negative voltage boosting circuit20may generate a boosted voltage used in programming or erasing data. For example, the boosted voltage may be obtained by boosting a power supply voltage Vcc. A positive voltage boosting circuit70may be further provided in the flash memory because a boosted positive voltage may also be used in programming or erasing data. The boosted positive voltage may be supplied to the ground line SL or the P and N-wells104and102through a source-well voltage control circuit60.

In addition, a source switch30may control connection/disconnection between the source-well voltage control circuit60and the ground line SL. Furthermore, a well switching unit80having a P-well switch40and an N-well switch50may control connection/disconnection between the P and N-wells104and102and the source-well voltage control circuit60.

FIG. 6is a plane view illustrating a cell array structure of an example NOR flash memory.FIG. 7is a sectional view illustrating the structure of the example NOR flash memory ofFIG. 6taken along line X-X′.

As transistors in memory cells progressively shrink in size, capacitances between respective nodes of the memory cells may be changed. For example, the capacitance between a word line connected to a control gate and a bit line connected to a drain may become significantly higher than capacitances of other nodes because the space between a bit line contact and the word line may be reduced.

FIG. 8is a timing diagram illustrating voltages applied during an erase operation performed in the flash memory device ofFIG. 4. An example erase operation will be described below.

The power supply voltage Vcc may be applied to a word line of a cell in a neutral state. At this point, the bit line BL, the ground line SL, the P-well and the N-well may each be grounded. The word line voltage may be set equal to the ground voltage so that a sector including cells to be erased may be unselected. This state may be denoted as ERS set inFIG. 8. A voltage boosted to a level in the range of approximately 5V to approximately 9.5V by the positive voltage boosting circuit70ofFIG. 5may be applied to the P-well104and the N-well102for well charge.

It may take a given amount of time for the voltage to be boosted to reach the voltage level in the range of approximately 5V to approximately 9.5V, and the boosting operation may be performed so that the voltage may gradually increase from the ground voltage level Gnd to reach the voltage level in the range of approximately 5 to approximately 9.5V.

At this point, the ground line SL and the bit line BL, which may be connected to the source and the drain of the memory cell, may be opened, and the PN junction between the substrate and the source and drain regions may be forward biased. Therefore, a current may flow from the substrate to the source and the drain, and a voltage, which may be reduced from the above boosted voltage by a forward bias voltage Vf of the PN junction, may be applied to the bit line BL and the ground line SL.

Subsequently, the negative voltage boosting circuit20may operate so that a negative voltage of approximately −9V may be applied to the word line though the row decoder RD. Thus, an erase operation may be performed such that charges accumulated in a floating gate FG of the cell transistor Tr may be put into the semiconductor substrate.

After erasing, a word line discharge and a well discharge may be performed for discharging the negative and positive voltages applied during the erase operation to the ground voltage level. For a word line discharge, the word line may be grounded so that the charges accumulated in the word line are discharged. After the word line discharge, the bit line, the ground line, the P-well and the N-well may be grounded for performing the well discharge. Therefore, charges in the bit line, the ground line, P-well and the N-well may be discharged to ground.

Thereafter, the unselected state of the word line may be terminated, and the word line voltage may return to the neutral state. For example, the word line voltage may return to the power supply voltage Vcc, whereby the erase operation may be completed.

In such an erase operation, because the coupling capacitance between the word line and the bit line may be higher as illustrated inFIGS. 6 and 7, the bit line voltage may become higher due to the overshoot of the bit line voltage when discharging the word line voltage. Consequently, this higher bit line voltage may exceed a breakdown voltage of a component connected to the bit line and cause the component to fail.

SUMMARY

Example embodiments are directed to an erase discharge control method of a flash memory.

An erase discharge control method may be used in conjunction with a nonvolatile memory device including a memory array in which a plurality of cells are arranged in a matrix form. Each of the cells may have a floating gate type field effect transistor, of which a control gate, a drain and a source may be respectively connected to a word line, a bit line and a ground line. The source and drain may be disposed in a P-well provided in an N-well of a semiconductor substrate.

According to an example embodiment, an erase discharge method may include drawing charges accumulated in a floating gate of the floating gate type field effect transistor into the semiconductor substrate to perform an erase operation by applying a first voltage to the word line, a second voltage to the N-well and the P-well, and/or opening the bit line and the ground line. The word line may be grounded, and a discharge transistor connected to the bit line may be turned on. The N-well and the P-well may be grounded to discharge charges accumulated in the N-well and P-well.

The method may also include reducing the second voltage applied to the N-well and the P-well to a voltage level lower than an erase voltage level. Reducing the second voltage may be performed before grounding the word line. Alternatively, reducing the second voltage may be performed simultaneously with grounding the word line. Turning on the discharge transistor may be performed before grounding the word line. The first voltage may be a negative voltage, and the second voltage may be a positive voltage.

According to another example embodiment, an erase discharge control method of a nonvolatile memory device may include drawing charges accumulated in a floating gate of the floating gate type field effect transistor into the semiconductor substrate to perform an erase operation by applying a first voltage to the word line, a second voltage to the N-well and the P-well, and/or opening the bit line and the ground line. The second voltage applied to the N-well and the P-well may be reduced to a voltage level lower than an erase voltage level. The word line may be grounded. Charges accumulated in the word line may be discharged. The N-well and the P-well may be grounded to discharge charges accumulated in the N-well and P-well.

The method may also include turning on a discharge transistor connected to the bit line. Reducing the second voltage may be performed before grounding the word line. Alternatively, reducing the second voltage may be performed simultaneously with grounding the word line. Turning on the discharge transistor may be performed before grounding the word line. The first voltage may be a negative voltage, and the second voltage may be a positive voltage.

According to another example embodiment, an erase discharge control method of a nonvolatile memory device may include drawing charges accumulated in a floating gate of the floating gate type field effect transistor into the semiconductor substrate to perform an erase operation by applying a first voltage to the word line, a second voltage to the N-well and the P-well, and/or opening the bit line and the ground line. The second voltage applied to the N-well and the P-well may be reduced to a voltage level lower than an erase voltage level, grounding the word line, and turning on a discharge transistor connected to the bit line. The N-well and the P-well may be grounded to discharge charges accumulated in the N-well and P-well.

Reducing the second voltage may be performed before grounding the word line. Alternatively, reducing the second voltage may be performed simultaneously with grounding the word line. Turning on the discharge transistor may be performed before grounding the word line. The first voltage may be a negative voltage, and the second voltage may be a positive voltage.

DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1is a view illustrating a portion of a flash memory structure for implementing an erase discharge control method according to an example embodiment. It will be understood that although a flash memory device is described herein, example embodiments may be implemented in many forms of nonvolatile semiconductor memory devices in addition to flash memory.

According to an example embodiment, a bit line BL may be discharged when a word line WL voltage is discharged. To extract charges from the bit line BL, a discharge transistor1may be connected to the bit line BL. A drain of the discharge transistor1may be connected to the bit line BL, and a source may be connected to a ground Gnd. In addition, a control voltage BL_DISCH may be applied to a gate of the discharge transistor1. The control voltage BL_DISCH may be applied to the gate of the discharge transistor1at the same time the word line WL beings to discharge. Thus, the discharge transistor1may be turned on so that the bit line BL may be grounded.

Furthermore, the discharge transistor1may be turned on until the discharge of a word line WL is terminated. Accordingly, the bit line BL may be discharged simultaneously with the word line WL, which may reduce or prevent an overshoot of the bit line voltage because the capacitance coupling between the word line and the bit line may have relatively little effect on the bit line voltage.

In order to discharge the bit line BL with the word line WL, a control voltage Ysel may be applied to a gate of a bit line select transistor2when the word line WL begins to discharge. Thus, the bit line BL may be connected to the drain of the discharge transistor1, and simultaneously a control voltage BL_DISCH may be applied to the gate of the discharge transistor1.

Although the discharge transistor1is illustrated as being provided in each bit line inFIG. 1, it may be unnecessary to select the bit line select transistor2when, for example, the discharge transistor1is provided for each local bit line of the memory array.

FIG. 2is a timing diagram of applied voltages according to an example embodiment.

Referring toFIG. 2, turning on the discharge transistor1connected to the bit line may be performed simultaneously with transitioning the word line voltage to ground. However, it may also be possible to turn on the discharge transistor1connected to the bit line before the word line voltage is transitioned to ground.

InFIG. 2, reference symbol PNWELLON illustrates applying of a positive voltage to the P-well and the N-well, and reference symbol PNWELLEN illustrates enabling of the P-well and the N-well. Furthermore, reference symbol DISCH illustrates discharging of the P-well and the N-well, and reference symbol SLEN illustrates driving of a transistor3connected to a ground line SL.

Because SLEN may maintain the ground voltage level during the erase operation, the transistor3may be opened so that the ground line SL is opened.

FIG. 3illustrates an example timing diagram of applied voltages according to another example embodiment.

According to the example embodiment illustrated inFIG. 3, the word line may be grounded such that a positive voltage applied to the P-well and the N-well may be reduced to a voltage level lower than an erase voltage level when the word line is discharged.

Although this may lead to an increase in the bit line voltage caused by the capacitance coupling between the word line and the bit line, the bit line voltage may actually be reduced because the voltage level of the positive voltage applied to the P-well and the N-well may be lowered. Therefore, the bit line voltage may be controlled to not exceed the breakdown voltage of a component connected to the bit line.

Alternatively, voltages of the P-well and the N-well may be reduced before the word line is discharged. Thus, reducing the positive voltage applied to the P-well and the N-well may be performed simultaneously with or before the word line is discharged.

According to another example embodiment, the positive voltage applied to the P-well and the N-well may be reduced to a voltage level lower than the erase voltage level by extracting the charges from the bit line when the word line voltage is discharged. An erase discharge control may be performed to weaken the coupling between the bit line and the word line, which may impede or prevent the breakdown of a component connected to the bit line.

A flash memory device is a kind of nonvolatile memory device which may retain stored data even if a power supply is deactivated. With the increased use of mobile devices, for example, cellular phones, PDAs, portable game consoles, and MP3 players, flash memory devices may be widely used in code storage as well as data storage. In addition, flash memory may be used in home applications, for example, HDTV, DVDs, routers, and GPS devices.

FIG. 9is a block diagram schematically illustrating a computing system including a flash memory device and a memory controller which may be used in conjunction with example embodiments disclosed herein.

Referring toFIG. 9, the computing system may include a microprocessor410, a user interface420, a modem430(for example, a baseband chipset or the like), a memory controller440, and/or a flash memory device450. The flash memory device450may operate based on the erase discharge control method according to above-described example embodiments. The flash memory device450may store N-bit data (N being a positive integer) processed/to be processed by the microprocessor410through the memory controller440(or without the memory controller). When the computing system is a mobile device, for example, a battery460may be additionally provided for supplying an operation voltage to the computing system. Although not shown, it will be obvious to one of ordinary skill in the art that an application chipset, a camera image processor (CIS), a mobile DRAM, etc., may be further provided in the computing system.

Because charges may be extracted from a bit line when a word line voltage is discharged, it may be possible to reduce the effect on the bit line voltage caused by the capacitance coupling between the bit line and the word line, and to reduce or remove the overshoot phenomenon of the bit line caused by the coupling accompanied by the discharge of the word line as well.

Furthermore, because the voltages of the P-well and the N-well may be reduced to a voltage level lower than an erase voltage level when the word line is discharged, the voltage applied to the bit line may be lowered in spite of the capacitance coupling between the bit line and the word line. Accordingly, the bit line voltage may not exceed a breakdown voltage of a component connected to the bit line, which may make it possible to prevent the component from being broken down.

Thus, example embodiments disclosed herein may provide an erase discharge control method of a nonvolatile semiconductor memory device, including flash memory, which may be suitable for high-speed operation without malfunction by discharging charges remaining in a memory cell, both relatively reliably and relatively quickly.