Flash memory with improved erasability and its circuitry

Abstract: A flash memory which includes a memory cell array (271) in which a plurality of nonvolatile memory cells that can be erased electrically are set in array and decoding units (273) that decode a plurality of signals and access said memory cell array (271), further comprises: drive units (274) each of which includes a first power terminal (275) and a second power terminal (276), inputs the output of the decoding unit (273), and selectively outputs a voltage applied to the first power terminal (275) or a voltage approximate to that voltage, and a voltage applied to the second power terminal or a voltage approximate to that voltage; the drive unit (274) assuming a first operation mode, in which a first voltage is applied to the first power terminal (275) and a second voltage that is lower than the first voltage is applied to the second power terminal (276), and a second operation mode, in which a third voltage is applied to the first power terminal (275) and a fourth voltage that is higher than the third voltage is applied to the second power terminal (276); and selecting an output voltage depending on whether the first or second operation mode is specified.

Description: The present invention relates to a so-called flash memory or a nonvolatile semiconductor memory from which existent information can be electrically erased concurrently or block by block. In particular, this invention is concerned with improvements in erasing a flash memory and improvements in the associated circuitry. In recent years, a variety of semiconductor memories such as an electrically erasable programmable read only memory (EEPROM), which can be rewritten by electrically erasing data stored therein, have been developed in the field of nonvolatile memories. The semiconductor memories are nonvolatile memories which are made rewritable by electrically erasing specified data previously written therein. As for this kind of nonvolatile memory, in consideration of the use as a substitute for a magnetic storage or the like, efforts have been made to increase the capacity and decrease the cost. For use in a portable information processing terminal or the like, however, it is required to lower the voltage requirements. Known semiconductor memories, which are nonvolatile memories that are made rewritable by erasing specified data previously written therein, include an erasable programmable read only memory (EPROM) and the EEPROM. The EPROM has the advantage of a small cell size. However, data erasing is a nuisance because ultraviolet light must be used to erase data. As for the EEPROM, data erasing is easy to do because data can be erased electrically. However, the cell size in the EEPROM is larger than that in the EPROM, which makes it difficult to increase the capacity of an EEPROM. As a semiconductor memory having the advantages of the foregoing memories, for example, what is referred to as a flash memory has been developed. The term "flash memory" will be adopted in the succeeding description. The nonvolatile semiconductor memory has an overall configuration, for example, as that shown in Fig. 1. In Fig. 1, N cell blocks 11 1 to 11 N (for example, eight blocks) each having multiple transistor cells are arranged in a row. The cell blocks 11 1 to 11 N are provided with bit line select circuits 12 1 to 12 N and sense amplifiers/sense buffers 13 1 to 13 N respectively. A row address signal is supplied from a row address buffer to each of the cell blocks 11 1 to 11 N via a row decoder 15. A column address signal is supplied from a column address buffer 16 to each of the bit line select circuits 12 1 to 12 N via a column address decoder 17. A voltage V S sent from a source power supply circuit 18 is applied to each of source electrodes of the transistor cells of the cell blocks 11 1 to 11 N . Fig. 2 shows the cell block 11, one of the cells blocks in the aforesaid nonvolatile semiconductor memory, and its peripheral circuitry. In Fig. 2, components identical to those in Fig. 1 bear the same reference numerals, of which a description will be omitted. In Fig. 2, the bit line select circuit 12 comprises n-channel MOS field-effect transistors Q1 to Qn. Column address signals Y1 to Yn are fed from the column decoder 17 to the gates of the transistors Q1 to Qn. The cell block 11 comprises a total of n by n field-effect transistors Q11 to Qnn having floating gates and control gates. The gates of n transistors Qil to Qin (where, i = 1, 2, etc., and n) arranged in tandem are provided with row address signals Xi sent from the row decoder 15 via word lines. The drains of the n transistors Qli to Qni arranged in tandem are connected to the drains of the transistors Qi in the bit line select circuit 12 via bit lines. Voltage from the source power circuit 18 is applied to each of the sources of the transistors Qll to Qnn. A cell amplifier 13a and a write buffer 13b are connected to each of the sources of the transistors Q1 to Qn. In the foregoing semiconductor memory, when row addresses Xi and column addresses Yi are selected for writing, data read from the write buffer 13 are written in the transistors Qij of the cell blocks 11 1 to 11 N . Writing is performed simultaneously on one bit per each of the cell blocks 11 1 to 11 N or a total of N bits designated with the row addresses and column addresses. Data erasing is performed concurrently on all transistors in the cell blocks 11 1 to 11 N . In a flash memory, information is retained depending on the presence or absence of a charge in a memory cell. Fig. 3 shows an example of a structure of a memory cell. As shown in Fig. 3, a gate has a two-layered structure consisting of a control gate (CG) 25 and a floating gate (FG) 24. The control gate 25 is connected to a word line WLi and a drain (D) 23 is connected to a bit line BLi. Reference numeral 26 denotes a tunneling oxide film. The flash memory is broadly divided into two types of what are referred to as NOR and NAND. These types differ from each other in a method of writing, reading, or erasing information into or from a memory cell. Taking the NOR type flash memory as an example, writing, reading, or erasing information into or from a memory cell will be described below. When information is to be written in a memory cell having the aforesaid structure, as shown in Fig. 4, the word line WLi is set to Vpp (approx. 12 V), the bit line BLi is set to approx. 6 V, and the source S is set to 0 V. High voltage is then applied to the control gate CG and drain D. Current then flows into the memory cell. Part of the electrons flowing through the memory cell are accelerated due to the high electric field in the vicinity of the drain D, gain energy, and then goes beyond the energy barrier of an insulating film of the floating gate. The electrons are finally injected into the floating gate FG. The floating gate FG is not electrically coupled with other circuits, so it therefore can retain charges on a semi-permanent basis. When information is to be read from a memory cell, as shown in Fig. 5, the word line WLi is set to Vcc (about 5 V), the bit line BLi is set to about 1 V, and the source S is set to 0 V. The memory cell is then selected by specifying the word line WLi and bit line BLi. The threshold value of the cell transistor varies depending on the charges retained in the floating gate FG. Current flowing through the selected memory cell varies depending on the information stored therein. The information therefore can be read out by detecting and amplifying the current. The voltage levels of the control gate CG, drain D, source S, and substrate PS in the aforesaid operative states are set to the values listed in Table 1. Voltages in modes in a prior art CG D S PS Reading Vcc to 1 V 0 V 0 V Writing Vpp to 6 V 0 V 0 V Erasing 0 V Float Vpp 0 V When information is to be erased from a memory cell, as shown in Fig. 6, the word line WLi is set to about 0 V and the bit line BLi is opened. In this state, the drain D is opened, about 0 volt is applied to the control gate CG, and a high voltage of about 12 volts is applied to the source S. Since a high voltage is applied to the source S, deep diffusion is required in order to increase the resistivity of the diffused layer in the source. This hinders reduction in cell area. For divided erasing, it is required that the Vss line in the source must partly have a different voltage. This leads to disconnection or an increased number of drive circuits. Eventually, chip size increases. A. solution to the above problem is to apply a negative voltage to the word line WLi. To be more specific, as shown in Fig. 7, a negative voltage (about - 10 V) is applied to the control gate CG and Vcc (about 5 V) is applied to the source S. The drain D is opened. Erasing is then executed. In this case, since a low voltage is applied to the source S, the resistivity of the source need not be intensified. This contributes to reduction in cell size. Partial erasing is enabled by selectively applying negative voltage to the control gates CG. The aforesaid erasing method is a source erasing method in which charges in the floating gate FG are routed to the source. A channel erasing method is also available, wherein charges in the floating gate are routed to a channel; that is, a substrate. Even in this method, negative voltage is applied to the control gate. The channel erasing method is sometimes employed for the aforesaid NAND-type flash memory. Figs. 8 to 11 show the states of a memory cell with voltage applied according to various erasing methods. In Figs. 8 to 11, the memory cell is an n-channel transistor. Fig. 8 shows a state in which positive voltage is applied according to a channel erasing method. The drain D and source S are opened, and the control gate CG is set to 0 V. The high voltage Vpp is applied to the P well equivalent to a channel. In channel erasing, a triple-well structure shown in Fig. 8 is adopted because positive bias is applied to the channel. Fig. 9 shows a state in which a positive voltage is applied according to the source erasing method. The drain D is opened, and then the control gate CG is set to 0 V. The high voltage Vpp is applied to the source S. The substrate is opened or set to 0 V. Fig. 10 shows a state in which a negative voltage is applied according to the channel erasing method. The drain D and source S are opened, and the control gate CG is set to a negative voltage V BB . A positive voltage Vcc is applied to the p well equivalent to a channel. V BB - Vcc is applied between the control gate CG and channel. Fig. 11 shows a state in which negative voltage is applied according to the source erasing method. The drain D is opened, and then the control gate CG is set to the negative voltage V BB . The source S is set to the positive voltage Vcc. The foregoing problems relate to the principle of erasing. The circuitry in a flash memory for performing the aforesaid erasing has several problems; such as, how to downsize the circuitry, reduce power consumption, or speed up processing. As mentioned above, the resistivity of a junction in a source region can be improved by applying negative voltage to a control gate during erasing. This has the advantage of enabling reduction of a cell area. It is, however, a big problem how to realize application of negative voltage to the control gate. It is, for example, conceivable to apply negative voltage from a row decoder to word lines, as in the flash memory disclosed in EP-A-0320916. This flash memory may be considered to comprise: an array of electrically-erasable nonvolatile memory cells; decoding units that decode a plurality of signals and access said array; and drive units, each having first and second power terminals and at least first and second conductive channel transistors, said first transistor being connected operatively to one of said power terminals, and each drive unit being operable to receive a decoding-unit output signal and to output selectively, in dependence upon the received output signal, a voltage applied to the first power terminal or a voltage approximate thereto, or a voltage applied to the second power terminal or a voltage approximate thereto. The voltage to be applied to a word line is changed depending on whether the word line is selected or not. In a flash memory, the voltage to be applied to a word line must be varied depending on whether the read mode or write mode is selected. When the row decoder is used to apply a negative voltage, the voltage to be applied to a word line must be changed to a negative voltage. A word line selected in the read or write mode has a higher voltage than other unselected word lines. For erasing, however, the selected word line must have a lower voltage than the unselected word lines. The level of applied voltage must therefore be reversed depending on the logic of selected or unselected word lines. This results in complex circuitry, making downsizing impossible. Accordingly, it is desirable to realize a simple structure that enables selective application of negative voltage to word lines. A flash memory embodying a first aspect of the present invention is characterised in that it is operable selectively in a first operation mode, in which a first voltage is applied to said first power terminal and a second voltage that is lower than said first voltage is applied to said second power terminal, and in a second operation mode, in which a third voltage is applied to said first power terminal and a fourth voltage that is higher than said third voltage is applied to said second power terminal; and in that the flash memory further comprises: a first well region of a first conductivity type enclosed in a second well region of a second conductivity type, different to said first conductivity type, formed on a semiconductor substrate of said first conductivity type, said first transistor being formed in said first well region, and said second transistor being formed in said second well region; and first well biasing means operable to apply to said first well region a potential different from a potential applied to said substrate. In a flash memory embodying a second aspect of the present invention said second transistor (instead of said first transistor) is connected operatively to one of said power terminals, and the flash memory further comprises (in place of said first well biasing means) second well biasing means operable to apply to said second well region a potential different from a potential applied to other such well regions of said flash memory having said second conductivity type. In a flash memory embodying a third aspect of the present invention at least one of said first and second transistors is connected operatively to one of said power terminals; each drive unit is operable to receive mutually-complementary decoding-unit output signals and to output selectively, in dependence upon the received output signals, a voltage applied to the first power terminal or a voltage approximate thereto, or a voltage applied to the second power terminal or a voltage approximate thereto; both of said first and second transistors are formed in said first well region (instead of said first transistor being in said first well and said second transistor being in said second well) and gates of said first and second transistors are connected respectively to receive said mutually-complementary decoding-unit output signals; and the flash memory has said first well biasing means. In such flash memories embodying the present invention, a row decoder outputs the negative voltage to be applied to word lines for erasing. Herein, the row decoder includes drive units for supplying decoded signals to word lines. The drive unit selectively outputs a voltage to be applied to a first power terminal and a voltage to be applied to a second power terminal. The drive unit can enter either a first operation mode in which a first voltage is applied to the first power terminal and a second voltage that is lower than the first voltage is applied to the second power terminal or a second operation mode in which a third voltage is applied to the first power terminal and a fourth voltage that is higher than the third voltage is applied to the second power terminal. Output voltages are thus changed depending on whether the first or second operation mode is selected. In such a flash memory, the levels of voltages to be applied to the power terminals of a drive unit in a row decoder are varied between two modes; that is, the levels of voltages to be applied to the power terminals of a drive unit in a row decoder are varied depending on which of the modes is specified. This obviates the need of changing the logic of each word line between selected and unselected. Simple circuitry ensues. Reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1 shows an overall configuration of a flash memory in which the present invention is implemented; Fig. 2 is a circuit diagram showing a major section of Fig. 1; Fig. 3 shows a structure of a memory cell; Figs. 4 to 7 are explanatory diagrams for methods of reading, writing, and erasing a flash memory; Fig. 4 shows the conditions for writing; Fig. 5 shows the conditions for reading; Fig. 6 shows the conditions for erasing; Fig. 7 shows the conditions for erasing based on negative-voltage application; Fig. 8 is an explanatory diagram for channel erasing based on high-voltage application; Fig. 9 is an explanatory diagram for source erasing based on high-voltage application; Fig. 10 is an explanatory diagram for channel erasing based on negative-voltage application; Fig. 11 is an explanatory diagram for source erasing based on negative-voltage application; Fig. 12 shows a structure of a transistor cell; Fig. 13 is an explanatory diagram for excessive erasing; Fig. 14 shows a configuration for applying negative voltage to a word line using an independent negative power supply; Fig. 15 shows a functional structure of a row decoder when the row decoder applies negative voltage; Fig. 16 shows an example of a circuitry of a row decoder in a flash memory according to an embodiment of the present invention; Fig. 17 shows an example of part of the circuitry in Fig. 16; Figs. 18 and 19 are diagrams for explaining problems occurring when negative voltage is applied to an example of a circuitry of a previously-considered row decoder; Figs. 20 and 21 show structures of a drive unit in a row decoder based on an embodiment of the present invention; Fig. 22 shows a configuration of a flash memory embodying the present invention; Figs. 23 and 24 show examples of circuitry for a decoding unit in the Fig. 22 embodiment; Fig. 25 shows a drive unit in the Fig. 22 embodiment; Fig. 26 shows another example of circuitry of a decoding unit in the Fig. 22 embodiment; Fig. 27 shows another example of circuitry of a drive unit in the Fig. 22 embodiment; Fig. 28 is a circuit diagram of a level change circuit in the Fig. 22 embodiment; and Fig. 29 shows circuitry of a power switching circuit for a drive unit in the Fig. 22 embodiment. Prior to a description of the embodiments, pre-erase writing will be described. Fig. 12 shows a structure of a memory cell shown in Fig. 3 in more detail. A transistor serving as the memory cell has, as shown in Fig. 12, such a structure that n-type diffusive regions 32 and 33 are formed in a p-type substrate 31 with a certain space between them. A floating gate (FG) 34 and a control gate (CG) 35 are formed above the p-type substrate 31, and coated with an oxide film 36. The n-type diffusive regions 32 and 33 are coupled with a source electrode 37 and a drain electrode 39 respectively. The control gate 35 is coupled with a gate electrode 38. An insulating film for isolating between the floating gate (FG) and control gate (CG) in the stacked gate type memory cell transistor may be an oxide film or oxide nitride film. However, an ONO film; that is, a three-layered structure of an oxide film over a nitride film over an oxide film is preferred in an effort to prevent a time-sequential leakage of charges from the floating gate (FG). Using the same process for producing the insulating film between gate electrodes, an insulating film for a gate in a single gate-type transistor which is used in any region of peripheral circuits may be produced. In this case, the gate insulating film for the single gate-type transistor formed in a region of peripheral circuits may be the ONO film or oxide nitride film. The gate insulating films for transistors except memory cell transistors are not necessarily oxide films. Writing in a memory cell is achieved by injecting electrons into the floating gate (FG) 34, whereby "0" will be written. Erasing is achieved by removing electrons from the floating gate (FG) 34, whereby "1" will be written. When a source erasing method based on high-voltage application is performed to erase the transistor cell (to write "1"), as described previously, high voltage is applied to the source electrode 37, zero volt is applied to the gate electrode 38, and the drain electrode 39 is opened. The electrons retained in the floating gate 34 are thus removed. When "0" has been written in the transistor cell; that is, when electrons have been retained in the floating gate 34, the threshold for the cell, I, in Fig. 13 decreases to be lower than a threshold level for identifying data "1" for the period of an erasing time T. In contrast, when "1" has been written in the transistor cell; that is, when no electrons have been retained in the floating gate 34, since the threshold level is low, as the threshold of the cell decreases as indicated with II in Fig. 13 due to the erasing, the threshold level becomes 0 for a shorter period of time than the erasing time T. During the above erasing, too many electrons are removed from transistor cells in which "1s" have been written. The transistor cells consequently enter a Normally On state. In order to prevent the foregoing excessive erasing, it is necessary for a flash memory, in which cells are NORed, to write data "0" in each of the transistor cells in all cell blocks before concurrent erasing is performed. Writing data "0" is performed on all transistor cells, wherein the data values written in the transistor cells are not checked. Specifically, a source voltage Vs for the source electrode 37 in the transistor cell shown in Fig. 12 is set to 0 V, a gate voltage Vg for the gate electrode 38 is set to a high voltage, and a drain voltage V D for the drain electrode 39 is set to a high voltage. High-energy electrons developed by applying high voltage between the source and drain then reach the floating gate 84 through the oxide film 36. Data "0" is thus written in the transistor cell. As described previously, negative-voltage application in which negative voltage is applied to a control gate during erasing is helpful for downsizing a flash memory. Various methods are conceivable for application of negative voltage to a control gate; that is, application of negative voltage to a word line on which the control gate is connected. For example, a row decoder is used to apply negative voltage to a word line on which a memory cell that is to be erased is connected. Alternatively, a negative voltage source is connected to a word line, and a connection switch circuit is installed to operate complementarily to both pairs of a word line and a row decoder, and a word line and a negative voltage source. The switch circuit disconnects a negative voltage generator in either a write or read mode, and disconnects the row decoder for erasing. Fig. 14 shows an example of circuitry, wherein a negative-voltage generator 223 is installed on the opposite side of a row decoder 222 with a memory cell array 221 between them. The negative voltage generator 223 is connected to a word line 226. P-channel transistors 224 and 225 are installed as disconnection switches between the row decoder 222 and memory cell array 221, and the memory cell array 221 and negative generator 223. To apply negative voltage selectively to word lines, a decoder similar to a row decoder must be installed in the negative voltage generator. The overall circuit thus becomes large in scale. To overcome this drawback, it is thought that a row decoder could be used to apply negative voltage to a word line. The row decoder applies a voltage indicated in Fig. 4 or 5 to a word line during writing or reading. Specifically, during writing, a high voltage Vpp is applied to a word line on which concerned memory cells are connected (selected word line), and a ground voltage Vss is applied to the other word lines (unselected word lines). About 6 V is applied to a selected bit line. Unselected bit lines are opened. Similarly, during reading, a positive voltage Vcc is applied to a selected word line, while a ground voltage Vss is applied to unselected word lines. About 1 V is applied to a selected bit line. Unselected bit lines are opened. In either case, the ground voltage Vss is applied to sources S. Memory cells are thus accessed independently. Information is then written into or read out from the memory cells. The row decoder selects a voltage to be applied to a word line from two supply voltages according to a row decode signal. The supply voltage must be changed from writing to reading. Specifically, a high voltage must be applied to a power terminal during writing, while a positive voltage must be applied during reading. The row decode signal to be fed to the row decoder is constant whichever mode is specified; a write or read mode. The row decoder has a level change function for changing a level of a signal to a different voltage level according to a select signal. The row decoder also applies negative voltage to a selected word line during erasing. For erasing, as shown in Fig. 7, drains D must be opened. A positive voltage Vcc must be applied to sources S, and a negative voltage V BB must be applied to control gates CG. Application of the negative voltage V BB to the control gates CG is performed block by block. The positive voltage Vcc is applied to the word lines in a block which are not subjected to erasing. In a flash memory, during reading or writing, a high voltage must be applied to selected word lines WLi, while a ground voltage is applied to unselected word lines WLj (j ≠ i). During erasing, the selected word lines WLi must be set to a negative voltage, while the unselected word lines WLj must be set to a positive voltage. In short, the following formula is established for reading or writing: Voltage for selected word lines WLi > Voltage for unselected word lines WLj The following formula is established for erasing: Voltage for selected word lines WLi < Voltage for unselected word lines WLj The levels of the voltages must be reversed. For supplying negative voltage that is to be applied to a word line, a row decoder must have a previously-considered level change function as well as a level change function that can change levels between a negative voltage V BB and a positive voltage Vcc, and be able to reverse the levels of voltages to be applied to a word line according to the logic value of "selected" or "unselected." Fig. 15 shows a functional structure of a row decoder for applying negative voltage to a word line. As illustrated, a row decoder 231 includes a decoding unit 232, a logic change unit 233, a level change unit 234, and a drive unit 235. The decoding unit 232 decodes a row decode signal RDC, and determines whether a word line connected to the row decoder 231 is selected or unselected. The drive unit 235 provides a large driving force for driving the word line. The decoding unit 232, logic change unit 233, and level change unit 234 except the drive unit can be arranged in any order. For example, the level change unit 234 may be located at a leading position. However, if the level change unit 234 is located at a leading position, the subsequent units must be able to operate at a voltage whose level has been changed. The logic change unit 233 may not be installed in the row decoder 231, but an address signal itself may be changed. In this case, a unit for changing the address signal is necessary. The row decoder for applying a negative voltage to a word line needs the foregoing functions, resulting in complex circuitry. Fig. 16 shows an example of a circuitry of a row decoder that is a feature of a flash memory embodying the present invention. Fig. 17 shows an example of a structure of a drive unit 247 shown in Fig. 16. Referring to these drawings, the basic components of a flash memory embodying the present invention will be described. A flash memory embodying the present invention includes a memory cell array, and decoding units 241 for decoding multiple signals and accessing the memory cell array. The flash memory further includes drive units 247 each of which has a first power terminal 250 and a second power terminal 251, inputs an output of the decoding unit 241, and selectively outputs a voltage applied to the first power terminal 250 or a voltage approximate to the voltage, and a voltage applied to the second power terminal 251 or a voltage approximate to the voltage. The drive unit 247 assumes a first operation mode in which a first voltage is applied to the first power terminal 250 and a second voltage that is lower than the first voltage is applied to the second power terminal 251, and a second operation mode in which a third voltage is applied to the first power terminal 250 and a fourth voltage that is higher than the third voltage is applied to the second power terminal 251. Depending on whether the first or second operation mode is specified, output voltages are changed. In this flash memory, the drive unit 247 in a row decoder performs a logic change. A logic change unit is thus eliminated. This realizes simpler circuitry. The drive unit 247 is designed to selectively output voltages V1 and V2 that are applied to the first and second power terminals 250 and 251. The levels of voltages applied to the first and second power terminals 250 and 251 are reversed depending on which operation mode is selected. The requirements for a drive unit to apply negative voltage to word lines will be described briefly. In Fig. 16, a level change circuit is arranged in front of the decoding unit 241 but not illustrated. Voltages V IH and V IN applied to the power terminals 245 and 246 of the decoding unit 241 vary depending on the level determined by the level change circuit. Voltages V1 and V2 are applied to power terminals 250 and 251 in the drive unit 247. The voltages V1 and V2 assume a high voltage Vpp and a zero voltage Vss respectively, a positive voltage Vcc and the zero voltage Vss respectively, or a negative voltage V BB and the positive voltage Vcc respectively depending on whether the writing, reading, or erasing mode is selected. In response to an input signal IDC, when the connected word line is selected, a signal ODC represents the voltage V IN . A p-channel transistor 248 is turned on. The voltage V1 is output from the power terminal 250. When the connected word line is unselected, an n-channel transistor 249 is turned on and the voltage V2 is output from the power terminal 251. The drive unit 247 has, as shown in Fig. 17, a dual-well structure. This structure resolves a problem that when negative voltage is applied to the power terminal 250 of the drive unit 247, a junction between a substrate and a diffused layer is forward-biased so that current flows and the output of a specified voltage is disabled. In Figs. 18 and 19, negative voltage is applied in an example of a structure of a drive unit in a previously-considered flash memory that does not use a row decoder to apply negative voltage. Fig. 18 shows a structure based on a p-type substrate. Fig. 19 shows a structure based on an n-type substrate having a p well. In Fig. 18, a p-type substrate is used and an n-type well is formed. In this case, if the voltage applied to the substrate itself decreases, the characteristics of portions operating at a normal voltage vary. This makes it impossible to apply negative voltage to only some portions, so a negative voltage generator must handle a heavy load. When an n-type substrate is used and a p-type well is formed, as shown in Fig. 19, negative voltage need be applied to only required portions of the p-type well. The above problem can thus be solved. However, the substrate bias of a p-channel transistor cannot be set partly to Vpp for writing. To avoid the above problems, the drive unit 247 for negative-voltage driving must have any of the following structures: (a) as shown in Fig. 20, a p-well region is formed within an n-well region on a p-type substrate, and an n-channel MOS transistor is formed in the p-well region; (b) as shown in Fig. 21, an n-well region is formed within a p-well region on an n-type substrate, and a p-channel MOS transistor is formed in the n-well region; or (c) a p-channel transistor and an n-channel transistor are formed on an insulating substrate and a p-well region is biased negatively, which is a silicon-on-insulator (SOI) structure. These structures enable application of negative voltage to a control gate CG of a memory cell. A drive unit according to an embodiment of the present invention has any of the above structures, which enables a row decoder to perform negative-voltage application. Fig. 22 shows a flash memory according to a first embodiment of the present invention. As shown in Fig. 22, a flash memory of this first embodiment comprises a memory cell array 271, a row decoder 272, an address buffer pre-decoder 279, a level change circuit 277, a drive unit power switching circuit 278, a high-voltage supply unit 279, a lower-voltage supply unit 280, and a power control circuit 281. In this embodiment, level change is not performed for each word line. A row decode signal RDC is changed in level by the level change circuit 277, and then input to each of decoding units. The level change circuit 277 can therefore be shared among the decoding units. Each of memory cells forming the memory cell array 271 has the same structure as a previously-considered one. The row decoder 272 comprises decoding units 273 and drive units 274 which are associated with word lines WLi. Figs. 23 and 24 are circuit diagrams of each decoding unit 273. Fig. 23 shows a NAND circuit having a depletion-type transistor 293 as a load. Fig. 24 shows a CMOS NAND circuit. Fig. 25 shows the circuitry of a drive unit 274. The decoding units 273 decode multiple signals IDC sent from the level change circuit 277 and access the memory cell array 271. Each of the drive units 274 has a first power terminal 275 and a second power terminal 276, inputs an output of the associated decoding unit 273, and selectively outputs a voltage V1 that is to be applied to the first power terminal 275 or a voltage approximately equal to that voltage, and a voltage V2 that is applied to the second power terminal 276 or a voltage approximately equal to that voltage. The drive unit 274 may have the structure shown in Fig. 20 in which a p-well region is formed within an n-well region on a p-type substrate and an n-channel MOS transistor is formed in the p-well region; the structure shown in Fig. 21 in which an n-well region is formed within a p-well region on an n-type substrate and a p-channel MOS transistor is formed in the n-well region; or the SOI structure in which a p-channel transistor and an n-channel transistor are formed on an insulating substrate, and a p-well region is biased negatively. Another decoding unit 273 provides, as shown in Fig. 26, two outputs ODC0 and ODC1 of different phases which are logically reversed from each other. The corresponding drive unit 274 (shown in Fig. 27) may consist of transistors 341 and 342 of the same (n) type, wherein the same function as that provided when the decoding unit 273 is composed of transistors of different types is still available. When the ODC0 signal is low, the ODC1 signal goes high, the transistor 341 is turned on, and the transistor 342 is turned off. When the ODC0 signal is high, the ODC1 signal goes low, the transistor 341 is turned off, and the transistor 342 is turned on. The high-voltage supply unit 279 selectively supplies a positive voltage Vcc and a high voltage Vpp under the control of a control signal Conl sent from the power control circuit 281. The negative-voltage supply unit 280 selectively supplies a zero (ground) voltage Vss and a negative voltage V BB under the control signal Con1 sent from the power control circuit 281. The positive voltage Vcc, high voltage Vpp, zero voltage Vss, and negative voltage V BB have the following relationship: Negative voltage V BB < Zero voltage Vss < Positive voltage Vcc < High voltage Vpp The level change circuit 277 changes the voltage level of a signal fed to the decoding unit 273. As shown in Fig. 28, the level change circuit 277 has a first terminal 359 for inputting an output V IH of the high-voltage supply unit 279 and a second terminal 360 for inputting an output V IN of the negative voltage supply unit 280. The level change circuit 277 further includes n-channel MOS transistors 353, 356, and 358, and p-channel MOS transistors 354, 355, and 357. The level change circuit 277 outputs the voltage V IH , which is applied to the first terminal 359, or a voltage approximate to the voltage V IH , when the output RDC of the address buffer pre-decoder 282 is high (Vcc < V IH ). When the input RDC is low (Vss > V IN ), the level change circuit 277 outputs the voltage V IN , which is applied to the second terminal 360, or a voltage approximately equal to the voltage V IN . The drive unit power switching circuit 278 switches the levels of supply voltages V1 and V2 which are to be applied to the drive unit 274, and comprises, as shown in Fig. 29, two level change circuits 371 and 372. Switching the levels of the outputs V1 and V2 is controlled with control signals Con2-1 and Con2-2 sent from the power control circuit 281 to the level change circuits 371 and 372. Specifically, when data is read from the memory cell array 271, the output V1 is set to a positive voltage Vcc and the output V2 is set to a zero voltage Vss. When data is written in the memory cell array 271, the output V1 is changed to a high voltage Vpp and the output V2 is changed to the zero voltage Vss. When data is erased from the memory cell array 271, the output V1 is changed to a negative voltage V BB and the output V2 is changed to the positive voltage Vcc. In the flash memory of this embodiment, reading from or writing to memory cells is done in a similar manner to that in a conventional memory. Specifically, for writing, the output voltage V1 of the drive unit power switching circuit 278 is set to the high voltage Vpp, and the output voltage V2 is set to the zero voltage Vss. Word lines WLi are set to Vpp (about 12 V), bit lines BLi are set to about 6 V, and sources S are set to 0 V. For reading, the output voltage V1 of the drive unit power switching circuit 278 is set to the positive voltage Vcc, and the output voltage V2 is set to the zero voltage Vss. Word lines WLi are set to Vcc (about 5 V), bit lines (BLi) are set to about 1 V, and sources S are set to 0 V. For erasing information from memory cells, the output voltage V1 of the drive unit power switching circuit 278 is set to the negative voltage V BB , and the output voltage V2 is set to the positive voltage Vcc. Word lines WLi are set to V BB , bit lines BLi are opened, and sources S are set to Vcc. The on or off operations of the transistors 310 and 311 in the drive unit 274 in the row decoder 272 are the same as those for reading or writing. In each of selected word lines WLi, the p-channel MOS transistor 310 is on and the n-channel MOS transistor 311 is off. On each of unselected word lines WLj (j ≠ i), the p-channel MOS transistor 310 is off and the n-channel MOS transistor 311 is on. A difference from erasing to reading or writing is a voltage .applied to a diffused layer (sources) in the opposite side (other end) of the word lines WLi. That is to say, the negative voltage V BB is applied to the sources of the p-channel MOS transistors 310, while the positive voltage Vcc is applied to the sources of the n-channel MOS transistors 311. On the selected word lines WLi, the p-channel MOS transistors 310 are on. The voltage on the word lines WLi is lower by a threshold voltage Vth of a p-channel MOS transistor 310 than the negative voltage V BB . On unselected word lines WLj, the n-channel MOS transistors 311 are on. The voltage on the word lines WLj is lower by the threshold voltage Vth of an n-channel MOS transistor 311 than the positive voltage Vcc. The negative voltage for erasing need not be equal to V BB . The voltage V BB is an internally-generated voltage. The voltage for erasing should therefore be a value with the threshold voltage Vth added so that voltage suitable for erasing will be applied to the word lines WLi. Due to the substrate bias effect, the threshold voltage Vth of a p-channel MOS transistor 310 and the threshold voltage Vth of an n-channel MOS transistor become relatively large values. The first embodiment has been described above. The level change circuit shown in Fig. 28 will now be described in more detail. In the subsequent description, since potentials are determined by applying voltages with respect to the zero (ground) voltage Vss, potentials Vpp, Vcc, Vss, and V BB are referred to as a high voltage Vpp, a positive voltage Vcc, a zero voltage Vss, and a negative voltage V BB . In this level change circuit, an input signal RDC is converted into an output signal IDC having a voltage reflecting the voltages V IH and V IN applied to the power terminals. When the high voltage Vpp and zero (ground) voltage V BB are fed to the power terminals, a signal whose amplitude varies from the high voltage Vpp to the zero voltage V BB is supplied. When the positive voltage Vcc and negative voltage V BB are fed to the power terminals, a signal whose amplitude varies from the positive voltage Vcc to the negative voltage V BB is supplied. As shown in Fig. 28, in the level change circuit, the pull-up p-channel transistor 355, the high-voltage cutoff n-channel transistor 353, the negative-voltage cutoff p-channel transistor 354, and the pull-down n-channel transistor 356 are connected in series with one another between the power terminals 359 and 360. The output p-channel transistor 357 and the output n-channel transistor 358 are also connected in series with one another between the power terminals 359 and 360. The gate of the output p-channel transistor 357 is connected to a contact point between the pull-up transistor 355 and high-voltage cutoff transistor 353. The gate of the output n-channel transistor 358 is connected to a contact point between the negative- voltage cutoff transistor 354 and pull-down transistor 356. The contact point between the output p-channel transistor 357 and output n-channel transistor 358 is connected to the gates of the pull-up transistor 355 and pull-down transistor 356. The positive voltage Vcc is applied to the gate of the high-voltage cutoff transistor 353, and the zero (ground) voltage Vss is applied to the channel thereof. The zero voltage Vss is applied to the gate of the low-voltage cutoff transistor 354, and the positive voltage Vcc is applied to the channel thereof. The input signal RDC is input to the contact point between the high-voltage cutoff transistor 353 and low-voltage cutoff transistor 354. The output signal is supplied from the contact point between the output p-channel transistor 357 and output n-channel transistor 358. Next, the operations of the level change circuit will be described. Assuming that a signal of the positive voltage Vcc (high) is fed as an input signal RDC to the input terminal 361 and a signal RDC of the zero voltage Vss is output, from the inverter 351, 352, the output signal IDC assures the voltage V IH applied to the power terminal 359. If the voltage of the signal RDC applied to the input terminal 361 changes from Vcc to Vss, the voltage of the input signal RDC of the level change circuit 277 changes to the positive voltage Vcc. The low-voltage cutoff transistor 354 is turned on, thus boosting the voltage at a point n23. At this time, the pull-down transistor 356 is on. A point n22 is still connected to the power terminal 360. The current flowing through the point n22 can be limited by lowering the capability of the pull-down transistor 356. The voltage at the point n23 is boosted so as to turn on the output n-channel transistor 358, which causes no problem. When the output n-channel transistor 358 is turned on, the voltage at a point n24 drops, and the pull-up transistor 355 is turned on. The voltage at a point n21 rises to be the voltage V IH corresponding to the voltage applied to the power terminal 359. The output p-channel transistor 357 shifts to the off state. The voltage at the point n24 further drops to approach the voltage V IN of the power terminal 360. Since the voltage at the point n24 drops, the pull-down transistor 356 is turned off. Transition thus terminates, and a stable state is attained. As long as the signal RDC of the zero voltage Vss is applied to the input terminal 361, this state is maintained. Contrary to the foregoing procedure, the procedure to be followed when the voltage applied to the input terminal 361 changes from the zero voltage Vss to the positive voltage Vcc begins with the drop of the voltage at the point n21. The above description can also apply, so it will be omitted here. Flash memories are expected to substitute for magnetic storages. For use in portable equipment on behalf of magnetic storages, in particular, the properties of flash memories; that is, small size and light weight will prove very helpful. For actual use in portable equipment in place of magnetic storage, the flash memory must further be innovated to achieve downsizing, an increase in operational speed, reduction in power consumption, and decrease in voltage requirements.Embodiments of the present invention can provide innovative flash memory and solve some of the problems obstructing the widespread use of the flash memory.

Claims: A flash memory comprising: an array (271) of electrically-erasable nonvolatile memory cells; decoding units (273; 241) that decode a plurality of signals and access said array (271); and drive units (274; 247), each having first and second power terminals (275, 276; 250, 251; 312, 313) and at least first and second conductive channel transistors (249, 248; 311, 310), said first transistor (249; 311) being connected operatively to one of said power terminals, and each drive unit being operable to receive a decoding-unit output signal (ODC) and to output selectively, in dependence upon the received output signal, a voltage (V 1 ) applied to the first power terminal or a voltage approximate thereto, or a voltage (V 2 ) applied to the second power terminal or a voltage approximate thereto; characterised in that the flash memory is operable selectively in a first operation mode, in which a first voltage (V PP /V CC ) is applied to said first power terminal and a second voltage (V SS ) that is lower than said first voltage is applied to said second power terminal, and in a second operation mode, in which a third voltage (V BB ) is applied to said first power terminal and a fourth voltage (V CC ) that is higher than said third voltage is applied to said second power terminal; and in that the flash memory further comprises: a first well region (257) of a first conductivity type enclosed in a second well region (256) of a second conductivity type, different to said first conductivity type, formed on a semiconductor substrate (255) of said first conductivity type, said first transistor (249; 311) being formed in said first well region, and said second transistor (248; 310) being formed in said second well region; and first well biasing means (280, 278) operable to apply to said first well region a potential (V IN ) different from a potential applied to said substrate. A flash memory according to claim 1, wherein said second transistor is also connected operatively to one of said power terminals, the flash memory further comprising second well biasing means (279, 278) operable to apply to said second well region a potential (V IH ) different from a potential applied to other such well regions of the flash memory having said second conductivity type. A flash memory comprising: an array (271) of electrically-erasable nonvolatile memory cells; decoding units (273; 241) that decode a plurality of signals and access said array (271); and drive units (274; 247), each having first and second power terminals (275, 276; 250, 251; 310, 311) and at least first and second conductive channel transistors (249, 248; 311, 310), said second transistor (248; 310) being connected operatively to one of said power terminals, and each drive unit being operable to receive a decoding-unit output signal (ODC) and to output selectively, in dependence upon the received output signal, a voltage (V 1 ) applied to the first power terminal or a voltage approximate thereto, or a voltage (V 2 ) applied to the second power terminal or a voltage approximate thereto; characterised in that the flash memory is operable selectively in a first operation mode, in which a first voltage (V PP /V CC ) is applied to said first power terminal and a second voltage (V SS ) that is lower than said first voltage is applied to said second power terminal, and a second operation mode, in which a third voltage (V BB ) is applied to said first power terminal and a fourth voltage (V CC ) that is higher than said third voltage is applied to said second power terminal; and in that the flash memory further comprises: a first well region (257) of a first conductivity type enclosed in a second well region (256) of a second conductivity type, different to said first conductivity type, formed on a semiconductor substrate (255) of said first conductivity type, said first transistor (249; 311) being formed in said first well region, and said second transistor (248; 310) being formed in said second well region; and second well biasing means (279, 278) operable to apply to said second well region a potential (V IH ) different from a potential applied to other such well regions of said flash memory having said second conductivity type. A flash memory according to claim 2 or 3, wherein said second well biasing means are operable to apply to said second well region a first potential (V PP ) in said first operation mode and a second potential (V CC ), different from that first potential, in said second operation mode. A flash memory according to claim 4, wherein: said first conductivity type is p-type and said second conductivity type is n-type; and said first potential applied to said second well region is equal or approximate to said first voltage and said second potential applied to said second well region is equal or approximate to said fourth voltage. A flash memory according to any preceding claim, wherein the gates of said first and second transistors are each connected to receive said decoding-unit output signal. A flash memory comprising: an array (271) of electrically-erasable nonvolatile memory cells; decoding units (273) that decode a plurality of signals and access said array (271); and drive units (274), each having first and second power terminals (275, 276; 343, 344) and at least first and second conductive channel transistors (342, 341), at least one of said first and second transistors being connected operatively to one of said power terminals, characterised in that : each drive unit is operable to receive mutually-complementary decoding-unit output signals (ODC 0 , ODC 1 ) and to output selectively, in dependence upon the received output signals, a voltage (V 1 ) applied to the first power terminal or a voltage approximate thereto, or a voltage (V 2 ) applied to the second power terminal or a voltage approximate thereto; the flash memory is operable selectively in a first operation mode, in which a first voltage (V PP /V CC ) is applied to said first power terminal and a second voltage (V SS ) that is lower than said first voltage is applied to said second power terminal, and a second operation mode, in which a third voltage (V BB ) is applied to said first power terminal and a fourth voltage (V CC ) that is higher than said third voltage is applied to said second power terminal; and in that the flash memory further comprises: a first well region of a first conductivity type enclosed in a second well region of a second conductivity type, different to said first conductivity type, formed on a semiconductor substrate of said first conductivity type, said first and second transistors (342, 341) being formed in said first well region, and gates of said first and second transistors being connected respectively to receive said mutually-complementary decoding-unit output signals (ODC 0 , ODC 1 ); and first well biasing means (280, 278) operable to apply to said first well region a potential (V IN ) different from a potential applied to said substrate. A flash memory according to claim 1, 2 or 7, or claim 4, 5 or 6 when read as appended to claim 1 or 2, wherein said first well biasing means are operable to apply to said first well region a first potential (V SS ) in said first operation mode and a second potential (V BB ), different from that first potential, in said second operation mode. A flash memory according to claim 8, wherein: said first conductivity type is p-type and said second conductivity type is n-type; and said first potential applied to said first well region is equal or approximate to said second voltage, and said second potential applied to said first well region is equal or approximate to said third voltage. A flash memory according to any preceding claim, wherein said first conductivity type is p-type and said second conductivity type is n-type. A flash memory according to any preceding claim, wherein said first transistor is connected operatively between said second power terminal and an output terminal (253; 315; 348) of the drive unit, and said second transistor is connected operatively between said first power terminal and said output terminal. A flash memory according to any preceding claim, wherein said first operation mode is a write or a read mode of the memory, and said second operation mode is an erase mode of the memory. A flash memory according to any preceding claim, wherein: said decoding units (273) and said drive units (274) constitute a row decoder (272) for selecting a row in said memory cell array; and said flash memory includes: a level change circuit (277) for changing the level of a signal to be fed to said decoding units (273) ; a drive unit power switching circuit (278) for switching power supplies for said drive unit (274); a high-voltage supply unit (279) for selectively supplying a positive voltage and a high voltage; and a negative-voltage supply unit (280) for selectively supplying a zero voltage and a negative voltage; said positive voltage, high voltage, zero voltage, and negative voltage having such a relationship that said negative voltage < said zero voltage < said positive voltage < said high voltage; and said drive unit power switching circuit (278) supplies said positive voltage to said first power terminals (275) of said drive units (274) and said zero voltage to said second power terminals (276) thereof, so that data will be read from said memory cell array (271); supplies said high voltage to said first power terminals (275) of said drive units (274) and said zero voltage to said second power terminals thereof, so that data will be written in said memory cell array (271); and supplies said negative voltage to said first power terminals (275) of said drive units (274) and said positive voltage to said second power terminals (276) thereof, so that data will be erased from said memory cell array (271). A flash memory according to claim 13, wherein said level change circuit (277) has a first terminal (359) to which the output (V IH ) of said high-voltage supply unit (279) is supplied and a second terminal (360) to which the output (V IN ) of said negative-voltage supply unit (280) is supplied; and said level change circuit (277) outputs a voltage applied to said first terminal (359) or a voltage approximate to that voltage, when the input of said level change circuit is at a first logic level, and outputs a voltage applied to said second terminal (360) or a voltage approximate to that voltage, when said input is at a second logic level opposite to said first logic level. A flash memory according to claim 14, wherein said first logic level is high and said second logic level is low. A flash memory according to claim 13, 14 or 15, wherein said drive unit power switching circuit (278) comprises two level change circuits (371, 372).

Inventors: AKAOGI TAKAO, HAGIWARA RYOJI, ITANO KIYOSHI, KASA YASUSHI, KAWAMURA SHOUICHI, KAWASHIMA HIROMI, OGAWA YASUSHIGE, TAKEGUCHI TETSUJI, AKAOGI, TAKAO, HAGIWARA, RYOJI, ITANO, KIYOSHI, KASA, YASUSHI, KAWAMURA, SHOUICHI, KAWASHIMA, HIROMI, OGAWA, YASUSHIGE, TAKEGUCHI, TETSUJI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: G11C16
IPC codes: G11C16, G05F3