1. Field of the Invention
The present invention relates to integrated nonvolatile memory circuits (preferably integrated flash memory circuits) which apply high voltage to selected transistors during one or more operating modes (e.g., during various stages of a memory erase operation), monitor one or more selected nodes to detect an illegal condition, and generate a halt signal (for causing the halting or aborting of circuit operation) in response to detection of an illegal condition. The nonvolatile memory circuit of the invention includes logic means for asserting a halt signal in response to an illegal condition, but only following application of a high voltage to components of the circuit (if the illegal condition occurs during application of the high voltage to such components).
2. Description of Related Art
Throughout the specification, including in the claims, the term "connected" is used (in the context of an electronic component being "connected" to another electronic component) in a broad sense to denote that the components are electrically or electromagnetically coupled with sufficient strength under the circumstances. It is not used in a narrow sense requiring that an electrically conducting element is physically connected between the two components.
Nonvolatile memory chips (integrated circuits) are becoming increasingly commercially important. The present invention pertains to a method and apparatus for halting operation of a nonvolatile memory chip in response to detection of an undesired ("illegal") operating condition of the chip. In order to appreciate the invention, it will be helpful initially to describe the structure and normal operating modes of a typical nonvolatile memory chip.
A typical nonvolatile memory chip includes an array of nonvolatile memory cells, each cell comprising a transistor having a floating gate capable of semipermanent charge storage. The current drawn by each cell depends on the amount of charge stored on the corresponding floating gate. Thus, the charge stored on each floating gate determines a data value that is stored "semipermanently" in the corresponding cell.
One particularly useful type of nonvolatile memory chip includes an array of flash memory cells, with each cell comprising a flash memory device (a transistor). The charge stored on the floating gate of each flash memory device (and thus the data value stored by each cell) is erasable by appropriately changing the voltage applied to the gate and source (in a well known manner).
FIG. 1 is a simplified block diagram of a conventional nonvolatile memory chip. Integrated circuit 3 of FIG. 1 includes at least one I/O pad 30 (for asserting output data to an external device or receiving input data from an external device), input/output buffer circuit 10 for I/O pad 30, address buffers A0 through Ap for receiving memory address bits from an external device, row decoder circuit (X address decoder) 12, column multiplexer circuit (Y multiplexer) 14, and memory array 16 (comprising columns of nonvolatile memory cells, such as column 16A). Each of address buffers A0 through Ap includes an address bit pad for receiving (from an external device) a different one of address bit signals X0 through Xn and Y0 through Ym.
I/O buffer circuit 10 includes a "write" branch and a "read" branch. The write branch comprises input buffer 18. The read branch comprises sense amplifier 19 and output buffer 20. Chip 3 executes a write operation by receiving data (to be written to memory array 16) from an external device at I/O pad 30, buffering the data in the write branch, and then writing the data to the appropriate memory cell. Chip 3 can also be controlled to execute a read operation in which it amplifies and buffers data (that has been read from array 16) in the read branch, and then assert this data to I/O pad 30.
Although only one I/O pad (pad 30) is shown in FIG. 1, typical implementations of the FIG. 1 circuit include a plurality of I/O pads, and each I/O pad is buffered by an I/O buffer circuit similar or identical to circuit 10. For example, one implementation of the FIG. 1 circuit includes eight I/O pads, eight buffer circuits identical to circuit 10, one line connected between the output of the output buffer 20 of each buffer circuit and one of the I/O pads (so that eight data bits can be read in parallel from buffers 20 to the pads), and one line connected between the input of the input buffer 18 of each buffer circuit and one of the I/O pads (so that eight data bits can be written in parallel from the pads to buffers 18). Each I/O pad (including I/O pad 30) typically has high impedance when the output buffer is not enabled.
Each of the cells (storage locations) of memory array circuit 16 is indexed by a row index (an "X" index determined by decoder circuit 12) and a column index (a "Y" index output determined by decoder circuit 14). FIG. 2 is a simplified schematic diagram of two columns of cells of memory array 16 (with one column, e.g., the column on the right, corresponding to column 16A of FIG. 1). The column on the left side of FIG. 2 comprises "n" memory cells, each cell implemented by one of floating-gate N-channel transistors N1, N3, . . . , Nn. The drain of each of transistors N1-Nn is connected to bitline 13, and the gate of each is connected to a different wordline (a different one of wordline 0 through wordline n). The column on the right side of FIG. 2 also comprises "n" memory cells, each cell implemented by one of floating-gate N-channel transistors N2, N4, . . . , Nm. The drain of each of transistors N2-Nm is connected to bitline 15, and the gate of each is connected to a different wordline (a different one of wordline 0 through wordline n). The source of each of transistors N1, N3, . . . , Nn, and N2, N4, . . . , Nm is held at a source potential (which is usually ground potential for the chip during a program or read operation).
In the case that each memory cell is a nonvolatile memory cell, each of transistors N1, N3, . . . , Nn, and N2, N4, . . . , Nm has a floating gate capable of semipermanent charge storage. The current drawn by each cell (i.e., by each of transistors N1, N3, . . . , Nn, and N2, N4, . . . , Nm) depends on the amount of charge stored on the corresponding floating gate. Thus, the charge stored on each floating gate determines a data value that is stored "semipermanently" in the corresponding cell. In cases in which each of transistors N1, N3, . . . , Nn, N2, N4, . . . , and Nm is a flash memory device (as indicated in FIG. 2 by the symbol employed to denote each of transistors N1, N3, . . . , Nn, N2, N4, . . . , and Nm), the charge stored on the floating gate of each is erasable (and thus the data value stored by each cell is erasable) by appropriately changing the voltage applied to the gate and source (in a well known manner).
In response to address bits Y0-Ym, circuit 14 (of FIG. 1) determines a column address which selects one of the columns of memory cells of array 16 (connecting the bitline of the selected column to Node 1 of FIG. 1), and in response to address bits X0-Xn, circuit 12 (of FIG. 1) determines a row address which selects one cell in the selected column. Consider an example in which the column address selects the column on the right side of FIG. 2 (the column including bitline 15) and the row address selects the cell connected along wordline 0 (the cell comprising transistor N2). To read the data value stored in the selected cell, a signal (a current signal) indicative of such value is provided from the cell's drain (the drain of transistor N2, in the example), through bitline 15 and circuit 14, to node 1 of FIG. 1. To write a data value to the selected cell, a signal indicative of such value is provided to the cell's gate and drain (the gate and drain of transistor N2, in the example).
More specifically, the FIG. 1 circuit executes a write operation as follows. Each of address buffers A0 through An asserts one of bits X0-Xn to decoder circuit 12, and each of address buffers An+1 through Ap asserts one of bits Y0-Ym to decoder circuit 14. In response to these address bits, circuit 14 determines a column address (which selects one of the columns of memory cells of array 16, such as column 6A), and circuit 12 determines a row address (which selects one cell in the selected column). In response to a write command (which can be supplied from control unit 29, or other circuitry not shown in FIG. 1), a signal (indicative of data) present at the output of input buffer 18 is asserted through circuit 14 to the cell of array 16 determined by the row and column address (e.g., to the drain of such cell). During such write operation, output buffer 20 may be disabled. A data latch (not shown) is typically provided between input buffer 18 and I/O pad 30 for storing data (to be written to a memory cell) received from I/O pad 30. When the latched data is sent to input buffer 18, input buffer 18 produces a voltage at Node 1 which is applied to the selected memory cell. Input buffer 18 is typically implemented as a tri-statable driver having an output which can be placed in a high impedance mode (and thus disabled) during a read operation. In some implementations, the functions of the latch and input buffer 18 are combined into a single device.
The FIG. 1 circuit executes a read operation as follows. Each of address buffers A0 through An asserts one of bits X0-Xn to address decoder circuit 12, and each of address buffers An+1 through Ap asserts one of bits Y0-Ym to address decoder circuit 14. In response to these address bits, circuit 14 asserts a column address to memory array 16 (which selects one of the columns of memory cells, such as column 16A), and circuit 12 asserts a row address to memory array 16 (which selects one cell in the selected column). In response to a read command (supplied from control unit 29, or from other circuitry not shown in FIG. 1), a current signal indicative of a data value stored in the cell of array 16 (a "data signal") determined by the row and column address is supplied from the drain of the selected cell through the bitline of the selected cell and then through circuit 14 to sense amplifier 19. This data signal is processed in amplifier 19 (in a manner to be described below), and the output of amplifier 19 is buffered in output buffer 20 and finally asserted at I/O pad 30.
When reading a selected cell of array 16, if the cell is in an erased state, the cell will conduct a first current which is converted to a first voltage in sense amplifier 19. If the cell is in a programmed state, it will conduct a second current which is converted to a second voltage in sense amplifier 19 (the "second current" flowing through a programmed cell is negligibly small when the cell is read by a typical, conventional read operation). Sense amplifier 19 determines the state of a cell (i.e., whether it is programmed or erased corresponding to a binary value of 0 or 1, respectively) by comparing the voltage indicative of the cell state to a reference voltage. The outcome of this comparison is an output which is either high or low (corresponding to a binary value of one or zero) which sense amplifier 19 sends to output buffer 20, which in turn asserts a corresponding data signal to I/O pad 30 (from which it can be accessed by an external device).
Nonvolatile memory chip 3 of FIG. 1 can also execute an erase operation in which all or selected ones of the cells of memory array 16 are erased in response to a sequence of one or more commands (e.g., an "Erase Setup" command followed by an "Erase Confirm" command), by discharging a quantity of charge stored on the floating gate of each cell. Typically, all cells of array 16 or large blocks of such cells are erased at the same or substantially the same time during an erase operation. Each erase operation comprises a sequence of steps, including "verification" steps for verifying that the cells have desired threshold voltages at each of one or more stages of the erase operation.
More specifically, if cells of memory array 16 of FIG. 1 are to be erased, an "Erase Setup" command and then an "Erase Confirm" command are sent from an external device to I/O pad 30. Where each such command comprises parallel bits, the different bits are sent in parallel to I/O pad 30 and to additional I/O pads identical to I/O pad 30. The command is transferred from I/O pad 30 (or from I/O pad 30 and additional I/O pads) to input buffer 18 (or input buffer 18 and input buffers connected to the other I/O pads), and then to control unit 29. Control unit 29, which typically includes command execution logic and a state machine, processes the command to generate instruction data, and supplies the instruction data to circuit 14 and sense amplifier 19 (and to other components of memory chip 3 of FIG. 1) to cause chip 3 to execute a sequence of steps required for erasing the specified cells of array 16. These steps include verification steps (e.g., the verification step discussed below with reference to FIG. 5) for verifying that one or more of the cells have desired threshold voltages at each of one or more stages of the erase operation.
During each verification step, verification data (denoted as "RAW VERIFY OK" in FIG. 1) is output from AND gate 22 (in response to the output of sense amplifier 19). This verification data can be fed back to control unit 29. Typically, an external device polls output pads of chip 3 in order to determine whether the erase operation has been completed and whether the erase operation was successful.
More specifically, during verification, the output of sense amplifier 19 is "Anded" with a verification enable signal. I.e., the output of sense amplifier 19 is supplied to one input of AND gate 22, verification enable signal "VERIFY ENABLE" is supplied to the other input of AND gate 22, and the AND gate 22 outputs the signal "RAW VERIFY OK". The signal RAW VERIFY OK is asserted to a state machine (e.g., a state machine within control unit 29) to trigger execution of the next chip operation. The level of signal VERIFY ENABLE is a logical "1" only during each verification cycle. Thus, if the sense amplifier output becomes valid (i.e., a logical "1") at any instant during a verification operation, the signal RAW VERIFY OK is a logical "1" at a corresponding instant (and this instantaneous value of RAW VERIFY OK can cause the state machine to trigger execution of the appropriate chip operation.
A conventional memory erase operation is next described in greater detail with reference to FIG. 3. FIG. 3 is a block diagram of a conventional flash memory integrated circuit 103 (also referred to herein as "system" 103) which is a variation on memory chip 3 of FIG. 1 which performs the same functions as does chip 3. The components of flash memory system 103 which correspond to components of memory chip 3 of FIG. 1 are identified by the same reference numerals as in FIG. 1. Memory array 16 of system 103 consists of flash memory cells arranged in rows and columns with there being a total of 256K of eight bit words in the array. The individual cells (not depicted) are addressed by eighteen address bits (A0-A17), with nine bits being used by X decoder circuit 14 to select the row of array 16 in which the target cell is located and the remaining nine bits being used by Y decoder circuit 14A (of Y-multiplexer 14) to select the appropriate columns of array 16.
Internal state machine 120 (a component of control unit 29 of system 103) controls detailed operations of system 103 such as the various individual steps necessary for carrying out cell programming, reading and erasing operations. State machine 120 thus functions to reduce the overhead required of the processor (not depicted) typically used in association with system 103.
In order for memory array 16 to be erased (typically, all or large blocks of the cells are erased at the same time), the processor must cause Output Enable (OE) pin to be inactive (high), and Chip Enable (CE) pin and Write Enable (WE or "-WE") pin to be active (low). The processor can then issue an 8 bit command 20H (0010 0000) on data I/O pins DQ0-DQ7, typically called an Erase Setup command (one of I/O pins DQ0-DQ7 corresponds to I/O pad 30 of FIG. 1). This is followed by issuance of a second eight bit command D0H (1101 0000), typically called an Erase Confirm command. Two separate commands are used so as to minimize the possibility of an inadvertent erase operation.
The commands are transferred to data input buffer 122 (input buffer 18 of FIG. 1 corresponds to a component of buffer 122 which receives one bit of each command) and the commands are then transferred to command execution logic unit 124. Logic unit 124 then instructs state machine 120 to perform all of the numerous and well known steps for erasing array 16. Once the erase sequence is completed, state machine 120 updates an 8 bit status register 126, the contents of which are transferred to data output buffer 128 which is connected to data I/O pins DQ0-DQ7 of the memory system (output buffer 20 of FIG. 1 corresponds to a component of buffer 128 which receives one bit from register 126). The processor will periodically poll the data I/O pins to read the contents of status register 126 in order to determine whether the erase sequence has been completed and whether it has been completed successfully.
FIGS. 4A and 4B are a flow chart showing a typical erase sequence as it is carried out by state machine 120. It should be noted that during any erase operation, there is a possibility that one or more cells of array 16 will become what is termed "overerased". The objective of the erase sequence is to erase all the cells of array 16 so that the threshold voltages are all within a specified voltage range. That range is typically a small positive voltage range such as +1.5 to +3.0 volts. If the erased cells fall within this range, the cell to be read (the "selected" or "target" cell) will produce a cell current in a read operation. The presence of cell current flow indicates that the cell is in an erased state (logic "1") rather than a programmed state (logic "0"). Cell current is produced in an erased cell because the voltage applied to the control gate of the cell, by way of the word line from the array connected to X decoder 12, will exceed the threshold voltage of the erased cell by a substantial amount. In addition, cells which are not being read ("deselected" cells) are prevented from producing a cell current even if such cells have been erased to a low threshold voltage state. By way of example, for cells located in the same row as the selected cell, by definition, share the same word line as the selected cell. However, the drains of the deselected cells will be floating thereby preventing a cell current from being generated. Deselected cells in the same column will not conduct cell current because the word lines of such deselected cells are typically grounded. Thus, the gate-source voltage of these cells will be insufficient to turn on these deselected cells even if they are in an erased state.
Once array 16 has been erased, the vast majority of its cells will have a proper erased threshold voltage. However, it is possible that a few (or even one) of the cells may have responded differently to the erase sequence and such cell(s) have become overerased. If a cell has been overerased, the net charge on the floating gate will be positive. The result will be that the threshold voltage will be negative to some extent. Thus, when the word line connected to such overerased deselected cell is grounded, the deselected cell will nevertheless conduct current. This current will interfere with the reading of the selected cell thereby preventing proper memory operation. A principal objective of the erase sequence of FIGS. 4A and 4B is to prevent the overerase condition from occurring.
With reference again to the FIG. 4A and 4B flow chart, the erase sequence is initiated (step 28) by the issuance of the two above-noted erase commands. Once the commands have been received by command execution logic 124 (shown in FIG. 3), state machine 120 will first cause all cells of array 16 to be programmed. This is done so that all cells are in essentially the same condition when they are subsequently erased, which reduces the likelihood that one or more of the cells will become overerased since all of the cells will have an increased tendency to respond to the subsequent erase sequence in the same manner. After step 28, an address counter (component 118 of FIG. 3) is initialized (as indicated by block 30) to the first address of the memory. Next, as indicated by block 32, the voltages used for programming are set to the proper level (including by providing high voltage Vpp, e.g. equal to +12 volts, from Vpp switch 121 of FIG. 3 to X and Y decoders 12 and 14A and other components of FIG. 3).
When switch 121 is in a state in which it provides high voltage Vpp to X decoder 12, Y decoder 14A, and other components of FIG. 3, the signal "High V" output from switch 121 is at a high (logical "one") level. Otherwise, the signal "High V" output from switch 121 is at a low (logical "zero") level.
Once the voltages are set, an internal program pulse counter (not depicted) is initialized as shown by block 34 of FIG. 4A. This counter will keep track of the number of programming pulses that have been applied to the cells of the word (byte) being programmed. Next, a programming pulse is applied to the cells of the word located at the first address of the memory, as indicated by block 36. The pulse counter is then incremented (block 38) and a determination is made as to whether a predetermined maximum number of pulses have been applied to the cells (block 40). If that is the case, the cells are read to determine whether the cells have, in fact, been programmed (verification operation 42). This is accomplished using sense amplifiers and associated components represented by block 100 of FIG. 3.
If the cells are still not programmed at this point, there has been a failure since he maximum number of programming pulses has been exceeded. Depending upon the particular memory, the sequence will be terminated or a record of the failed word will be made and the sequence continued. This information will then be transferred to status register 126 (FIG. 3) so that it can be read by the processor. One potential cause of such a failure is that the memory endurance may have been exceeded. In other words, the memory has been cycled too many times.
Assuming that the maximum count has not been exceeded, the byte is verified as indicated by operation 44. If the byte has not been programmed, a further programming pulse is applied (block 36) and the counter is incremented (block 38). Assuming that the maximum count has still not been exceeded, the byte is again verified (operation 44). This sequence will continue until the byte finally passes the verification test or until the pulse counter is at the maximum.
Assuming that the first byte is eventually successfully programmed, a determination is made as to whether the last address of array 16 has been programmed (step 46). If that is not the case, address counter 118 (of FIG. 3) is incremented to the second address (block 48) and the internal pulse counter reset (block 34). A first programming pulse is applied to the byte of the second address (block 36) and the sequence is repeated. This process will continue until all cells of array 16 have either been programmed or until a determination is made that there is a programming failure.
Assuming that all of the cells have been successfully programmed and verified, state machine 120 will continue the erase sequence by setting the appropriate voltages used for erasing, including the initialization of the address counter 118 (block 49 of FIG. 4B) and the setup of the appropriate voltages for erasing, including voltage Vpp (block 50).
Next, an internal erase pulse counter is reset (block 52) and a single erase pulse is applied (block 54) to all of the cells of array 16 (or to the block of the array being erased in the event that capability is provided). The cells of array 16 will then be sequentially read (erase verification step 58) in order to determine whether all cells have been successfully erased. Before step 58, the conditions necessary for erase verification, namely those for cell reading, are set up (block 56) and the first cell of array 16 is read.
A single erase pulse is almost never sufficient to accomplish an erasure so that the test (step 58) will almost always fail. The state of the erase pulse counter is then examined (step 60) and a determination is made that the maximum count has not been exceeded. Accordingly, a second erase pulse is applied to the entire array 16 (step 54) and the first byte is again tested (step 58).
Once the byte has received a sufficient number of erase pulses and has passed the verification test (step 58), the address is incremented (block 60) and the second byte is tested (steps 56 and 58) to determine whether the second byte has been successfully erased. Since the cells are not always uniform, it is possible that the second byte has not be erased even though it has received the same number of erase pulses received by the first byte. In that event, a further erase pulse is applied to the entire array 16 and the second byte is again tested for a proper erase. Note that the address is not reset at this point since it is not necessary to retest those bytes that have already been erased. However, there is a possibility that those earlier erased bytes will become overerased, as will be explained.
Once it has been established that the second byte has been properly erased, a determination is made as to whether the last address of array 16 has been verified (step 62). If that is not the case, address counter 118 is incremented (step 64) and the third byte is tested. Additional erase pulses will be applied if necessary. The internal erase pulse counter (step 60) will monitor the total number of erase pulses applied in the erase sequence. If a maximum number has been exceeded, the sequence will be terminated and one of the bits of status register 126 will be set to reflect that an erase error has occurred.
Assuming that the second byte of cells has been properly erased, the remaining bytes will be verified and any necessary additional erase pulses will be applied. Once the last address has been verified, the erase sequence is ended and status register 126 is updated to indicate that the erase sequence has been successfully completed.
With reference again to FIG. 3 (and to FIG. 5), memory chip 103 includes a conventional circuit 130 (shown in FIG. 3) for monitoring the status of one or more nodes of the chip. In response to detecting a condition which requires that the normal operational flow of the state machine be stopped (denoted below as an "illegal" condition), monitoring circuit 130 asserts a signal (labeled "ILLEGAL" in FIG. 3) having a high level (a logical "one") to command execution logic 124. In the absence of an illegal condition, the signal "ILLEGAL" is at a low level (a logical "zero"). In response to a high level of the signal "ILLEGAL," logic unit 124 (and in particular, logic circuitry 124A shown in FIG. 5 within unit 124) generates a halt signal (labeled "HALT" in FIG. 5) for causing the halting of circuit operation). Conventional logic circuitry 124A is typically implemented using combinational logic. Logic unit 124 asserts the halt signal to state machine 120 and optionally also to other components of chip 103. In response to the halt signal, state machine 120 halts (aborts or otherwise stops) any ongoing memory erase operation.
However, a problem can arise as a result of the described conventional technique for halt signal generation. This problem occurs when the halt signal is generated at a time when high voltage (e.g., voltage V.sub.pp) is applied across the source and drain of one or more transistors of the FIG. 3 chip.
It is well known that many typical transistors (e.g., most MOSFET transistors) can withstand high voltage (e.g., 12 volts) across their terminals. However, when a circuit attempts to switch a typical transistor (e.g., a typical NMOS or PMOS transistor) which is a component thereof by changing the voltage on the transistor's gate while a high voltage is applied across its source and drain, the well-known (and undesirable) phenomenon known as the "snap back bipolar" effect can disrupt operation of the circuit, and can also damage the transistor being switched.
For convenience, the phrase "high voltage mode" is used below to denote an operating mode of a nonvolatile memory chip in which high voltage is applied across source and drain terminals of one or more transistors of the chip (where such "high voltage" is sufficiently high to subject the transistors to the snap back bipolar effect if gates of such transistors are switched with the high voltage across their source and drain terminals). An example of such a "high voltage" is the below-discussed voltage V.sub.pp (equal to 12 volts) which is sometimes applied across PMOS or NMOS transistors within X decoder circuit 12 of FIG. 3, which voltage is much greater than the supply voltage (V.sub.cc =5.0 volts or V.sub.cc =5.5 volts) applied across the terminals of the same transistors during reads of data from memory cells of the same chip.
Also for convenience, the phrase "low voltage mode" (with reference to the same nonvolatile memory chip and the same transistors as in the definition of "high voltage mode") is used below to denote an operating mode of the chip in which "low voltage" is applied across the same terminals of the same transistors (where "low voltage" denotes a voltage, substantially lower than a "high voltage" applied across the terminals in a high voltage mode, and thus sufficiently low so as not to subject the transistors significantly to the snap back bipolar effect if the transistors are switched with the low voltage across their terminals). A typical value of "low voltage" is a supply voltage, V.sub.cc =5.5 volts, applied across the terminals of the transistors during reads of data from memory cells of the chip (where a high voltage of 12 volts is applied across the terminals of the transistors during a high voltage mode). In the example of the previous sentence, the operation of reading data from the memory cells (with V.sub.cc =5.5 volts applied across the terminals of the transistors) is a "low voltage mode" of the chip.
The above-mentioned snap back bipolar effect occurs because of a bipolar effect on a MOS device. This effect is an avalanche hole generated phenomenon which can occur in relatively short channel devices. As source-to-drain separation is reduced, some hole current can flow to the source. If the drain voltage is low, most hole current flows out the substrate terminal. However, with large drain voltages, a substantial hole current can flow to the source, and the product of current flow and substrate resistance becomes large enough (e.g., 0.6 volts) to forward bias the source-substrate junction, thereby causing electron injection into the substrate from the source. This injection leads to an "n-p-n" (source-substrate-drain) bipolar transistor action or a snap back phenomenon. The emitter-base junction of the transistor is self biased. Any current across the reverse-biased collector-base junction is multiplied by current gain .beta. of the bipolar transistor. With reference to FIG. 8 (which shows a family of "drain current" v. "drain voltage" curves, each curve for a different indicated value of gate voltage), this current multiplication results in both a negative resistance characteristic and lower voltage of the BV.sub.CEO.
If a halt signal of the type described above is generated during a high voltage mode of the FIG. 3 chip, at least some NMOS or PMOS transistors (having high voltage across their sources and drains) will be switched in the course of the chip's response to the halt signal. As a result, the snap back bipolar effect will not only disrupt operation of the chip, but may also damage each transistor that is switched during the high voltage mode with high voltage applied across its terminals.
One possible solution to this problem (the problem of damage and disruption due to switching one or more transistors during a high voltage mode) is to ignore any illegal condition and avoid generating any halt signal during a high voltage mode. However, it is impractical to implement this "solution" in a nonvolatile memory chip for the following reason.
During almost the entire duration of a memory erase operation of a nonvolatile memory chip (in which memory array cells of the chip are erased), the chip is in a high voltage mode. More specifically, as described above with reference to FIGS. 4A and 4B, an erase operation includes setup steps (such as steps 30, 32, and 34 of FIG. 4A and steps 49, 50, and 52 of FIG. 4B), programming and erasing steps (step 36 of FIG. 4A and step 54 of FIG. 4B), and verification steps (such as steps 42 and 44 of FIG. 4A and steps 56 and 58 of FIG. 4B). The chip is in a high voltage mode during the programming and erasing steps, and these steps typically consume almost the entire duration of an overall erase operation (programming step 36 typically requires on the order of ten microseconds while the setup and verification steps of FIG. 4A typically consume a total of about one microsecond, and erase step 54 typically requires on the order of tens of milliseconds while the setup and verification steps of FIG. 4B typically consume a total of about one microsecond). Thus, an undesirable consequence of operating a nonvolatile memory chip while ignoring any illegal condition that occurs (and without generating any halt signal) during a high voltage mode is that the chip could not generate a halt signal during almost the entire duration of an erase operation.-As a result, the chip would almost always fail to halt an erase operation despite the occurrence of an illegal condition during the erase operation, and it would thus be impractical to operate the chip in this manner.
Until the present invention, it had not been known to design or operate a nonvolatile memory chip so that the chip generates a halt signal (and responds to the halt signal) each time an illegal condition has occurred (whether or not the illegal condition has occurred during a high voltage mode) while also avoiding the described snap back bipolar effect problem.