Sense amplifier for semiconductor memory device

Abstract: A semiconductor device includes a nonvolatile memory cell and a current detecting type sense amplifier for detecting a current flowing through a data line into the memory cell. The semiconductor device is further provided with an element for outputting a first voltage detecting signal when a detected supply voltage exceeds a set value and outputting a second voltage detecting signal when the detected supply voltage does not exceed the set value. The sense amplifier includes an element for switching a dependent characteristic of a level sensing current upon the supply voltage to be higher in response to the second voltage detecting signal that in response to the first voltage detecting signal. Thus, error reading of a data from the memory cell, which can be otherwise caused under application of a low supply voltage, can be avoided.

Description: The present invention refers to a semiconductor device comprising a memory cell, a data line connected to the memory cell, voltage supplying means for supplying a supply voltage, control signal input means and current detecting means. From US-A-4 758 748 a sense amplifier for a programmable read only memory having a sense current source circuit for supplying memory cells in the memory with the currents is known. The sense current source circuit is adapted to supply memory cells with a cell current both for irregular reading and a verify reading. A reference value of the cell current for the regular reading is selected to be higher than a reference value of the cell current for the verify reading, whereby sufficient value of margin of voltage of a power source for the memory is endured. The arrangement comprises a memory cell, a data line connected to the memory cell, voltage supplying means for supplying supply voltage, supply voltage detecting means connected to the voltage supplying means for detecting at least two supply voltage regions separated by at least one set value and outputting a plurality of kinds of voltage detecting signals corresponding to the respective supply voltage regions, control signal input means for entering a first control signal for giving an instruction to read a data from the memory cell, current detecting means connected to the control signal input means, the voltage supply means and the data line, for detecting, in response to the first control signal, a current flowing from the voltage supplying means to the data line by using a level sensing current in accordance with the supply voltage, the current detecting means having a plurality of different dependent characteristics as a dependent characteristic of the level sensing current upon the supply voltage, wherein a reference value of a current for irregular reading in operation of a flash memory is selected to be higher than a reference value of the current for a verify reading, i.e. the reference value is switched dependent of the level sensing current. US-A-4 774 692 describes a sense circuit of a semiconductor memory transistor including a bit line connected to a memory cell which stores "1" and "0". The sense circuit includes a MOS transistor which has its gate connected to the bit line, its source connected to ground voltage and its ? connected to supply voltage through a load MOS transistor. The sense circuit also includes a compensating circuit for compensating the voltage at the bit line when the ground voltage has fluctuated. Recently, there is increasing demand for a nonvolatile memory that is electrically writable under application of a low voltage. A sense amplifier is one of the important technique of such a low voltage operable nonvolatile memory. A conventional sense amplifier will be herein exemplified. A circuit diagram for the sense amplifier shown in Figure 16 will be described first. The sense amplifier receives a control signal SAE and a data line signal DL at its two input terminals, and outputs an output signal OUT through its output terminal. In this sense amplifier, an inverter INV1 receives the control signal SAE at its input terminal and outputs a signal N1 generated by inverting the control signal SAE. A CMOS inverter including a P-channel MOS transistor Qp1 and an N-channel MOS transistor Qn1 inverts the output signal N1 from the inverter INV1. The output signal N1 from the inverter INV1 is applied to the gates of the transistors Qp1 and Qn1. The source of the P-channel MOS transistor Qp1 is connected to a terminal for supplying a supply voltage VDD, and the source of the N-channel MOS transistor Qn1 is grounded. The drains of the transistors Qp1 and Qn1 are connected to a common signal line, through which a signal N2 generated by inverting the signal N1 is transferred. P-cannel MOS transistors Qp2 and Qp3 receive the output signal N1 from the inverter INV1 at their gates, and N-channel MOS transistors Qn2 and Qn3 receive the output signal N2 from the CMOS inverter at their gates. The sources of the P-channel MOS transistors Qp2 and Qp3 are respectively connected to terminals for supplying a supply voltage VDD, and the sources of the N-channel MOS transistors Qn2 and Qn3 are connected to a common data line (a signal line L1). The drains of the transistors Qp2 and Qn2 are connected to each other, and the drains of the transistors Qp3 and Qn3 are connected to each other. A signal N3 is output through a signal line connecting the transistor Qp3 to the transistor Qn3. A P-channel MOS transistor Qp4 receives a ground potential at its gate, an N-channel MOS transistor Qn4 receives the signal N3 at its gate, and an N-channel MOS transistor Qn5 receives the control signal SAE at its gate. The source of the transistor Qp4 is connected to a terminal for supplying a supply voltage VDD, and the drain thereof is connected to the drain of the transistor Qn4. The source of the transistor Qn4 is connected to the drain of the transistor Qn5, and the source of the transistor Qn5 is grounded. These three transistors Qp4, Qn4 and Qn5 together serve as an output unit. The output signal OUT of the sense amplifier is output through a signal line connecting the drain of the transistor Qp4 to that of the transistor Qn4. An N-cannel MOS transistor Qn6 is disposed between the gate of the transistor Qn4 and the ground so as to release charge from the gate of the transistor Qn4. An N-cannel MOS transistor Qn8 is disposed between the gate of the transistor Qn2 and the ground, and the gate of the transistor Qn8 is connected to the data line (the signal line L1). An N-channel MOS transistor Qn7 is disposed between the signal line L1 and the ground, and the gate of the transistor Qn7 is supplied with the output signal N1 from the inverter INV1. The operation of this sense amplifier will now be described. When the control signal SAE is at a high level and no current flows from the data line (signal line L1) to the ground, the sense amplifier is operated as follows: Since the output signal N1 of the inverter INV1 is at a low level, the transistor Qp1 of the CMOS inverter is turned off and the transistor Qn1 is turned on, resulting in increasing the potential level of the output signal N2 from the CMOS inverter. This turns on the subsequent transistors Qp2, Qp3, Qn2 and Qn3, but the transistor Qn8 starts to be turned on because the potential level of the data line signal DL is high. Because the transistor Qn8 starts to be turned on, the potential level of the signal N2 is decreased and the transistors Qn2 and Qn3 start to be turned off. As a result, the potential level of the signal N3 becomes high. This makes the gate potential of the transistor Qn4 high, thereby allowing the transistor Qn4 to operate. Thus, an output signal OUT is output at a low level. When the control signal SAE is at a high level and a current flows from the data line signal DL to the ground, the sense amplifier is operated as follows: Since the potential level of the output signal N2 from the CMOS inverter becomes high as in the aforementioned case, the subsequent transistors Qp2, Qp3, Qn2 and Qn3 are all turned on. Nevertheless, the potential of the data line signal DL becomes low because the memory cell connected to the data line (signal line L1) is in an on state, and the transistor Qn8 is kept in an off state. This results in a high potential level of the signal N2 and a low potential level of the signal N3, thereby turning off the transistor Qn4. Thus, an output signal OUT is output at a high level. When the control signal SAE is at a low level, all the transistors Qp2, Qp3, Qn2 and Qn3 are turned off, resulting in turning off the transistor Qn5. Therefore, the output unit outputs the output signal OUT at a high level. In this case, since both the transistors Qn6 and Qn7 are on, charge is released to the ground from the gates of the transistors Qn4 and Qn8, resulting in keeping low the gate potential of the transistors Qn4 and Qn8. For attaining a high speed operation, the sizes of the transistors Qp3 and Qn3 are made small so as to minimize the capacitance of the signal N3. Since the transistors Qp3 and Qn3 are disposed previously to the output unit, the load on the transistor Qn4 in the output unit is decreased. Since the transistors Qp2 and Qn2 are disposed previously to the transistors Qp3 and Qn3, the charge is rapidly supplied to the data line. This increases the operation speed of the circuit together with the operation of the transistor Qn8. Figure 17 is a characteristic diagram showing the relationship between a supply voltage VDD and a level sensing current in this sense amplifier. The level sensing current herein indicates a current that flows through the data line when the logical voltage of the output signal OUT of the sense amplifier undergoes a transition, i.e., when the transistor Qn4 is turned on/off. As described above, when a current with a level exceeding a predetermined level flows through the data line, the logical voltage of the output signal OUT undergoes a high transition, and when the current with a level exceeding the predetermined level does not flow through the data line, the logical voltage of the output signal OUT undergoes a low transition. In other words, a memory cell having a memory cell on current exceeding a level sensing current is in a low threshold state, and a memory cell having a memory cell on current not exceeding the level sensing current is in a high threshold state. As is shown with a curve VSALC1 in Figure 17, the level sensing current increases as the supply voltage VDD increases in the conventional sense amplifier. In addition, the increase of the supply voltage VDD also causes the increase of the increasing ratio of the level sensing current. Figure 18 is a characteristic diagram showing the relationship between a supply voltage and a data line voltage. As is shown with a characteristic line VDLC1 in Figure 18, the data line voltage increases as the supply voltage VDD increases. Figure 19 is a characteristic diagram showing the relationship among a supply voltage VDD, a level sensing current and a memory cell current. A curve SALVO shows the variation characteristic of a level sensing current, and a curve MCONO shows the variation characteristic of a memory cell on current. The memory cell on current herein indicates a characteristic against a supply voltage of a current, which flows between the source and the drain of a FET forming a nonvolatile memory cell when the floating gate is not charged negatively (i.e., in an erasing state). As is shown in Figure 19, the characteristic curve SALEVO of the level sensing current has a shape swelling downward similarly to the characteristic curve shown in Figure 17. The memory cell current generally has a similar characteristic to a current flowing between the source and the drain of a MISFET, and hence, the characteristic curve MCONO of the memory cell current increases as the supply voltage increases but eventually reaches a saturation state, i.e., has a shape swelling upward. There exists a memory cell off current (or a memory cell off leak current) in contrast with a memory cell on current. A memory cell off current herein indicates a current that flows through a memory cell when it is not supposed to flow. Such a current is caused in the following two cases: First, when a FET forming a nonvolatile memory cell is negatively charged (i.e., in a writing state), namely, when the threshold value of a memory cell transistor is set to be high, the control gate of the memory cell is selected. In such a case, since the negative charge is insufficient and the threshold value is insufficiently high, a current flows through the memory cell. Secondly, although the control gate of the memory cell is not selected, a current flows through the memory cell because the memory cell transistor is of a depletion mode. In a sense amplifier, the level sensing current is adjusted to have an intermediate value between the memory cell on current and the memory cell off current by adjusting the threshold values of the transistors. Furthermore, a flash memory stores a data by setting the threshold value of a memory cell transistor within a predetermined range by charging (for a writing operation) or discharging (for an erasing operation) a floating gate. For example, when two kinds of data are desired to be stored using one memory cell, two kinds of threshold values, "H" and "L", suffice. When three kinds of data are desired to be stored using one memory cell, an intermediate threshold value is required to be set as well as "H" and "L". At this point, it is necessary to confirm whether or not the threshold value has been appropriately set, which operation is designated as verification. The verification is performed as follows: In the verification of a memory cell in a writing state, i.e., having a high threshold value, it is determined whether or not a current flows through the memory cell using the level sensing current, with the word line of the memory cell selected. When it is determined that the level sensing current does not flow and the sense amplifier outputs an output signal OUT at a low level, it is confirmed that the desired high threshold value has been set in the memory cell. In the verification of a memory cell in an erasing state. i.e., having a low threshold value, it is also determined whether or not a current flows through the memory cell using the level sensing current, with the word line of the memory cell selected. When it is determined that the level sensing current flows and the sense amplifier outputs an output signal OUT at a high level, it is confirmed that the low threshold value has been set in the memory cell. In the verification of the memory cell in an erasing state, however, it is necessary to further confirm that the level sensing current does not flow when the memory cell is not selected. The reason is as follows: An off leak current can flow through an unselected memory cell if the memory cell is of a depletion mode as described above. A plurality of memory cells are connected to one data line, and during the verification of one memory cell in a writing state, an off leak current can flow through another memory cell that is in an erasing state and is not selected. Such an off leak current can decrease the reliability of the verification. A semiconductor device including the above-mentioned sense amplifier has the following problems: First, the sense amplifier has a characteristic that, as the supply voltage VDD increases, the level sensing current increases and in addition the increasing ratio of the level sensing current also increases. This results in that the level sensing current is extremely small when the supply voltage is low. In, for example, a flash EEPROM that stores a data by varying the threshold voltage of a memory cell transistor, a slight current can flow through the memory cell transistor even when the threshold voltage thereof is set to be so high that a current is prevented from flowing therethrough. Accordingly, when the level sensing current is small under application of a low supply voltage as described above, it is difficult to distinguish from a current flowing through a memory cell transistor having a low threshold value from a leak current flowing through a memory cell transistor having a high threshold value, in reading a data from the memory cell. This can cause error discrimination of a data stored in the memory cell by the sense amplifier. Furthermore, when the supply voltage VDD is low, the memory cell on current is decreased, and hence, error discrimination can also be caused. There also arises a problem that a longer time is required for the discrimination by the sense amplifier. Secondly, the data line voltage increases as the supply voltage increases. In, for example, a flash EEPROM that stores a data by varying the threshold voltage of a memory cell transistor, a current is allowed to flow through the memory cell transistor when a voltage is applied to the gate of the memory cell transistor whose threshold voltage is set to be low. In such a case, a high data line voltage can increase the threshold voltage of the memory cell transistor. The increase of the threshold voltage can decrease the current flowing under application of a voltage to the gate of the memory cell transistor. This also can lead to error discrimination by the sense amplifier as in the aforementioned case. Thirdly, a value of the level sensing current is univocally defined with regard to one supply voltage value. Therefore, it is impossible to determine with margin included whether or not a memory cell current in an on state is larger than a level sensing current value during verification. Accordingly, in, for example, a flash EEPROM that stores a data by varying the threshold voltage of a memory cell transistor, it is impossible to determine whether or not the memory cell current in the on state has sufficient margin even when the threshold voltage is set to be sufficiently low to allow a current to flow. Furthermore, in such a flash EEPROM, there arises another problem that frequent change of the threshold voltage of the memory cell transistor can cause a failure of the flash EEPROM because the memory cell current in the on state decreases as compared with that in the initial state. Fourthly, since a value of the level sensing current is univocally defined with regard to one supply voltage value, it is impossible to determine with margin included whether or not a memory cell current in an off state is smaller than a level sensing current value. Therefore, in, for example, a flash EEPROM that stores a data by varying the threshold voltage of a memory cell transistor, a slight off current (a leak current) can flow through the memory cell transistor when the memory cell transistor is turned off by grounding the gate thereof. Such an off current increases at a higher temperature, which can disadvantageously causes error discrimination and a failure in the conventional sense amplifier. SUMMARY OF THE INVENTION The present invention was devised in order to overcome the aforementioned problems, and has the following objectives: The object is providing a semiconductor device mounting a sense amplifier that can attain a high level sensing current even under application of a low supply voltage. In order to attain the object, the semiconductor device of this invention comprises the features of claim 1. Preferred embodiments thereof are subject matter of the dependent claims. Owing to this configuration, it is possible to switch the dependent characteristic of the level sensing current of the current detecting means upon the supply voltage in accordance with the kind of the voltage detecting signals corresponding to the respective supply voltage regions. Accordingly, error discrimination due to the variation of the supply voltage can be prevented in reading a data from the memory cell in the semiconductor device. In one aspect, when the current detecting means receives a voltage detecting signal corresponding to a lower supply voltage region from the supply voltage detecting means, a dependent characteristic with a higher level sensing current is selected than a level sensing current selected when a voltage detecting signal corresponding to a higher supply voltage region is received. Owing to this configuration, the level sensing current of the current detecting means is higher in the lower supply voltage region, and hence, the semiconductor device is stably operated even when a larger leak current is present in the memory cell, and error discrimination is effectively prevented. Furthermore, the supply voltage detecting means can allow the voltage detecting signal to have a hysteresis characteristic. In this case, the semiconductor device can be stably operated even when a voltage with a value approximate to a set value is used as the supply voltage. Preferably, the supply voltage detecting means changes the kind of a voltage detecting signal to be output when the current detecting means is not operated. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an electric circuit diagram of a sense amplifier according to Embodiment 1; Figure 2 is an electric circuit diagram of a voltage detector of Embodiment 1; Figure 3 is a characteristic diagram showing the relationship between a level sensing current and a supply voltage in the sense amplifier of Embodiment 1; Figure 4 is a characteristic diagram showing the relationship between an output signal and a supply voltage in the voltage detector of Embodiment 1; Figure 5 is an electric circuit diagram of a voltage detector of Embodiment 2; Figure 6 is a characteristic diagram showing the relationship between a level sensing current and a supply voltage in a sense amplifier of Embodiment 2; Figure 7 is a characteristic diagram showing the relationship between an output signal and a supply voltage in the voltage detector of Embodiment 2; Figure 8 is an electric circuit diagram of a sense amplifier of Embodiment 3; Figure 9 is a characteristic diagram showing the relationship between a data line voltage and a supply voltage in Embodiment 3; Figure 10 is an electric circuit diagram of a sense amplifier of Embodiment 4; Figure 11 is a characteristic diagram showing the relationship between a data line voltage and a supply voltage in Embodiment 4; Figure 12 is an electric circuit diagram of a sense amplifier of Embodiment 5; Figure 13 is a characteristic diagram showing the relationship between a data line voltage and a supply voltage in Embodiment 5; Figure 14 is an electric circuit diagram of a sense amplifier of Embodiment 6; Figure 15 is an electric circuit diagram of a voltage detector of Embodiment 7; Figure 16 is an electric circuit diagram of a conventional sense amplifier; Figure 17 is a characteristic diagram showing the relationship between a level sensing current and a supply voltage in the conventional sense amplifier; Figure 18 is a characteristic diagram showing the relationship between a data line voltage and a supply voltage in the conventional sense amplifier; and Figure 19 is a characteristic diagram showing the relationship among a level sensing current, a memory cell current and a supply voltage in the conventional sense amplifier. DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described by way of examples. (Embodiment 1) The configuration of a semiconductor device of Embodiment 1 will be first described referring to Figures 1 and 2, which are electric circuit diagrams showing the configurations of a sense amplifier and a voltage detector of this embodiment, respectively. The configuration of the sense amplifier of Figure 1 is fundamentally identical to that of the conventional sense amplifier shown in Figure 16. This sense amplifier receives a control signal SAE and a data line signal DL at its input terminals and outputs an output signal OUT through its output terminal. As does the conventional sense amplifier of Figure 40, this sense amplifier comprises an inverter INV1, two transistors Qp1 and Qn1 together working as a CMOS inverter, two transistors Qp2 and Qn2 for attaining a rapid operation of the circuit, two transistors Qp3 and Qn3 for decreasing the load on an output unit, three transistors Qp4, Qn4 and Qn5 together working as the output unit, and transistors Qn6, Qn7 and Qn8 operated for releasing charge from each signal line to the ground. These elements are disposed and operated in the same manner as described above referring to Figure 16, and hence the description is omitted. This sense amplifier further comprises, as one of the characteristics of this embodiment, an element for varying the characteristic of a level sensing current by using a voltage detecting signal. Specifically, the sense amplifier further comprises an inverter INV2 that receives a voltage detecting signal VDTO and outputting a signal N7 generated by inverting the voltage detecting signal VDTO, a NAND circuit NAND1 that receives an output signal N7 of the inverter INV2 and the control signal SAE and outputs a signal N6 generated through the NAND operation of these signals, and a P-channel MOS transistor Qp11 that receives an output signal N6 of the NAND circuit NAND1 at its gate. The drain of the P-channel MOS transistor Qp11 is connected to a terminal for supplying a supply voltage VDD, and the source thereof is connected to a signal line leading to the gate of the transistor Qn4 in the output unit. Owing to this configuration, the transistor Qp11 is operated in accordance with the voltage detecting signal VDTO, thereby increasing the potential level of the signal N3. Figure 2 is a circuit diagram of the voltage detector for generating the voltage detecting signal VDT0 to be input to the sense amplifier. As is shown in Figure 2, between a terminal for supplying a supply voltage VDD and the ground are disposed three P-channel MOS transistors, Qp21, Qp22 and Qp23 and an N-channel MOS transistor Qn21 connected in series. The gates of the transistors Qp21, Qp22 and Qp23 are connected to their own drains, respectively, and the gate of the transistor Qn21 is connected to a terminal for supplying a supply voltage VDD. The drains of the transistors Qp23 and Qn21 are connected to a common signal line, through which a signal N23 is transferred. The voltage detector further comprises a CMOS inverter including a P-channel MOS transistor Qp24 and an N-channel MOS transistor Qn24 that receive the signal N23 at their gates. The source of the transistor Qp24 in the CMOS inverter is connected to a terminal for supplying a supply voltage VDD, and the source of the N-channel MOS transistor Qn24 is grounded. The CMOS inverter outputs a signal N24 generated by inverting the signal N23 input to the gates of the transistors Qp24 and Qn24. An inverter INV21 disposed subsequently to the CMOS inverter generates the voltage detecting signal VDTO by inverting the signal N24. This voltage detector switches the level of the voltage detecting signal VDTO, by adjusting the threshold values of the respective transistors, between the cases where the supply voltage VDD is below and exceeds a set value of 4 V. Referring to Figures 3 and 4, the operation of the sense amplifier of this embodiment will be described. Figure 3 is a characteristic diagram showing the relationship between a level sensing current and a supply voltage VDD in the sense amplifier. Figure 4 is a characteristic diagram showing the relationship between the output signal VDT0 of the voltage detector and the supply voltage VDD. As is shown in Figure 4, when the supply voltage VDD is below 4 V, the logical value of the output signal VDT0 of the voltage detector is "L". When the supply voltage VDD exceeds 4 V, the logical value of the output signal VDTO is switched to be "H". The level sensing current is discontinuously varied in accordance with whether the logical value of the voltage detecting signal VDT0 is "H" or "L". Specifically, when the logical value of the voltage detecting signal VDTO is "H" in this sense amplifier, the logical value of the signal N6 is "H" and the transistor Qp11 is not operated. In such a case, this sense amplifier has an output characteristic similar to that of the conventional sense amplifier of Figure 16, which is indicated as a curve VSALC1 in Figure 3. When the logical value of the control signal SAE is "H" and the logical value of the voltage detecting signal VDTO is "L", the logical value of the signal N6 is "L" and the transistor Qn11 is operated, thereby increasing the potential level of the signal N3. Through this operation, the characteristic of the sense amplifier is switched to be that shown as a curve VSALC2 in Figure 3, which is obtained by moving the curve VSALC1 upward. Specifically, when the supply voltage VDD is as low as 4 V or less (i.e., in a low voltage region), the level sensing current exhibits the characteristic according to the curve VSALC2, and when the supply voltage VDD is as high as to exceed 4 V (i.e., in a high voltage region), the level sensing current exhibits the characteristic according to the curve VSALC1. Accordingly, the sense amplifier of this embodiment can effectively prevent error data discrimination even when a memory cell transistor has a slight leak current because the level sensing current is sufficiently large even when the supply voltage is low. (Embodiment 2) Now, Embodiment 2 will be described referring to Figures 5 through 7. The configuration of a sense amplifier of this embodiment is identical to that of Embodiment 1 shown in Figure 1, and the configuration of a voltage detector is different from that of Embodiment 1 shown in Figure 2. Figure 5 is a circuit diagram of the voltage detector for the semiconductor device of this embodiment. This voltage detector is similar to that of Embodiment 1 shown in Figure 2 in comprising three P-channel MOS transistors Qp21 through Qp23, an N-channel MOS transistor Qn21, a CMOS inverter including a P-channel MOS transistor Qp24 and an N-channel MOS transistor Qn24 and an inverter INV21. The voltage detector further comprises a signal line connecting the drain of the transistor Qp22 to the source of the transistor Qp23, and a CMOS inverter including a P-channel MOS transistor Qp25 and an N-channel MOS transistor Qn25 that receive a signal N22 transferred through the signal line at their gates. The voltage detector further comprises an inverter INV22 that receives an output signal N25 of the CMOS inverter and outputs a signal VDT2 generated by inverting the output signal N25. The voltage detector is additionally provided with an N-channel MOS transistor Qn26 and a P-channel MOS transistor Qp26 that receive the output signals VDT1 and VDT2 of the inverters INV21 and INV22 at their gates, respectively. The source of the transistor Qp26 is connected to a supply voltage VDD, the source of the transistor Qn26 is grounded, and a signal N26 is output through a signal line commonly connected to the drain of the transistor Qp26 and that of the transistor Qn26. Furthermore, the voltage detector is provided with an inverter INV23 that receives the signal N26 and outputs an output signal VDT0 generated by inverting the signal N26, and an inverter INV24 that inverts the output signal VDT0 of the inverter INV23 again and transfers the resultant signal to the input port of the inverter INV23. It is noted that, through the adjustment of the threshold values of the respective transistors, the output signal VDT1 of the inverter INV21 is at a low level when the supply voltage VDD is 3.5 V or less, and is at a high level when the supply voltage VDD exceeds 3.5 V, as is shown in Figure 7 described in detail below. It is also noted that, through the adjustment of the threshold values of the respective transistors, the output voltage VDT2 of the inverter INV22 is at a low level when the supply voltage VDD is 4.5 V or less, and is at a high level when it exceeds 4.5 V. The voltage detector is functionally characterized by that it outputs the voltage detecting signal VDTO having a hysteresis characteristic against the supply voltage by using the two voltage detecting signals VDT1 and VDT2. Figure 6 is a characteristic diagram showing the relationship between the supply voltage and the level sensing current obtained when the voltage detecting signal VDTO output by the voltage detector of Figure 5 is supplied to the sense amplifier of Figure 1, and Figure 7 is a characteristic diagram showing the relationship between the supply voltage for the voltage detector and the voltage detecting signal. As is shown in Figure 7, the voltage detecting signal VDTO ultimately output by the voltage detector has the following logical voltages: (1) When VDD≤ 3.5 V, the voltage detecting signal VDTO is at a low level; (2) When VDD > 4.5 V, the voltage detecting signal VDTO is at a high level; and (3) When 3.5 V < VDD ≤ 4.5 V, (A) in the case where the supply voltage VDD has been increased from a voltage below 3.5 V, the voltage detecting signal VDTO is at a low level, and (B) in the case where the supply voltage VDD has been decreased from a voltage exceeding 4.5 V, the voltage detecting signal is at a high level. The characteristic of the level sensing current is discontinuously varied as is shown in Figure 6 in accordance with the aforementioned operation of the voltage detecting signal VDTO. Specifically, the level sensing current curve is switched between the curves VSALC1 and VSALC2 in accordance with the change of the supply voltage as follows: (1) When VDD ≤ 3.5 V, the level sensing current curve VSALC2 is exhibited; (2) When VDD > 4.5 V, the level sensing current curve VSALC1 is exhibited; and (3) When 3.5 < VDD ≤ 4.5, (A) in the case where the supply voltage VDD has been increased from a voltage below 3.5 V, the level sensing current curve VSALC2 is exhibited, and (B) in the case where the supply voltage VDD has been decreased from a voltage exceeding 4.5 V, the level sensing current curve VSALC1 is exhibited. In this manner, similarly to the sense amplifier of Embodiment 1, the sense amplifier of Embodiment 2 including such a voltage detector can discriminate a data without an error even when a memory cell transistor has a slight leak current since the level sensing current is not decreased as low as that in the conventional sense amplifier even under application of a low voltage. In addition, the voltage at which the level sensing current curves are switched is allowed to have a hysteresis characteristic against the supply voltage VDD. Therefore, when two types of devices respectively adopting a supply voltage of, for example, 3 V and 5 V are provided by using this semiconductor device, the 3V device has a fixed level sensing current curve of VSALC2 at a supply voltage of 4.5 V or less, and the 5 V device has a fixed level sensing current curve of VSALC1 at a supply voltage of 3.5 V or less. As a result, the resultant devices can be stably operated even when the supply voltages are somewhat varied. (Embodiment 3) Now, Embodiment 3 will be described referring to Figures 8 and 9. Figure 8 is an electric circuit diagram of a sense amplifier for a semiconductor device of this embodiment. Also in this embodiment, the configuration and operation of a fundamental portion of the sense amplifier is similar to that of the conventional sense amplifier shown in Figure 16. This sense amplifier additionally comprises a P-channel MOS transistor Qp12 and an N-channel MOS transistor Qn12 that receive a voltage detecting signal VDTO at their gates. The source of the transistor Qp12 is connected to a terminal for supplying a supply voltage VDD, and the drain thereof is connected to the source of a transistor Qp2 disposed in the fundamental portion. The drain of the transistor Qn12 is connected to a terminal for supplying a supply voltage VDD, and the source thereof is connected to the source of a transistor Qp3 disposed in the fundamental portion. A signal line connecting the drain of the transistor Qp12 to the source of the transistor Qp2 is connected to a signal line connecting the source of the transistor Qn12 to the source of the transistor Qp3 via a common signal line L11. The case where, for example, the voltage detector shown in Figure 2 is used for the sense amplifier of this embodiment will be exemplified. As is described in Embodiment 1 referring to Figure 4, the voltage detecting signal VDTO is at a low level under application of a low voltage, and is at a high level under application of a high voltage. When the voltage detecting signal VDT0 is at a high level, the transistor Qp12 is off and the transistor Qn12 is on. Therefore, the source potential of the transistors Qp2 and Qp3 is decreased by the threshold value of the transistor Qn12. When the voltage detecting signal VDT0 is at a low level, the transistor Qp12 is on and the transistor Qn12 is off. Therefore, the source potential of the transistors Qp2 and Qp3 is equal to the supply voltage VDD. Thus, the source potential of the transistors Qp2 and Qp3 is varied by turning on/off the transistors Qp12 and Qn12. The voltage characteristic of a data line (i.e., a signal line L1) is changed by using this variation of the source potential. Therefore, similarly to the conventional sense amplifier, when a current flows through the data line (signal line L1), the output signal OUT is output at a high level, and when no current flows through the data line (signal line L1), the output signal OUT is output at a low level. The data line voltage, however, is discontinuously varied depending upon the logical value of the voltage detecting signal VDTO. Figure 9 is a characteristic diagram showing the relationship between the data line voltage and the supply voltage VDD obtained when the voltage detecting signal VDT0 output by the voltage detector of Figure 2 is used as an input signal for the sense amplifier. As is shown in Figure 9, when the supply voltage VDD is as low as 4 V or less, the data line voltage is changed in accordance with a characteristic line VDLC1. When the supply voltage VDD is as high as to exceed 4 V, the data line voltage is changed in accordance with a characteristic line VDLC2 obtained by moving the characteristic line VDLC1 downward. Thus, in the semiconductor device of Embodiment 3, the threshold voltage of a memory cell transistor is prevented from increasing because the data line voltage does not become excessively high. As a result, a current under application of a voltage to the gate of the memory cell transistor never decreases, resulting in effectively preventing error discrimination by the sense amplifier. (Embodiment 4) Now, Embodiment 4 will be described referring to Figures 10 and 11. Figure 10 is an electric circuit diagram of a sense amplifier for a semiconductor device of Embodiment 4. Also in this embodiment, the configuration and operation of a fundamental portion of the sense amplifier is similar to that of the conventional sense amplifier of Figure 16. The sense amplifier of this embodiment additionally comprises a P-channel MOS transistor Qp31 and an N-channel MOS transistor Qn31 that receive a voltage detecting signal VDTO at their gates. The source of the transistor Qp31 is connected to a terminal for supplying a supply voltage VDD and the drain thereof is connected to the source of a transistor Qp1 disposed in the fundamental portion. The drain of the transistor Qn31 is connected to a terminal for supplying a supply voltage VDD, and the source thereof is connected to the source of the transistor Qp1 together with the drain of the transistor Qp31. The case where, for example, the voltage detector of Figure 2 is used for this sense amplifier will be exemplified. As is described in Embodiment 1 referring to Figure 4, the voltage detecting signal VDTO is at a low level under application of a low voltage, and is at a high level under application of a high voltage. When the voltage detecting signal VDTO is at a high level, the transistor Qp31 is off and the transistor Qn31 is on. When the voltage detecting signal VDTO is at a low level, the transistor Qp31 is on and the transistor Qn31 is off. Thus, the source potential of the transistor Qp1 is varied by turning on/off the transistors Qp31 and Qn31. The voltage characteristic of a data line (i.e., a signal line L1) is changed by using this variation of the source potential of the transistor Qp1. The sense amplifier of this embodiment is operated similarly to that of Embodiment 3. Figure 11 is a characteristic diagram showing the relationship between the data line voltage and the supply voltage VDD in this embodiment. As is shown in Figure 11, the data line voltage is changed in accordance with a characteristic line VDLC1 under application of a low voltage of 4 V or less, and is changed in accordance with a characteristic line VDLC2 obtained by moving the characteristic line VDLC1 downward under application of a high voltage exceeding 4 V. Therefore, the sense amplifier of this embodiment can attain the same effect as that of Embodiment 3. (Embodiment 5) Now, Embodiment 5 will be described referring to Figures 12 and 13. Figure 12 is an electric circuit diagram of a sense amplifier for a semiconductor device of Embodiment 5. Also in this embodiment, the configuration and operation of a fundamental portion of the sense amplifier is similar to that of the conventional sense amplifier of Figure 16. The sense amplifier of this embodiment additionally comprises an inverter INV41 that receives a voltage detecting signal VDTO and generates a signal N41 by inverting the voltage detecting signal VDTO, a NAND circuit NAND41 that receives the signal N41 and a control signal SAE and outputs a signal N42 generated through the NAND operation of these signals, and a P-channel MOS transistor Qp41 that receives the signal N42 at its gate. The source of the transistor Qp41 is connected to a terminal for supplying a supply voltage VDD, and the drain thereof is connected to a signal line through which an output signal N2 of a CMOS inverter including transistors Qp1 and Qn1 in the fundamental portion flows. The case where, for example, the voltage detector of Figure 2 is used for this sense amplifier will be exemplified. As is described in Embodiment 1 referring to Figure 4, the voltage detecting signal VDTO is at a low level under application of a low voltage, and is at a high level under application of a high voltage. Merely when the control signal SAE is at a high level and the voltage detecting signal VDTO is at a low level, the transistor Qp41 is on, and in the remaining cases the transistor Qp41 is off. The potential of the signal N2 is varied by turning on/off the transistor Qp41, and the voltage characteristic of a data line (i.e., a signal line L1) is changed by using this potential variation of the signal N2. Also in this embodiment, the operation of the sense amplifier is similar to that of Embodiment 3. Figure 13 is a characteristic diagram showing the relationship between the data line voltage and the supply voltage VDD in this embodiment. As is shown in Figure 11, the data line voltage is changed in accordance with a characteristic line VDLC1 under application of a low voltage of 4 V or less, and is changed in accordance with a characteristic line VDLC2 obtained by moving the characteristic line VDLC1 downward, under application of a high voltage exceeding 4 V. As a result, the sense amplifier of this embodiment also exhibits the same effect as that of Embodiment 3. It is also possible to optionally change the data line voltage by changing the size of the P-channel MOS transistor Qp41. (Embodiment 6) Now, Embodiment 6 will be described referring to Figure 14. Figure 14 is an electric circuit diagram of a sense amplifier for a semiconductor device of Embodiment 6. Also in this embodiment, the configuration and operation of a fundamental portion of the sense amplifier is similar to that of the conventional sense amplifier of Figure 16. The sense amplifier of this embodiment additionally comprises an inverter INV11 that receives a voltage detecting signal VDTO and outputs a signal N17 generated by inverting the voltage detecting signal VDTO, a NAND circuit NAND11 that receives the signal N17 and a control signal SAE and outputs a signal N16 generated through the NAND operation of these signals, a P-channel MOS transistor Qp13 and an N-channel MOS transistor Qn14 that receive the signal N16 at their gates, and an N-channel MOS transistor Qn13 disposed between the transistors Qp13 and Qn14. The source of the transistor Qp13 is connected to a terminal for supplying a supply voltage VDD and the drain thereof is connected to a signal line through which an output signal N2 of a CMOS inverter including transistors Qp1 and Qn1 in the fundamental portion flows. The source of the transistor Qn14 is grounded, and the drain thereof is connected to the source of the transistor Qn13. The drain of the transistor Qn13 is connected to the signal line through which the output signal N2 of the CMOS inverter including the transistors Qp1 and Qn1 flows, together with the drain of the transistor Qp13, and the gate thereof is connected to the gate of a transistor Qn8 disposed in the fundamental portion. The case where, for example, the voltage detector of Figure 2 is used for this sense amplifier will be exemplified. As is described in Embodiment 1 referring to Figure 4, the voltage detecting signal VDTO is at a low level under application of a low voltage, and is at a high level under application of a high voltage. Merely when the control signal SAE is at a high level and the voltage detecting signal VDTO is at a low level, the transistor Qp13 is on and the transistor Qn14 is off. In the remaining cases, the transistor Qp13 is off and the transistor Qn14 is on. The transistor Qn13 is operated similarly to the transistor Qn8. The potential of the signal N2 is varied by turning on/off the transistors Qp13, Qn13 and Qn14, and the voltage characteristic of a data line (i.e., a signal line L1) is changed by using this potential variation of the signal N2. In the operation of this sense amplifier, the data line voltage is varied in accordance with the voltage detecting signal VDT0 in the same manner as in Embodiment 5 shown in Figure 13. Specifically, the data line voltage exhibits a characteristic according to the characteristic line VDLC1 under application of a low voltage, and a characteristic according to the characteristic line VDLC2 under application of a high voltage. Thus, the sense amplifier of this embodiment also can attain the same effect as that of Embodiment 3. Furthermore, it is also possible to optionally change the data line voltage by varying the sizes of the P-channel MOS transistor Qp13 and the N-channel MOS transistor Qn14. (Embodiment 7) Embodiment 7 will now be described referring to Figure 15. The configuration of a voltage detector alone will be described herein, and any sense amplifier described in the foregoing embodiments can be adopted in this embodiment. Figure 15 is an electric circuit diagram of the voltage detector for a semiconductor device of this embodiment. As is shown in Figure 15, the configuration of the voltage detector is basically identical to that of Embodiment 1 shown in Figure 2. Specifically, the voltage detector comprises transistors Qp21 through Qp23, a CMOS inverter including transistors Qp24 and Qn24 and an inverter INV21. This voltage detector further comprises an inverter INV51 that receives a control signal XCE and outputs a signal N51 generated by inverting the control signal XCE, a P-channel MOS transistor Qp51 that receives the signal N51 at its gate, an N-channel MOS transistor Qn51 that receives the control signal XCE at its gate, and an inverter INV52 for inverting an output signal VDTO of the inverter INV21 and supplying the inverted signal to the inverter INV21. The source of the transistor Qp51 is connected to a terminal for supplying a supply voltage VDD, and the drain thereof is connected to the source of the transistor Qp24. The source of the transistor Qn51 is grounded, and the drain thereof is connected to the source of the transistor Qn24. Owing to this configuration, when the control signal XCE is at a high level, both the transistors Qp51 and Qn51 are on, thereby allowing the CMOS inverter to output a signal N24. When the control signal XCE is at a low level, the transistors Qp51 and Qn51 are turned off, thereby inhibiting the CMOS inverter from outputting the signal N24. When the signal N24 is not output, the output signal VDTO is kept to have a current logical value due to the feed back function of the inverter INV52. Thus, by turning on/off the transistors Qp51 and Qn51, the voltage detecting signal VDT0 is allowed to undergo a low to high or a high to low transition merely when the control signal XCE is at a high level. In the voltage detector of this embodiment, the logical voltage of the voltage detecting signal VDTO undergoes a transition merely when the logical voltage of the control signal XCE is "H". Accordingly, when, for example, the logical voltage of the control signal XCE is "L" and the semiconductor device is placed in an operable state, the logical voltage of the voltage detecting signal VDTO does not undergo a transition. In this manner, the malfunction of the circuit using the voltage detecting signal VDTO during the operation can be avoided. It is noted that the sense amplifiers and the voltage detectors described in the foregoing embodiments can be freely combined, and that the combinations described in the embodiments do not limit the invention.

Claims: A semiconductor device comprising: a memory cell; a data line (L1) connected to the memory cell; voltage supplying means for supplying a supply voltage (VDD) in a reading operation of the memory cell; supply voltage detecting means (INV21) connected to the voltage supplying means for detecting at least two supply voltage regions separated by at least one set value and outputting a plurality of voltage detecting signals (VDTO) corresponding to the respective supply voltage regions; control signal input means for entering a first control signal (SAE) for giving an instruction to read a data from the memory cell; current detecting means connected to the control signal input means, the voltage supplying means and the data line (L1), for detecting, in response to the first control signal (SAE), a current flowing from the voltage supplying means to the data line (L1) by using a level sensing current in accordance with the supply voltage, the current detecting means having a plurality of different relations between the level sensing current and the supply voltage (VDD); and switching means in the reading operation of the memory cell connected to the supply voltage detecting means (INV21) and the current detecting means, for switching the level sensing current between two of the plurality of different relations (VSALC1, VSALC2) in accordance with are of the voltage detecting signals output by the supply voltage detecting means (INV21). The semiconductor device of claim 1, wherein, when the current detecting means receives a voltage detecting signal corresponding to the lower supply voltage region from the supply voltage detecting means (INV21), the relation between the level sensing current and the supply voltage having a higher level sensing current is selected than a level sensing current selected when a voltage detecting signal (VDTO) corresponding to the higher supply voltage region is received. The semiconductor device of claim 1, wherein the current detecting means has a first relation (VSALC2) between the level sensing current and the supply voltage and a second relation (VSALC1) between the level sensing current and the supply voltage respectively having different level sensing currents, the supply voltage detecting means detects three supply voltage regions separated by a first set value and a second set value smaller than the first set value, the supply voltage detecting means outputting a first voltage detecting signal in one of the supply voltage regions exceeding the first set value, outputting a second voltage detecting signal in another of the supply voltage regions not exceeding the second set value, and in the other of the supply voltage regions exceeding the second set value but not exceeding the first set value, continuously outputting a voltage detecting signal that has been output before entering this supply voltage region, and the switching means selects the first relation between the level sensing current and the supply voltage in response to the first voltage detecting signal and selects the second relation between the level sensing current and the supply voltage in response to the second voltage detecting signal. The semiconductor device of claim 3, wherein the current detecting means is formed so that the second relation between the level sensing current and the supply voltage has a higher level sensing current than the second relation between the level sensing current and the supply voltage. The semiconductor device of claim 1, wherein the supply voltage detecting means changes the voltage detecting signal to be output in response to the first voltage control signal when the current detecting means is not operated. The semiconductor device of claim 2, further comprising a word line connected to the memory cell; another voltage supplying means for supplying the supply voltage to the word line; and boosting means connected to the supply voltage detecting means and the signal line, for boosting a potential of the signal line to be higher than the supply voltage in response to the second voltage detecting signal.

Inventors: HIRANO HIROSHIGE, HONDA TOSHIYUKI, HIRANO, HIROSHIGE, HONDA, TOSHIYUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1995
Main classification: G11C7
IPC codes: G11C5, G11C16, G11C7