Programmable Read Only Memory With Means for Discharging Bit Line Before Program Verifying Operation

The present invention relates to a semiconductor memory wherein each of memory cells includes an insulated gate field effect transistor having a floating gate, and more particularly to such a memory having a function of verifying that a memory cell supplied with a programming voltage is actually programmed.

A semiconductor memory employing an insulated gate field effect transistor having a floating gate as a memory transistor is widely used as a programmable read only memory (PROM). The memory transistor further has a control gate connected to a word line and a drain-source path connected to a bit line. In a data programming (write) operation, a programming high voltage is applied to the control gate and the drain-source path to inject carriers into the floating gate, so that the threshold voltage of the memory transistor is shifted from a first, low value to a second, high value. The memory transistor has the first, low threshold voltage, if no carrier is injected into the floating gate or if the carriers are released from the floating gate.

Since the carrier injection depends on the value of the programming voltage and/or a time period for applying the programming voltage and further on electrical characteristics of the memory transistor, it is required to verify that the memory transistor applied with the programming voltage is actually programmed. For this purpose, the PROM is brought into a data read operation immediately after the data programming operation without changing address data to read out data of the memory transistor which has been supplied with the programming voltage. This operation mode is called "program verifying operation". In the program verifying operation, the control gate of the memory transistor is supplied with a reading-out voltage that is larger than the first threshold voltage but smaller than the second threshold voltage. When the memory transistor is actually programmed, therefore, it is not turned ON by the reading-out voltage, so that no current flows through the drain-source path of the memory transistor. The bit line is thereby held at a high level and a data read-out circuit coupled to the bit line produces output data of "1", for example. On the other hand, in case where the memory transistor is failed to be programmed, it is turned ON by the reading-out voltage. In this case, if the threshold voltage of the memory transistor is shifted to a third value that is slightly smaller than the reading-out voltage, the conductive resistant of the memory transistor is considerably large, so that only a small current flows through the drain-source path thereof. The potential level of the bit line is thereby lowered with a large time constant. For this reason, the data read-out circuit produces erroneously the output data of "1" during the program verifying operation period. Thus, there occurs error in judgement.

Therefore, an object of the present invention is to provide a semiconductor memory including an improved circuit arrangement for a verifying operation.

Another object of the present invention is to provide a semiconductor memory which makes possible to surely verify that a memory transistor supplied with a programming voltage is actually programmed.

A semiconductor memory according to the present invention comprises a plurality of word lines, a plurality of bit lines intersecting the respective word lines, a plurality of memory transistors each disposed at a different one of the intersections of the word and bit lines, means responsive to a set of address signals for selecting one of the word lines and one of the bit lines, means responsive to a program control signal for supplying a programming voltage to the selected word line and the selected bit line, means responsive to a disappearance of the program control signal for discharging the selected bit line, means for supplying a reading-out voltage to the selected word line after the selected bit line is discharged, and a data read-out circuit coupled to the selected bit line for producing output data.

The present invention is characterized by provision of means for discharging the selected bit line before a program verifying operation starts. The selected bit line is thereby changed to a low level. Accordingly, even if the programmed memory transistor has a considerably large conductive resistance through insufficient programming, the data read-out circuit produces output data of "0", for example. In that case, the transistor is again programmed by additional data programming operation. On the other hand, when the memory transistor is well programmed, no current flows through the drain-source path thereof. The selected bit line is charged by the data read-out circuit to produce output data of "1".

In a preferred embodiment of the present invention, the memory transistor discharges the selected bit line before the program verifying operation starts. For this purpose, the selected word line is maintained at the programming voltage during a predetermined period of time from the disappearance of the program control signal.

The above and other objects, features and advantages of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which

FIG. 1 is a circuit diagram representative of a semiconductor memory according to an embodiment of the present invention;

FIG. 2 is a circuit diagram representative of one example of a row programming voltage supply circuit shown in FIG. 1;

FIG. 3 is a circuit diagram representative of a column programming voltage supply circuit shown in FIG. 1;

FIG. 4 is a timing chart representative of a data programming operation and a program verifying operation of the semiconductor memory shown in FIG. 1; and

FIG. 5 is a circuit diagram representative of another example of the row programming voltage circuit shown in FIG. 1.

Referring to FIG. 1, a semiconductor memory 1 according to an embodiment of the present invention includes a plurality of memory transistors QM. Sixteen memory transistors QM.sub.00 to QM.sub.33 are shown in this embodiment and arranged in a matrix form. Each of the memory transistors QM has a control gate, a floating gate and a drain-source path. The control gates of the memory transistors arranged in the same row, QM.sub.00 to QM.sub.03, QM.sub.10 to QM.sub.13, QM.sub.20 to QM.sub.23 and QM.sub.30 to QM.sub.33, are connected in common to respective word lines W.sub.0 to W.sub.3 which are in turn connected to a row programming voltage supply circuit 16 via load transistors QRL.sub.0 to QRL.sub.3 of a depletion type, respectively. The word lines W.sub.0 to W.sub.3 are further connected to a row address decoder 14. The drains of the memory transistors arranged in the same column, QM.sub.00 to QM.sub.30, QM.sub.01 to QM.sub.31, QM.sub.02 to QM.sub.32 and QM.sub.03 to QM.sub.33, are connected in common to respective bit lines B.sub.0 to B.sub.3 which are in turn connected to a circuit node N via switching transistors QS.sub.0 to QS.sub.3, respectively. The sources of all the memory transistors QM.sub. 00 to QM.sub.33 are connected to a ground terminal 8. The gates of the switching transistors QS.sub.0 to QS.sub.3 are connected to a column address decoder 13 and further to a column programming voltage supply circuit 15 via load transistors QCL.sub.0 to QCL.sub.3 of a depletion type, respectively. The row programming voltage supply circuit 16 is applied with a power voltage Vcc via a terminal 7, a programming voltage Vpp via a terminal 6 and a program control signal PC via a terminal 4. The circuit 15 is applied with the power voltage Vcc, the programming voltage Vpp and an inverted signal PC of the program control signal PC.

Row address signals RA0 to RAi are supplied respectively to row address terminals 2-0 to 2-i and then introduced into a row address buffer 13. Column address signals CA0 to CA.sub.j are supplied respectively to column address terminals 3-0 to 3-j and then introduced into a column address buffer 11. A row address decoder 14 responds to outputs of the buffer 13 and selects one of the word lines W.sub.0 to W.sub.3. A column address decoder 12 responds to outputs of the buffer 11 and turns ON one of the switching transistors QS.sub.0 to QS.sub.3 to select one of the bit lines B.sub.0 to B.sub.3. Thus, one of the memory transistors QM.sub.00 to QM.sub.33 is designated in response to a set of address signals RA0 to RAi and CA0 to CAj.

In a data programming (or writing) operation, a high level program control signal PC is supplied to the terminal 9 and the programming voltage Vpp having a predetermined high value of about 12.5 V is supplied to the terminal 5. At this time, a terminal 6 is used as a data input terminal supplied with input data D.sub.IN. When the input data is "1", a program control circuit 9 turns a programming transistor QP ON to transfer the programming voltage Vpp to the node N. On the other hand, the input data D.sub.IN of "0" causes the circuit 9 to turn the transistor QP OFF.

In a program verifying operation, the program control signal PC is changed to the low level and the programming voltage Vpp is held at the predetermined high value. The program control circuit 9 maintains the transistor QP in a nonconductive state irrespective of the data at the terminal 6. A data read-out circuit 10 is then activated so that a sense amplifier 100 detects the level of the node N to produce output data D.sub.OUT at the terminal 6.

In a data reading-out operation, a low level program control signal PC is supplied to the terminal 4 and the power voltage Vcc of 5 V is supplied to the terminal 5 in place of the programming voltage Vpp. The programming control circuit 9 is thereby inactivated and the data readout circuit 10 is activated.

Referring to FIG. 2, a row programming voltage supply circuit 16 includes three depletion type transistors Q.sub.160, Q.sub.161 and Q.sub.162, one enhancement type transistor Q.sub.163, a NOR gate 160 and a delay circuit 161 which are connected as shown. Referring to FIG. 3, a column programming voltage supply circuit 15 includes three depletion type transistors Q.sub.150 to Q.sub.152 and one enhancement type transistor Q.sub.153 which are connected as shown. In FIGS. 1 to 3, all the transistors are of an N-channel type.

The data programming operation and program verifying operation will be described below with reference to FIGS. 1 to 4. Since the high level program control signal PC is supplied to the terminal 4, the NOR gate 160 produces the low level output, as shown in FIG. 4. The transistors Q.sub.160 and Q.sub.163 are thereby turned OFF and the transistors Q.sub.161 and Q.sub.162 are turned ON. As a result, the circuit 16 supplies the programming voltage Vpp to the respective one ends of the load transistors QRL.sub.0 to QRL.sub.3. Since the inverted program control signal PC takes the low level, the transistors Q.sub.150 and Q.sub.153 are turned OFF and the transistors Q.sub.151 and Q.sub.152 are turned ON. Consequently, the circuit 15 supplies the programming voltage Vpp to the respective one ends of load transistors QCL.sub.0 to QCL.sub.3. Assuming that the word line W.sub.0 is selected in response to the valid row address signals RA.sub.0 to RA.sub.i and the bit line B.sub.0 is selected in response to the valid column address signals CA.sub.0 to CAj, the row address decoder 14 disconnects the word line W.sub.0 from the ground point and connects the remaining non-selected word lines W.sub.1 to W.sub.3 to the ground point. Similarly, the column address decoder 12 disconnects the gate of the switching transistor QS.sub.0 from the ground point and connects the gates of the remaining switching transistors QS.sub.1 to QS.sub.3 to the ground point. As a result, the selected word line W.sub.0 is raised up to the programming voltage Vpp and the transistor QS.sub.0 is turned ON by the programming voltage Vpp applied to the gate thereof. Since the input data of "1" is supplied to the terminal 6, the program control circuit 9 turns the transistors QP 0N to supply the programming voltage Vpp to the selected bit line B.sub.0. Thus, the memory transistor QM.sub.00 is designated and the selected word and bit lines W.sub.0 and B.sub.0 are supplied with the programming voltage Vpp, as shown in FIG. 4. Since the programming voltage Vpp is applied to the control gate and the drain-source path of the designated memory transistor QM.sub.00, electrons as carriers are injected and trapped into the floating gate thereof. If the designated memory transistor QM.sub.00 is well programmed, the threshold voltage thereof is shifted from a first value, typically 2 V, to a second value, typically 10 V.

In order to verify that the designated memory transistor QM.sub.00 is well programmed, a program verifying operation is carried out successively. In this program verifying operation, the row and column address signals RA.sub.0 --RAi and CA.sub.0 --CAj are not changed and the programming voltage Vpp is held at the predetermined high voltage, 12.5 V. However, the program control signal PC is changed to the low level, as shown in FIG. 4. The program control circuit 9 turns the transistor QP OFF. Since the inverted program control signal PC takes the high level, the transistors Q.sub.150 and Q.sub.153 are turned ON and the transistor Q.sub.151 is turned OFF. The power voltage Vcc of 5 V is thereby supplied to the load transistors QCL.sub.1 to QCL.sub.3. On the other hand, since the delay circuit 161 delays the low level program control signal PC to be supplied to the NOR gate 160, the output of the NOR gate 160 is maintained at the low level during a predetermined period of time which is determined by a delay time of the delay circuit 161, as shown in FIG. 4. Accordingly, the output of the circuit 16, i.e. the selected word line W.sub.0, is held at the programming voltage Vpp during this period, as shown in FIG. 4. Since the programming voltage Vpp is larger than the second threshold level (10 V) of the well programmed memory transistor QM, the memory transistor QM.sub.00 is maintained in a conductive state to discharge the bit line B.sub.0. Thus, the bit line B.sub.0 is discharged to the low level before reading-out data of the memory transistor QM.sub.00.

Since the delay circuit 161 thereafter supplies the low level to the NOR gate 160, the output of the NOR gate 160 changes to the low level and the circuit 16 produces the power voltage Vcc, as shown in FIG. 4. The selected word line W.sub.0 is thereby supplied with the power voltage Vcc as a reading-out voltage. Assuming that the designated memory transistor QM.sub.00 has been failed in well programming, the threshold voltage thereof is not shifted to the second value (10 V), but to a third value, 4 V for example, that is slightly smaller than the reading-out voltage (the power voltage Vcc). The memory transistor QM.sub.00 is thereby turned ON, but the conductive resistance thereof is considerably large. Only a small current flows through the drain-source path of the transistor QM.sub.00. However, the bit line B.sub.0 has been discharged before data-reading, and therefore the bit line B.sub.0 maintaines a level near the ground voltage, as shown in FIG. 4. The sense amplifier 100, which is activated by the inverted program control signal PC, detects the potential level of the selected bit line B.sub.0 through the switching transistor QS.sub.0 and produces the output data D.sub.OUT of "0", as shown in FIG. 4. By checking the output data D.sub.OUT, it is judged that the memory transistor QM.sub.00 is not well programmed. In response to this, the data programming is repeated to the memory transistor QM.sub.00, so that the threshold voltage thereof is shifted to the second value.

If the bit line B.sub.0 is not discharged before the verifying operation, i.e. if the circuit 16 produces the power voltage Vcc in synchronism with the changing point of the program control signal PC, the bit line B.sub.0 is discharged from the level of the programming voltage Vpp by a small current flowing through the drain-source path of the memory transistor QM.sub.00. As a result, the level of the bit line B.sub.0 is lowered at a very small rate and the sense amplifier 100 produces erroneously the data D.sub.OUT of "1", as shown by dotted lines in FIG. 4. Thus, there occurs a misjudgement that the memory transistor QM.sub.00 has been well programmed.

In the above-mentioned verifying operation, when the threshold voltage of the memory transistor QM.sub.00 is shifted to the second value, the transistor QM.sub.00 is not turned ON by the reading-out voltage. No currents flows through the memory transistor QM.sub.00 via the bit line B.sub.0. The bit line B.sub.0 is thereby charged up by the sense amplifier 100 and the output data D.sub.OUT of "1" is produced.

When the data programming and program verifying operations complete with respect to the memory transistor QM.sub.00, new row and column address signals RA.sub.0 --RAi and CA.sub.0 --CAj are supplied to designate another memory transistor, and the data programming and verifying operations are carried out for the newly designated memory transistor.

Referring to FIG. 5, another example of the row programming voltage supply circuit 16 includes one depletion type transistor Q.sub.200, four enhancement type transistors Q.sub.201 to Q.sub.204, a capacitor C.sub.200, a NOR gate 165 and a delay circuit 166 which are connected as shown. A clock signal .phi. is supplied to one end of the capacitor C.sub.200. In the data programming operation wherein the program control signal PC takes the high level, the transistors Q.sub.201 and Q.sub.204 are turned OFF and the transistors Q.sub.202 and Q.sub.203 are turned ON. A charge pumping circuit composed of the transistors Q.sub.202 and Q.sub.203 and the capacitor C.sub.200 operates in response to the clock signal .phi., so that the circuit 16 produces the boosted level of the programming voltage Vpp. When the program control signal PC changes to the low level to carry out the program verifying operation, the transistor Q.sub.201 is turned ON to inactivate the charge pumping circuit. By the delay circuit 166, the transistor Q.sub.204 is maintained in the nonconductive state during a predetermined time, so that the boosted level of the programming voltage Vpp is derived from the output of the circuit 16. Since the boosted level make small the conductive resistance of the memory transistor QM, the selected bit line B is discharged rapidly to the low level. The delay circuit 166 thereafter supplies the low level to the NOR gate 165, the output of the NOR gate 165 changes to the high level to turn the transistor Q.sub.204 ON. As a result, the circuit 16 produces the power voltage Vcc as a reading-out voltage.

The present invention is not limited to the above embodiments but may be changed and modified without departing from the scope and spirit of the present invention. For example, the programming voltage Vpp can be obtained internally by boosting the power voltage Vcc. The output voltage from the column programming voltage supply circuit 15 can be changed in the same timing as the output voltage from the row programming supply circuit 16. In this case, the circuit 16 can be used in common for the circuit 15.