Programmable Memory Circuit

The present invention relates to a programmable memory circuit and, more particularly, to a complementary type programmable memory circuit in which a selected one of a plurality of non-volatile semiconductor memory cells is accessed (read out therefrom or written thereinto).

Since read and write voltages for the programmable memory have different levels, it is necessary to provide a circuit which can deal with voltages of two levels and control the supply of the two-level voltages to the memory. Conventionally, such a circuit is very complicated.

In order to decrease power consumption, it has been proposed to use complementary insulated gate field effect transistors (CMOSFETs) for a circuit adapted to access non-volatile semiconductor memory cells. However, many insulated gate field effect transistors (hereinafter referred to as IGFETs) must be used to constitute a multi-input circuit such as a decoder of the complementary IGFET type, and this circuit is not suited for high speed operations.

It is, therefore, an object of the present invention to provide a novel complementary type programmable memory circuit which can be constituted by a small number of IGFETs to ensure a highly integrated structure.

According to the present invention, a programmable memory circuit comprises a decoder responsive to address input signals for accessing the memory, an access line for the memory, a transfer gate connected between the decoder and the access line, and a selective voltage supply circuit connected to the access line. The transfer gate is responsive to a read/write switch signal to connect the decoder to the access line in the read mode and disconnect the decoder from the access line in the write mode. The selective voltage supply circuit is also responsive to the read/write switch signal to provide on the access line different voltage levels in accordance with the read/write mode. The access line assumes, in the read mode, one voltage level which is equal to the level of the output signal from the decoder and in the write mode the other voltage level.

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

FIG. 2 is a waveform diagram useful in explaining read and write operations; and

FIG. 3 is a block diagram showing the programmable memory circuit of the present invention applied to a PROM.

FIG. 1 shows a 3-input complementary type programmable memory circuit according to an embodiment of the present invention. A 2-input NAND gate includes P-channel IGFETs M1 and M2 and N-channel IGFETs M3 and M4. Two address input signals A1 and A2 are supplied to the 2-input NAND gate. An output terminal 1 of the 2-input NAND gate is set to be logic "0" (corresponding to the ground potential) only when both the address input signals A1 and A2 are set to be logic "1" (corresponding to a read power supply voltage Vcc of +5V for non-volatile semiconductor memory cells). Otherwise, the output terminal 1 of the NAND gate is set to be logic "1". Thus, the 2-input NAND gate constitutes a main decoder 10.

A complementary logic circuit comprises a P-channel IGFET M5 and an N-channel IGFET M6 and receives the outlet logic signal from the output terminal 1. The source electrode of the IGFET M6 is grounded, while the source electrode of the IGFET M5 receives an address input signal A3. The drain electrodes of the IGFETs M5 and M6 are connected to each other. An output from the complementary logic circuit appears at the common node (output terminal 2) of the drain electrodes of the IGFETs M5 and M6. The drain of an N-channel IGFET M7 is connected to the output terminal 2, the gate electrode thereof receives an address signal A3 which is an inverted signal of the address signal A3, and the source electrode thereof is grounded. The complementary logic circuit and the IGFET M7 constitutes a sub-decoder 11, and a 3-input decoder is constituted by the main decoder 10 and the sub-decoder 11.

An N-channel depletion-type IGFET M8 is connected between the output terminal 2 of the complementary logic circuit, that is, the decoder 11 and a word line WL. The gate electrode of the IGFET M8 is controlled by a read/write switch signal PG. Thus, the depletion-type IGFET M8 constitutes a transfer gate means 12.

A complementary inverter circuit has a P-channel IGFET M9 and an N-channel IGFET M10 which are connected between a power supply voltage Vpp' (high in the write mode and low in the read mode) and ground. A P-channel IGFET M11 is further arranged to receive an output from an output terminal 4 of the complementary inverter circuit. The source electrode of the IGFET M11 is connected to the power supply voltage Vpp' and the drail electrode thereof is connected to the word line WL. The complementary inverter circuit having its input terminal 3 connected to the word line WL and the IGFET M11 constitute a selective voltage supply circuit 13. A load capacitor C1 is connected between the word line WL and ground.

The power supply voltage Vpp' is fed from a drive circuit 15 so as to bear, in accordance with the logic of the read/write switch signal PG, one of two levels Vcc and Vpp respectively corresponding to a read power supply voltage of +5V and a write power supply voltage of +25V for the non-volatile semiconductor memory cells. The drive circuit 15 comprises a P-channel depletion type IGFET M12 and an N-channel IGFET M13. The source electrode of the IGFET M12 receives the write power supply voltage Vpp and the source electrode of the IGFET M13 receives the read power supply voltage Vcc. The drain electrodes of the IGFETs M12 and M13, on which the power supply voltage Vpp' appears, are connected in common and coupled to the source electrodes of the IGFETs M9 and M11 included in the selective voltage supply circuit 13. The gate electrodes of the IGFETs M12 and M13 are connected in common to receive the read/write switch signal PG which is fed from a PG signal generator 14. An external signal PGM which is "0" during the read cycle and "1" during the write cycle is inverted by an inverter INV included in the PG signal generator 14 so that the read/write switch signal PG which assumes "1" of Vcc level in the read mode and "0" of ground level in the write mode as shown at section (A) in FIG. 2 is produced from the PG signal generator 14. A read/write signal as shown at section (B) in FIG. 2 is generated from a circuit not shown, alternately bearing Vcc and Vpp and it is selectively applied to the source electrode of the IGFET M13 or M12.

Now assume that one of the address input signals A1 and A2 is set to be at logic "0" so that the potential at the output terminal 1 is set to be logic "1". The P-channel IGFET M5 is turned off and the N-channel IGFET M6 is turned on. Therefore, the potential at the output terminal 2 is kept at logic "0" irrespective of the logic state of the address input signal A3.

In the read mode, the read/write switch signal PG is set to be logic "1", assuming a potential corresponding to the read power supply voltage Vcc. Accordingly, in the drive circuit 15, the N-channel IGFET M13 is turned on and the P-channel depletion type IGFET M12 is turned off to cause the power supply voltage Vpp' to assume the read power supply voltage Vcc as shown at section (c) in FIG. 2. In addition, the depletion-type IGFET M8 is kept on in the read mode, and the word line WL is set to be at the same potential as that at the output terminal 2, i.e., the ground potential.

When the word line WL is set to be "0", the IGFET M9 is turned on and the IGFET M10 is turned off. As a result, the potential at the output terminal 4 is set to be "1". The IGFET M11 is then turned off, so that the transfer of the power supply voltage Vpp' (which is at this time the read power supply voltage Vcc) to the word line WL is interrupted.

The time required for changing the logic state of the word line WL from "1" to "0" is determined by conductances gm of the IGFETs M6 and M8 and the capacitance of the capacitor C1.

In the write mode, the read/write switch signal PG is set to be "0". In the drive circuit 15, the P-channel depletion type IGFET M12 is turned on and the N-channel IGFET M13 is turned off to cause the power supply voltage Vpp' to assume the write power supply voltage Vpp as shown at section (c) in FIG. 2. The N-channel depletion-type IGFET M8 is kept on when the potential at the output terminal 2 is "0", and the potential at the word line WL becomes the same as that at the output terminal 2, i.e., the ground potential.

As previously described, only when both the address input signals A1 and A2 are set to be "1", the potential at the output terminal 1 is set to be "0". The P-channel IGFET M5 is turned on and the N-channel IGFET M6 is turned off. Thus, the potential at the word line WL is determined by the logic state of the address input signal A3. When the address input signal A3 is set to be "1", the N-channel IGFET M7 receiving the inversion A3 is turned off. The potential at the output terminal 2 is the same as that of the address input signal A3. In other words, the potential at the output terminal 2 is set to be logic "1" (i.e., the read power supply voltage Vcc).

In the read mode, since the read/write switch signal PG is set to be "1", the IGFET M8 is turned on and the word line WL is set to "1" of the same potential as at the output terminal 2 and which corresponds to the read power supply voltage Vcc. On the other hand, when the word line WL is "1", the P-channel IGFET M9 is turned off, the N-channel IGFET M10 is turned on, and the IGFET M11 is turned on. Since, in the drive circuit 15, the IGFETs M13 and M12 respond to the read/write switch signal PG now being "1" and only the IGFET M13 is turned on, the read supply voltage Vcc appearing at the source electrode of the IGFET M13 is transmitted to the word line WL through the IGFET M11. Thus, the logic state of the word line WL determined by the potential appearing at the output terminal 2 is not disturbed. In this manner, the level of logic "1" of the word line WL in the read mode can the Vcc level or +5V which is suitable for reading the non-volatile semiconductor memory cells. The time required for changing the logic state of the word line WL from "0" to "1" is determined by conductances gm of the IGFETs M5 and M8 and the capacitance of the capacitor C1.

In the write mode, since the read/write switch signal PG goes to logic "0", the voltage of +5V (Vcc) is applied to the source electrode of the IGFET M8, and the voltage of 0 V is applied to the gate electrode thereof. The IGFET M8 is then turned off. More particularly, when the IGFET M5 is turned on and the potential at the output terminal 2 becomes the read power supply voltage Vcc of +5V, the potential at the word line WL is increased through the IGFET M8 until the IGFET M8 is turned off. In this case, the potential at the word line WL corresponds to an absolute value of a threshold voltage V.sub.T of the IGFET M8. Consequently, the IGFET M10 is turned on to decrease the potential at the output terminal 4. The IGFET M11 is then turned on, so that the word line WL is applied with the power supply voltage Vpp' which now corresponds to the write power supply voltage Vpp since, in the drive circuit, the P-channel IGFET M12 is turned on in response to the logic "0" of the read/write switch signal PG, with the N-channel IGFET M13 rendered off, and the write power supply voltage Vpp is passed. In this case, the IGFET M8 is kept off, and the word line WL is separated or disconnected from the decoder 11 having the IGFETs M1 to M7. As a result, in the write mode, the word line WL becomes "1" of a level equal to the write power supply voltage Vpp of +25V.

It should be noted that the threshold voltage V.sub.T of the depletion-type IGFET M8 must be less than -5 V to turn off the IGFET M8 when the voltage of +5V is applied to the source electrode thereof and the voltage of 0 V is applied to the gate electrode thereof.

With the output terminal 1 set at "0", when the address input signal A3 is set to "0", the IGFET M7 is turned on, and then the output terminal 2 is set to be logic "0". As a result, the word line WL is set to be logic "0".

Charge/discharge time in the read mode is determined by the conductances gm of the IGFETs M5 and M8 and the IGFETs M6 and M8. In this embodiment, since the IGFET M8 is of the depletion type, and its gate voltage is set at the read power supply voltage Vcc, the conductance gm of the IGFET M8 is high. For this reason, the charge/discharge time is short, and high-speed operation can be performed.

In the write mode, the decoder can be separated or disconnected from the word line WL by the IGFET M8, so that the decoder circuit can be prevented from being exposed to the write voltage and can be easily disconnected from the write circuit. Therefore, a high-speed operation can be performed by the circuit having IGFETs each having the minimum channel length and the low breakdown voltage.

As described previously, the logic at the output terminal 1 of the main decoder 10 can be divided into two logics by the address input signals A3 and A3 applied to the sub-decoder 11. When applying the programmable memory circuit to non-volatile semiconductor memory cells, first and second sub-decoders 11-1 and 11-2 are connected to one main decoder 10-1 to provide two channels of memory access circuit which branch from the output terminal 1 of the main decoder 10-1 as shown at a dotted line block in FIG. 3. The second sub-decoder 11-2 receives address input signal which is complementary to that applied to the first sub-decoder 11-1. In other words, when it is assumed that the sub-decoder 11 shown in FIG. 1 stands for the first sub-decoder 11-1, the second sub-decoder 11-2 has a P-channel IGFET M5 whose source receives the inverted address input signal A3 and an N-channel IGFET M7 whose gate electrode receives the address input signal A3. Advantageously, the two channels of the memory access circuit branching from the one main decoder can reduce the number of main decoders to half the number of access lines for the memory cells. Accordingly, the number of IGFETs used to arrange the programmable memory circuit can be decreased. In the foregoing embodiment, the output logic from the output terminal 1 is divided into two logics by address input signals A3 and A3; however, it may be divided into four or eight logics. In proportion to the number of division, the reduction in the number of IGFETs used can be amplified.

Furthermore, in the programmable memory circuit of the above embodiment, the power is not substantially steadily consumed. Therefore, the programmable memory circuit of the present invention is suitable for a low-power configuration.

In summary, according to the present invention, one multi-input circuit having a number of IGFETs is used for several access lines. Therefore, as the number of access lines is increased, the reduction in the number of required IGFETs is amplified as compared with the conventional decoder circuit. Furthermore, according to the present invention, since the depletion-type IGFET is used, a high-speed read operation can be performed. In the write mode, since the word line is separated from the decoder, no voltage drop occurs. As a result, a sufficient write voltage can be obtained.

The present invention has the above-mentioned advantages and may be suitably applied in the design of a high-capacity memory.

In an application, a number of the programmable memory circuits of the present invention are connected to a PROM, for example, having non-volatile semiconductor memory cells as shown in FIG. 3. Where the main decoder is exemplified as a 2-input NAND gate, four main decoders 10-1 to 10-4 are provided. The main decoder 10-1 is followed by a first channel of a memory access circuit including sub-decoder 11-1, transfer gate means 12-1, and selective voltage supply circuit 13-1 and a second channel of a memory access circuit including sub-decoder 11-2, transfer gate means 12-2, and selective voltage supply circuit 13-2. Similarly, the main decoder 10-4 is followed by a seventh channel of a memory access circuit including sub-decoder 11-7, transfer gate means 12-7, and selective voltage supply circuit 13-7 and an eighth channel of a memory access circuit including sub-decoder 11-8, transfer gate means 12-3, and selective voltage supply circuit 13-8. A PROM 30 has a number of memory cells arranged in matrix. For simplicity of illustration, only row access lines connected to cells 31-1-1, 31-1-2 . . . through 31-8-1, 31-8-2 . . . are shown in FIG. 3 but obviously, a decoding arrangement for column access lines can be constructed in a similar manner. First to eighth row access lines respectively leading from the first to eighth channels are connected to cells 31-1-1, 31-1-2 . . . through 31-8-1, 31-8-2 . . . , respectively. Each of the cells 31-1-1, 31-1-2 . . . through 31-8-1, 31-8-2 . . . has a non-volatile semiconductor element with a floating gate. A semiconductor element of each of the cells 31-1-1, 31-1-2 . . . has its gate electrode connected to the first row access line, its source electrode connected to an input/output circuit 32 and its drain electrode grounded. Similarly, a semiconductor element of each of the cells 31-8-1, 31-1-2 . . . has its gate connected to the eighth row access line, its souce electrode connected to the input/output circuit 32 and its drain electrode grounded.

In operation, the first to fourth main decoders 10-1 to 10-4 are applied with a set of address input signals A1, A2, a set of address input signals A1, A2, a set of address input signals A1, A2, and a set of address input signals A1, A2, and one of the two sub-decoders associated with one main decoder is applied with a set of address input signals A3, A3 and its complement is applied to the other sub-decoder. Obviously, in accordance with combinations of logic states of the address signals applied to the first to fourth main decoders and to the first to eighth sub-decoders, one of the access lines can be set to "1" with the remaining seven access lines set to "0". The access line set at "1" can have the Vcc level of +5V in the read mode and the Vpp level of +25V in the write mode.