Circuit for Generating Internal Supply Voltage

The present invention relates to a circuit for generating internal supply voltage for use in a high density semiconductor memory device. In particular, the present invention relates to a circuit for generating internal supply voltage wherein the output voltage generated therefrom increases in response to increase of the temperature.

At recent, in a semiconductor memory device with high memory capacity, it is required to supply a metal oxide silicon (hereafter referred to as MOS) transistor below a micron unit level with low supply voltage lower than the external supply voltage of 5 V which is generally supplied in computer systems. For this end, an internal supply voltage generator must be prepared in semiconductor chips in addition to a memory circuit so as to supply the low internal supply voltage. For instance, a semiconductor dynamic random access memory (hereinafter referred to as "DRAM" device over 16 Mbit level necessarily includes the internal supply voltage generator in order to obtain a high reliability of the memory device.

A prior art internal supply voltage generator and the characteristics thereof are well disclosed with reference to FIGS. 1 to 3. Referring to FIG. 1, the conventional internal supply voltage generator 100 includes of a reference voltage generator 50, a comparator 60 and an output circuit 70. Shown in FIG. 2 are the characteristics of the internal supply voltage generator of FIG. 1, in comparison with the external supply voltage. Furthermore, another embodiment of the reference voltage generator 50 of FIG. 1 is described in FIG. 3.

Referring again to FIG. 1, the internal supply voltage generator 100 has the reference voltage generator 50 and the output circuit 70 which includes a p-channel metal oxide semiconductor (hereinafter referred to as "PMOS") transistor 10 serving as a variable resistor. The voltage output from the reference voltage generator 50 and the output circuit 70 are then compared at the comparator 60 which is a differential amplifier for controlling the voltage applied to the gate of the PMOS transistor 10. The reference voltage generator 50 has first and second resistors R.sub.1, R.sub.2 connected in series between the external supply voltage and the ground level, to generate reference voltage V.sub.ref through a connection node 3. The comparator 60 has first and second NMOS transistors 6, 7 forming a differential amplifier, a third NMOS transistor 8 serving as a constant current source, and a first and second PMOS transistors 4, 5 forming a current mirror load stage. Moreover, the PMOS transistor 10 has the source connected to the external supply voltage Vcc.sub.ext, and the drain connected to the internal supply voltage Vcc.sub.int of an output node 11. In the drawing, the reference voltage V.sub.ref is applied to the gate to the first NMOS transistor 6 of the comparator 60. In the event that the load current flow is formed from the output node 11 to memory circuit (not shown), a voltage drop occurs at the PMOS transistor 10 of the output circuit 70. As a result, the internal supply voltage is set to a voltage level lower than the external supply voltage. At the same moment, the comparator 60 controls the gate voltage of the PMOS transistor 10 so as to keep the internal supply voltage level identical to the reference voltage V.sub.ref level.

The internal supply voltage generator must keep a constant internal supply voltage regardless of the change of the external supply voltage in order to obtain high reliability of the semiconductor memory device. Undesirably, however, the conventional internal supply voltage generator 100 of FIG. 1 has the voltage difference .DELTA.V as shown in FIG. 2, in response to the increase of the external supply voltage. The foregoing problem is caused by the fact that the reference voltage V.sub.ref from the reference voltage generator 50 is ##EQU1## therefore, the reference voltage V.sub.ref increases as the external supply voltage increases, thereby increasing the internal supply voltage. Accordingly, the reliability of the semiconductor may be reduced.

Referring to FIG. 3, the reference voltage generator 50 includes of a first to third PMOS transistors 12, 13, 14 connected in series to each other, and fourth and fifth PMOS transistors 15, 16 connected in series to each other, the first to third PMOS transistors being connected in parallel to the fourth and fifth PMOS transistors. The gates and drains of first to fifth PMOS transistors 12-16 each are diode-connected and, further, the gate of the fourth PMOS transistor 15 is connected to the source of the third PMOS transistor 14. The source of the third PMOS transistor 14 is coupled so as to set the gate voltage level of the fourth PMOS transistor 15 to ##EQU2## the source of the fourth PMOS transistor 15 becoming an output node 17, by which the reference voltage generator 50 generates the reference voltage V.sub.ref, through the output mode 17. As the temperature increases, however, the threshold voltage V.sub.th of the respective PMOS transistors in the reference voltage generator 50 shown in FIG. 3 is reduced. Therefore, the reference voltage V.sub.ref will also be reduced. If the reference voltage is reduced, the internal supply voltage will also be reduced, causing the semiconductor memory device to operate at a low speed.

It is accordingly an object of the present invention to provide a circuit for generating internal supply voltage which allows a semiconductor memory device to operate at a stable, constant speed regardless of the change of the temperature, wherein the output voltage thereof increases as the temperature increases.

In order to achieve the above and other objects and features of the invention, an inventive internal supply voltage generator includes a voltage sharing circuit which has a first and second variable load resistors connected in series between the internal supply voltage output and the ground level, a common connection of the variable load resistors becoming an output node thereof, in order to allow the internal supply voltage output to increase in response to increase of the temperature.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which:

FIG. 1 shows a conventional internal supply voltage generator;

FIG. 2 shows the output characteristics of the internal supply voltage generator of FIG. 1;

FIG. 3 shows another embodiment of the reference voltage generator of FIG. 1;

FIG. 4 shows an internal supply voltage generator according to the present invention;

FIG. 5 shows the output characteristics of the internal supply voltage generator of FIG. 4;

FIG. 6 shows an embodiment of the internal supply voltage generator of FIG. 4; and

FIG. 7 is a table for illustrating the decrease ratio of the current driving power of MOS transistors in response to change of the temperature.

Referring to FIG. 4, how a voltage sharing circuit 80 is connected to a comparator 60 and an output circuit 70 is well illustrated. A reference voltage generator 50 to which an external supply voltage is applied, generates a reference voltage V.sub.ref. The comparator 60 connected to the output of the reference voltage generator 50, compares a first input voltage coupled to the reference voltage V.sub.ref with a second input voltage. The output of the comparator 60 is coupled to an input terminal of the output circuit 70 for generating internal supply voltage. The voltage sharing circuit 80, connected to the output circuit 70, generates the second input voltage at an output node thereof, thereby to allow the output voltage level of the output circuit 70 to increase in response to increase of the temperature. It should be further noted that the voltage sharing circuit 80 has a first and second variable load resistors R.sub.1 ', R.sub.2 ' of which resistances increase in response to increase of the temperature wherein resistance of the first variable load resistor R.sub.1 ' being higher than that of the second variable load resistor R.sub.2 '. Furthermore, the resistance increase ratio according to increase of the temperature, of the first variable load resistor R.sub.1 ' is higher than that of the second variable load resistor R.sub.2 '.

The output voltage Vcc.sub.int of the internal supply voltage generator 100 of FIG. 4, can be written as ##EQU3## where the ratio of resistance variation to temperature change is R.sub.1 '>R.sub.2 '. It is therefore apparent from the foregoing that as the temperature increases, the resistance increase ratio of the first variable load resistor R.sub.1 ' becomes higher than that of the second variable load resistor R.sub.2 ', thereby increasing the internal supply voltage Vcc.sub.int.

As well illustrated with reference to FIG. 5, it is understood that the internal supply voltage Vcc.sub.int increases stably to keep it constant as the temperature increases. Accordingly, the problem found in the conventional circuits, that as the temperature increases, the reference voltage V.sub.ref of the reference voltage generator is reduced, resulting in the undesirably low internal supply voltage, as well as the problem that as the external supply voltage increases, the reference voltage also increases, resulting in the unstable internal supply voltage, are both solved out.

An embodiment of the internal supply voltage generator 50 of FIG. 4 will now be described in detail hereinbelow, with reference to FIG. 6. The reference voltage generator 50 has a constant current source 31 an input thereof being coupled to the external supply voltage Vcc.sub.ext, an output node 38 being coupled to the output of the constant current source 31, and a circuit connected between the output node and the ground level, for dropping the voltage level of the output node 38 to a predetermined level.

The voltage dropping circuit has a first resistor 35 connected to the output node 38, another end of the first resistor 35 being commonly coupled to the collector and base of a first bipolar transistor 32 of which emitter is coupled to the ground level. A second resistor 36 an end thereof being coupled to the output node 38, is coupled to the collector of a second bipolar transistor 33 having the base coupled to the collector of the first bipolar transistor 32 and the emitter connected the ground level via a third resistor 37. Moreover, the output node 38 is connected to the collector of a third bipolar transistor 34 of which the base is connected to the collector of the second bipolar transistor 33 and the emitter is coupled to the ground level.

The comparator 60 has a first PMOS transistor 39 of which the source is connected to the external supply voltage, and a second PMOS transistor 40 of which the source is connected to the external supply voltage. The gate of the first PMOS transistor 39 is commonly connected to the gate and drain of the second PMOS transistor 40. Then, the gate of a first NMOS transistor 41 is coupled to a first input voltage, i.e., the reference voltage V.sub.ref. Furthermore, the first NMOS transistor 41 has the drain connected to the train of the first PMOS transistor 39 and the source connected to the source of a second NMOS transistor 43 of which the drain is connected to the drain of the second PMOS transistor 40 and the gate is coupled to a second input voltage. The sources of the first and second NMOS transistors 41, 43 are connected to the drain of a third NMOS transistor 42 of which the source is connected to the ground level and the gate is coupled to he first input voltage. The connection node 44 of the drains of the first PMOS transistor 39 and the first NMOS transistor 41 serves as an output node of the comparator 60.

The output circuit 70 includes of a PMOS transistor with the source being connected to the external supply voltage Vcc.sub.ext and the gate being connected to the output node 44 of the comparator 60. The drain of the PMOS transistor 45 is connected to an output node 49 through which the internal supply voltage Vcc.sub.int is generated.

The voltage sharing circuit 80 includes of a first PMOS transistor 46 with the source being connected to the output node 49 of the output circuit 70, and the gate and drain being diode-connected. A second PMOS transistor 47 has the source connected to the drain of the first PMOS transistor 46, and the gate and drain diode-connected to the ground level. Moreover, an output node 48 to which the drain and source of the first and second PMOS transistors are respectively connected, is coupled to the second input voltage.

In order to generate a constant reference voltage V.sub.ref regardless of change of the temperature, the reference voltage generator 50 includes of bipolar transistors. For reference, the output voltage of the inventive reference voltage generator 50 is ##EQU4## where V.sub.BE is base-emitter voltage of the third bipolar transistor 34; V.sub.t the thermoelectric voltage; R.sub.b and R.sub.c the second and third resistors 36, 37 respectively; and I.sub.S1 and I.sub.S2 the collector saturation current of the first and second bipolar transistors 32, 33 respectively. The reference voltage generator 50 is made such that the base-emitter voltage V.sub.BE with a negative temperature coefficient of -2.2 mV/.degree.C. and the thermoelectric voltage V.sub.t with a positive temperature coefficient of 0.085 mV/.degree.C. are combined with each other so as to obtain a zero temperature coefficient. Therefore, unlike the conventional reference voltage generator using PMOS transistors having a negative temperature coefficient of -3 mV/.degree.C., the inventive reference voltage generator 50 can generate a stable, constant reference voltage V.sub.ref, regardless of change of the temperature.

Besides, the voltage sharing circuit 80 is so connected to increase the reference voltage V.sub.ref, to thereby increase the internal supply voltage Vcc.sub.int according to increase of the temperature. For this end, the channel conductance ##EQU5## of the first PMOS transistor 46 is made lower than the channel conductance g.sub.2 of the second PMOS transistor 47; therefore, the channel resistance of the first PMOS transistor 46 is higher than that of the second PMOS transistor 47. It is known that the conductance is a reciprocal of the resistance. From the foregoing, it can be understood that the current driving power is reduced for a MOS transistor with a high channel resistance. In general, a MOS transistor with a long channel length is under greater influence of the temperature than a MOS transistor with a short MOS transistor; therefore, variation of the channel resistance according to the temperature, of the MOS transistor with a long channel length is comparatively considerable.

The internal supply voltage generator of FIG. 6 operating at the normal temperature of 25.degree. C. will now be considered hereinbelow. The reference voltage V.sub.ref from the reference voltage generator 50 is applied to the gates of the first and third NMOS transistors 41, 42. At this moment, if this voltage is higher than the voltage applied to the gate of the second NMOS transistor 43 of the comparator 60, a given voltage will be charged at the output node 49 of the output circuit 70. In the meantime, in the case where the reference voltage V.sub.ref is at the same level as the output voltage of the voltage sharing circuit 80, the internal supply voltage Vcc.sub.int is maintained constant by the voltage sharing circuit 80. In a little while, if the temperature is increased up to over 83.degree. C., the current flow in the first PMOS transistor 46 of the voltage sharing circuit 80 will be reduced; therefore, a voltage lower than that during the normal temperature is applied to the gate of the second NMOS transistor 43 of the comparator 60. Thus, a voltage at the output node 44 of the comparator 60 is charged to be lower than that during the normal temperature and, thereby, a voltage at the output node 49, i.e., the internal supply voltage Vcc.sub.int, of the output circuit 70 will be increased. Furthermore, as the temperature increases more and more, the current flow formed at the channel of the first PMOS transistor 46 of the voltage sharing circuit 80 is reduced more and more, so that the internal supply voltage of the output node 49 of the output circuit 70 increases with reference to increase of the temperature. As a result, since the deterioration of MOS transistor due to variation of the temperature is prevented, it can be possible to have a semiconductor memory device operating stably.

For a better understanding of the voltage sharing circuit 80 according to the present invention, a table in FIG. 7 will be described hereinbelow, wherein the current driving power decrease ratio is shown in detail for MOS transistors with the gate oxide layer thereof being 160 .ANG. thick. In the following, as a matter of convenience, the PMOS transistor will be described together with the NMOS transistor; however, the matters related to the PMOS transistor are described in a parenthesis. In the table, the current driving power for the NMOS (PMOS) transistor is measured under the condition that the gate and drain voltages applied thereto are both +4.0 V (-4.0 V), and the substrate-source voltage is -2.0 V (0 V). The current driving power decrease ratio at 85.degree. C. is illustrated in comparison with the current driving power decrease ratio at 25.degree. C. It will be understood to a person skilled in the art that the high decrease ratio of the current driving power means the high increase ratio of the channel resistance in response to increase of the temperature, of the MOS transistor. Accordingly, as the channel conductance of the MOS transistor is made low, the channel resistance thereof is high and, thus, the resistance increase ratio thereof is also high.

From the foregoing descriptions, it is understood that the inventive internal supply voltage generator can compensate for the decrease of the current driving power due to increase of the temperature as well as for the decrease of the operational speed thereof due to the decrease of the current driving power. Therefore, the semiconductor memory device employing the inventive circuit can be operated stably regardless of the variation of the temperature.

Although specific constructions and procedures of the invention have been illustrated and described herein, it is not intended that the invention be limited to the elements and constructions disclosed. One skilled in the art will easily recognize that the particular elements or subconstructions may be used without departing from the scope and spirit of the invention.