SEMICONDUCTOR DEVICE

Abstract: This semiconductor device has power source terminals (P1, P2) for receiving a power source voltage (VCC) and a volt­ age converting circuit (12) for generating an output voltage lower than the power source voltage (VCC) on a semiconduc­ or chip. In particular, the voltage converting circuit (12) has a constant voltage circuit (16) for generating a reference voltage (VR) lower than the power source voltage, a voltage drop cir­ cuit (20) which is connected to the power source terminal (P1), and a differential amplifying circuit (18) for controlling the con­ ductivity of the voltage drop circuit (20) in accordance with a difference between the output voltage (VDD) of the voltage drop circuit (20) and the reference voltage (VR) from the con­ stant voltage circuit (16). This differential amplifying circuit (18) generates a control voltage which linearly varies at a gentle slope in response to a change in feedback voltage near the ref­ erence voltage (VR).

Description: The present invention relates to a semiconductor device and, more particularly, to a semiconductor device having a voltage converting circuit to supply power to insulated gate MOS transistors each of which has a very short effective channel length. Recently, manufacturing technology for MOS LSIs has developed remarkably, so that very small MOS transistors can be formed on a semiconductor chip with high integration. A very large scale integrated circuit (i.e., VLSI) having tens of thousands of MOS transistors, each having an effective channel length of about 1.5 µm, has already been developed. It is speculated that a MOS transistor having an effective channel length of less than 1 µm will be formed on a semiconductor chip in the future. In the case of increasing the degree of integration of a MOS LSI without changing the operation voltage, as the effective channel length of a MOS transistor is reduced, the electric field in this MOS transistor generally increases more than that before such a reduction. The following phenomena are induced due to this increase in the electric field and become obstacles, particularly in the development of a MOS LSI having submicron MOS transistors (effective channel length is less than 1 µm). These phenomena include: 1. Generation of hot electron or hot hole due to impact ionization. 2. Increase in substrate current. 3. Punch through. 4. Change in threshold voltage with the elapse of time that is caused since a gate insulation film traps the hot carrier and the like. Especially, the time-change of the threshold voltage shown in the fourth item largely affects the performance and reliability of a MOS transistor. As countermeasures to these phenomena, it is important to prevent the occurrence of a high electric field in the channel on the drain side. For example, it can be considered to use LDD (Lightly Doped Drain) transistor technology whereby the drain of the MOS transistor is formed at a low density. Although this low-density drain can lighten the electric field in the channel, the conductance of the MOS transistor will have decreased due to this drain. In the case where it is intended to form a further fine MOS transistor than the current MOS transistor, this reduction of conductance cannot be ignored. At the same time, its manufacturing process will become difficult. Therefore, the usefulness of such a counter plan will become low in the future, although this method is presently adopted. As another countermeasure, it can be considered to set the operating voltage of the MOS transistor to a low value. However, there is a case where this counter-measure is still impractical since a submicron MOS LSI has not come into wide use yet. For instance, in the case where the MOS LSI is one of the circuit parts of a computer system, this MOS LSI is ordinarily organically connected to other circuit parts, e.g., TTL devices on a printed circuit board. Therefore, to realize miniaturization and cost reduction of a computer system, it is desirable that electricity be commonly supplied from a single power source to these circuit parts. If the operating voltage of the MOS LSI is lower than those of other circuit parts, the computer system will need an additional power source for this MOS LSI. Practically speaking, in current computer systems, the operating voltage (i.e., 5 V) of the TTL device is adopted as the standard power source voltage. Therefore, it is desirable that the MOS LSI operate at 5 V irrespective of the effective channel length of the MOS transistors therein. Earlier application EP-A-0 113 458 which falls within the field of Article 54(3) EPC discloses a MIS semiconductor integrated circuit comprising a differential amplifying circuit used for a switching operation in which a MOS transistor of a voltage drop circuit is selectively turned on and off. The MOS transistor is turned on when it is detected that a power source voltage is lower than a reference voltage generated by voltage generating means and turned off when it is detected that the power source voltage is higher than the reference voltage. It is an object of the present invention to provide a semiconductor device having a voltage converting circuit which can convert a power source voltage that is supplied to a semiconductor chip from the outside to a voltage at which an MOS transistor having a very short effective channel length can stably operate. To solve this object, the present invention provides a semiconductor device as defined in claim 1. According to the present invention, even if the output voltage of the voltage drop circuit varies due to, for example, a variation in temperature, power source voltage or the like, the feedback loop in the voltage converting circuit can always recover the output voltage to a set voltage. If this set voltage is sufficiently low, a MOS transistor, which is capable of operating at this voltage, can be formed in a sufficient short effective channel length. Therefore, the obstacle to an increase in the integration of a MOS LSI can be reduced. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 shows a structure of an MOS LSI device according to an embodiment of the present invention; Fig. 2 is a detailed circuit diagram of a voltage converting circuit in the MOS LSI of Fig. 1; Fig. 3 is a graph showing the input/output operation characteristics of a differential amplifying circuit shown in Fig. 2; Fig. 4 is a graph showing a time-change of an output voltage of a voltage drop circuit shown in Fig. 2; and Fig. 5 shows a part of a voltage converting circuit, which is similar to the voltage converting circuit in Fig. 2, in a MOS LSI according to another embodiment of the present invention. A semiconductor device of one embodiment of the present invention will now be described with reference to Figs. 1 to 4. The MOS LSI device of Fig. 1 has an LSI circuit 10 (serving as, for example, a memory or a data processor) and a voltage converting circuit 12 to supply power to the LSI circuit 10 on the same semiconductor chip 14. Power source terminals P1 and P2 are formed in the outer peripheral portions on the semiconductor chip 14 together with data input and output terminals IN and OUT of the LSI circuit 10. The power source terminals P1 and P2 are connected to an external power source (not shown) for generating a power source voltage VCC, e.g., 5 V, and are set to the potentials of 5 V and 0 V, respectively. The LSI circuit 10 is constituted by MOS transistors each having a very short (e.g., less than 1 µm) effective channel length and is designed so as to operate at a voltage, e.g., 3 V, which is lower than the power source voltage VCC. The voltage converting circuit 12 is connected to the power source terminals P1 and P2 in order to receive the power source voltage VCC and is also connected to power lines L1 and L2 of the LSI circuit 10 in order to supply an output voltage VDD. The voltage converting circuit 12 includes MOS transistors, each having a sufficiently long effective channel length (e.g., 2 µm or more). In the voltage converting circuit 12, the output voltage VDD is controlled such that it is maintained at an operating voltage (i.e., 3 V) of the LSI circuit 10. Fig. 2 shows a detailed structure of the voltage converting circuit 12 shown in Fig. 1. This voltage converting circuit 12 includes a constant voltage generating circuit 16, a differential amplifying circuit 18, and a voltage drop circuit 20. The constant voltage generating circuit 16 is provided to supply a reference voltage VR to the differential amplifying circuit 18. The reference voltage VR is set to a voltage (i.e., 3 V) which is equal to the operating voltage of the LSI circuit 10. The constant voltage generating circuit 16 has a load 22 and a diode series 24. One end of the load 22 is connected to the power source terminal P1. The diode series 24 is connected between the other end of the load 22 and the power source terminal P2 in the forward direction. The differential amplifying circuit 18 has p-channel MOS transistors T1 and T2 as loads, n-channel MOS transistors T3 and T4 as a differential pair, and an n-channel MOS transistor T5 as a constant current source. In the differential amplifying circuit 18, a control voltage to the voltage drop circuit 20 is generated in correspondence to the difference between the gate voltages of the MOS transistors T3 and T4. A gate of the MOS transistor T3 is connected to an output terminal of the voltage drop circuit in order to receive the output voltage VDD. A gate of the MOS transistor T4 is connected to a junction between the load 22 and the diode series 24 in order to receive the reference voltage VR. A bias voltage VB is supplied to a gate of the MOS transistor T5. When the bias voltage is 5 V, a gate of the MOS transistor T5 is connected to the power source terminal P1. Gates of the MOS transistors T1 and T2 are commonly connected to a drain of the MOS transistor T3. A source of the MOS transistor T5 is connected to the power source terminal P2. A drain of the MOS transistor T5 is connected to the power source terminal P1 through the current path of the MOS transistors T3 and T1 in this order and is also connected to the power source terminal P1 through the current path of the MOS transistors T4 and T2 in this order. Back gates of the MOS transistors T1 and T2 are commonly connected to the power source terminal P1. Back gates of the MOS transistors T3, T4 and T5 are commonly connected to the power source terminal P2. The voltage drop circuit 20 has a p-channel MOS transistor T6. A gate of the MOS transistor T6 is connected to a drain of the MOS transistor T4 in order to receive the control voltage. A source and a back gate of the MOS transistor T6 are commonly connected to the power source terminal P1. A drain of the MOS transistor T6 is connected through the power line L1 to the LSI circuit 10 in order to supply the output voltage VDD and is also connected to the gate of the MOS transistor T3. A gate width of the MOS transistor T6 is set to a value wide enough to supply a load current to the LSI circuit 10. The differential amplifying circuit 18 will now be further explained in detail. Channel widths W1, W2, W3, and W4 and channel lengths L1, L2, L3, and L4 of the respective MOS transistors T1, T2, T3, and T4 are set so as to have the following relation: W1 L1 · L3 W3 = W2 L2 · L4 W4 (1) Namely, equation (1) denotes that the differential amplifying circuit 18 is of the current mirror type. Fig. 3 shows the input/output operation characteristics of the differential amplifying circuit 18. FEEDBACK VOLTAGE in Fig. 3 corresponds to the output voltage VDD fed back to the gate of the MOS transistor T3 from a drain of the MOS transistor T6. The differential amplifying circuit 18 generates the control voltage shown in Fig. 3 in response to this feedback voltage. In addition, the control voltage increases or decreases depending on an increase or decrease in the feedback voltage, respectively. In particular, when the feedback voltage is about 3 V, the differential amplifying circuit 18 allows the control voltage to change linearly at a gentle slope, thereby controlling the MOS transistor T6 such that the output voltage VDD is maintained at the operating voltage 3V of the LSI circuit 10. The operation of the voltage converting circuit 12 will now be described with reference to Fig. 4. When the power source voltage VCC (= 5 V) is supplied between the power source terminals P1 and P2, the output voltage VDD rises and is then stably set at 3 V. Upon an application of this output voltave VDD to the LSI circuit 10, the load current starts flowing to the LSI circuit 10 through the current path of the MOS transistor T6. Immediately after this, the drain voltage (i.e., output voltage VDD) of the MOS transistor T6 drops as shown in Fig. 4 due to the charging of the LSI circuit 10. The output voltage VDD is supplied as the feedback voltage to the gate of the MOS transistor T3. A current corresponding to this feedback voltage flows through the current path of the MOS transistors T1 and T3. Then, a mirror current equal to this current flows through the current path of the MOS transistors T2 and T4. The constant voltage generating circuit 16 supplies the reference voltage VR (= 3V) to the gate of the MOS transistor T4, thereby maintaining the resistance of the current path of the MOS transistor T4 constant. Due to this, the voltage drop at the MOS transistor T4 is proportional to the mirror current. The MOS transistor T6 receives the voltage corresponding to the voltage drop at the MOS transistor T4 as the control voltage. The relation between the control voltage and the feedback voltage is shown in Fig. 3. Assuming that the feedback voltage is 2.75 V, the differential amplifying circuit 18 generates the control voltage of about 1 V, and this control voltage causes the conductivity of the MOS transistor T6 to increase. Then, the voltage drop at the MOS transistor decreases, so that the drain voltage (i.e., output voltage VDD) of the MOS transistor T6 increases due to the reduction in voltage drop. The output voltage VDD is again supplied as the feedback voltage to the differential amplifying circuit 18. As the feedback voltage VDD approaches the reference voltage VR (= 3 V), the differential amplifying circuit 18 gently drops the conductivity of the MOS transistor T6. When the feedback voltage reaches a value equal to the reference voltage VR (= 3 V), the control voltage of 2.5 V is supplied to the MOS transistor T6 from the differential amplifying circuit 18. The conductivity of the MOS transistor T6 is controlled by the control voltage which gently varies and this control does not cause the output voltage VDD to be increased to a value higher than the reference voltage VR. However, the output voltage VDD could be a value higher than the reference voltage VR due to the influence of, for instance, variation in load. If it should happen, the differential amplifying circuit 18 generates a control voltage higher than 2.5 V; however, this control voltage exceeds the threshold voltage of the p-channel MOS transistor T6, so that the MOS transistor T6 is made non-conductive. Consequently, the output voltage VDD assuredly drops to 3 V due to the charging of the LSI circuit 10. As described above, in this embodiment, the voltage converting circuit 12 is formed on the same semiconductor chip 14 as that of the LSI circuit 10 and serves to convert the external power source voltage VCC to the output voltage VDD lower than that voltage VCC. In this conversion, the output voltage VDD is set to a value equal to the operating voltage (i.e., reference voltage VR) of the LSI circuit 10. Therefore, no malfunction occurs in the LSI circuit 10. In other words, by adding the voltage converting circuit to the LSI circuit 10, the operating voltage of the MOS transistors in this LSI circuit 10 can be set to a low enough value. Thus, in case of increasing integration of the MOS LSI, the effective channel length of the MOS transistor can be further reduced without deteriorating performance and reliability. The feedback loop of, the voltage converting circuit 12 surely recovers the output voltage VDD to the operating voltage even if the power source voltage VCC or characteristics of the MOS transistors T1 to T6 temporarily changes to some extent due to a variation of, e.g., temperature, or the like. Thereby, the output voltage VDD is maintained at a set voltage. Thus, stable electric power can be supplied to the LSI circuit 10. The points to which attention should be paid, concerning the regulation of the output voltage VDD, will now be explained. For example, in the case where the output voltage VDD makes a larger drop from the reference voltage VR due to the starting operation of the LSI circuit 10, the conductivity of the MOS transistor T6 has to be set to a high value. In the feedback loop in this embodiment, the conductivity of the MOS transistor gently drops from this high value in accordance with the decrease in voltage difference between the output voltage VDD and the reference voltage VR. Thus, the output voltage VDD is regulated to a value equal to the reference voltage VR without exceeding the reference voltage VR. On the contrary, in the case where the loop gain of the feedback loop is high, the control voltage rapidly changes. That is, the conductivity of the voltage drop circuit 20 is maintained to a high value during the time when the output voltage VDD is lower than the reference voltage VR. When the output voltage VDD becomes equal to the reference voltage VR, the conductivity of the voltage drop circuit 20 is reduced instantaneously. In this case, the output voltage VDD is easily affected by the time-delay in the feedback loop. For example, when the output voltage VDD increases from the low voltage state to the reference voltage VR, the conductivity of the voltage drop circuit 20 is not reduced by the control voltage. After the delay time during which the output voltage VDD increases to a value exceeding the reference voltage VR, the conductivity of the voltage drop circuit 20 is reduced by the control voltage. Thereafter, the output voltage VDD changes to a value lower than the reference voltage VR due to the charging of the LSI circuit 10; then, it again changes to a value higher than the reference voltage VR. Namely, when the loop gain of the feedback loop is high, this loop serves to perform the oscillating operation as described above. Therefore, it is difficult for the output voltage VDD to be stabilized to a value equal to the reference voltage VR. In the foregoing embodiment, since the constant voltage generating circuit 16 is used to drive a single MOS transistor in the differential amplifying circuit 18, a large current driving capability is not required. Therefore, the present is not limited to the circuit shown in Fig. 2, but any other circuit can be used if it can only generate a reference voltage VR which is stable against variation in external power source voltage or temperature. Further, in the foregoing embodiment, the differential amplifying circuit 18 is of the current mirror type, but it may be substituted by a differential amplifying circuit of the non-current mirror type if the change characteristic of the output voltage is gentle. Fig. 5 shows a voltage converting circuit of a semiconductor device according to another embodiment of the present invention. In the semiconductor device of this embodiment, similar parts and components as those shown in the foregoing embodiment are designated by the same reference numerals. In addition, the portions which are not shown in the drawing are the same as those in the foregoing embodiment. In this embodiment, the p-channel MOS transistor T6 in Fig. 2 is replaced by an n-channel MOS transistor T7. Further, the reference voltage VR is supplied to the gate of the MOS transistor T3 and the output voltage VDD is supplied as the feedback voltage to the gate of the MOS transistor T4. According to this embodiment, the stable output voltage VDD can be generated similar to the foregoing embodiment. However, in a comparison of the current driving capability of the voltage drop circuit between the n-channel and p-channel MOS transistors under the same conditions, the p-channel MOS voltage drop circuit is superior to the n-channel MOS one. In the differential amplifying circuits in Figs. 2 and 5, the p-channel MOS transistors may be replaced by the n-channel MOS transistors, and the n-channel MOS transistors may be replaced by the p-channel MOS transistors, respectively. Further, the polarities of the power source terminals P1 and P2 may be reversely set. For instance, the power source terminals P1 and P2 may be set to the potentials of 0 V and 5 V.

Claims: A semiconductor device including a voltage converting circuit (12) for supplying a voltage to a load circuit (10), the device having first and second power source terminals (P1, P2) for receiving a power source voltage (VCC) and the voltage converting circuit (12) comprising: voltage generating means (16) for generating a constant voltage lower than said power source voltage (VCC), as a reference voltage (VR); voltage drop means (20), connected in series with said load circuit (10) between said first and second power source terminals (P1, P2), for supplying a regulated voltage (VDD) to said load circuit (10), the resistance of said voltage drop means (20) being variable by a control voltage; and differential amplifying means (18) for detecting a voltage difference between said reference voltage (VR) and regulated voltage (VDD) and generating, as said control voltage, an output voltage in accordance with the detected voltage difference, the output voltage of said differential amplifying means (18) being determined so that said voltage drop means (20) is switched off when the regulated voltage (VDD) rises above said reference voltage, and conducts to varying degrees depending on the value of the control voltage when the regulated voltage (VDD) falls below the reference voltage (VR), wherein: said differential amplifying means (18) has an input/output characteristic in which the output voltage changes at an essentially constant ratio to the regulated voltage (VDD) within a limited range of the regulated voltage (VDD), in which range said reference voltage (VR) is present, and changes at a ratio lower than said essentially constant ratio with respect to the regulated voltage (VDD) in a range below said limited range, so that the regulated voltage (VDD) is maintained at a set voltage. A semiconductor device according to claim 1, characterized in that said voltage drop means (20) includes a MOS transistor (T6; T7) having a gate connected to receive said control voltage and a current path connected between said first power source terminal (P1) and said load circuit (10). A semiconductor device according to claim 2, characterized in that the output voltage of said differential amplifying means (18) is determined such that the MOS transistor (T6; T7) of said voltage drop means (20) is made nonconductive when said regulated voltage (VDD) rises above the reference voltage (VR). A semiconductor device according to claim 3 , characterized in that said differential amplifying means includes a differential amplifying circuit (18) having a current mirror load (T1, T2), a differential pair (T3, T4), and a constant current source (T5) which are connected in series between said first and second power source terminals (P1, P2). A semiconductor device according to claim 4, characterized in that said MOS transistor of said voltage drop means is a MOS transistor (T6) of a first conductivity type, said current mirror load is constituted by first and second MOS transistors (T1, T2) of a first conductivity type, said differential pair is constituted by third and fourth MOS transistors (T3, T4) of a second conductivity type, a gate of said fourth MOS transistor (T4) is connected to an output terminal of said voltage generating means (16), a gate of said third MOS transistor (T3) is connected to the junction of said voltage drop means (20) and load circuit (10), one end of said constant current source (T5) is connected to said second power source terminal (P2), current paths of said first and third MOS transistors (T1, T3) are connected in series with each other between said first power source terminal (P1) and the other end of said constant current source (T5), current paths of said second and fourth MOS transistors (T2, T4) are connected in series with each other between said first power source terminal (P1) and said other end of said constant current source (T5), gates of said first and second MOS transistors (T1, T2) are connected to the junction of the current paths of said first and third MOS transistors (T1, T3), and the gate of said MOS transistor (T6) of said voltage drop means (20) is connected to the junction of the current paths of said second and fourth MOS transistors (T2, T4) (Fig. 2). A semiconductor device according to claim 4, characterized in that said MOS transistor of said voltage drop means is a MOS transistor (T7) of a first conductivity type, said current mirror load is constituted by first and second MOS transistors (T1, T2) of a second conductivity type, said differential pair is constituted by third and fourth MOS transistors (T3, T4) of a first conductivity type, a gate of said fourth MOS transistor (T4) is connected to the junction of said voltage drop means (20) and load circuit (10), a gate of said third MOS transistor (T3) is connected to an output terminal of said voltage generating means (16), one end of said constant current source (T5) is connected to said second power source terminal (P2), current paths of said first and third MOS transistors (T1, T3) are connected in series with each other between said first power source terminal (P1) and the other end of said constant current source (T5), current paths of said second and fourth MOS transistors (T2, T4) are connected in series with each other between said first power source terminal (P1) and said other end of said constant current source (T5), gates of said first and second MOS transistors (T1, T2) are connected to the junction of the current paths of said first and third MOS transistors (T1, T3), and the gate of said MOS transistor (T7) of said voltage drop means (20) is connected to the junction of the current paths of said second and fourth MOS transistors (T2, T4)(Fig. 5). A semiconductor device according to claim 1, characterized in that said voltage generating means (16) has resistive means (22) and a diode series (24) which are connected in series between said first and second power source terminals (P1, P2) A semiconductor device according to claim 1, characterized in that said first and second power source terminals (P1, P2), said voltage generating means (16), said voltage drop means (20), and said differential amplifying means (18) are formed on a semiconductor chip (14). A semiconductor device according to claim 8, characterized in that said load circuit (10) is formed on said semiconductor chip (14). A semiconductor device according to claim 9, characterized in that said load circuit is an LSI circuit (10) containing at least one submicron MOS transistor.

Inventors: OHSAWA, TAKASHI C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1985
Main classification: G05F1
IPC codes: H01L21, G11C11, H03F1, H01L27, G05F1, G11C5, H03F3