Abstract:
A semiconductor circuit has first and second MOS transistors which are connected between an output terminal and a positive and a reference power source terminal, respectively, a bootstrap capacitor connected between the output terminal and the gate of the first MOS transistor, an inverter which inverts the input signal and which supplies the inverted signal to the gate of the second MOS transistor after a predetermined delay timne, and a switching MOS transistor having a current path connected between the input terminal and the gate of the first MOS transistor. The switching MOS transistor has a threshold voltage greater than that of the second MOS transistor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor circuit with a bootstrap circuit. 
     Various studies on semiconductor circuits have been made recently in order to achieve higher integration density and high speed operation. Improvements in the read/write speed of a static MOS memory are particularly notable; the access time is almost comparable to that of a bipolar memory. Such improvements in the operation speed of the static MOS memory are mostly attributed to improvements in techniques of reducing the size of the MOS transistors and in circuit design of MOS transistors. For example, a semiconductor circuit which may be operated with less power consumption and at a high speed is being developed by using a bootstrap circuit, which is widely used in a dynamic semiconductor circuit, in a static semiconductor circuit. 
     A conventional driver circuit for driving a node having a large stray capacitance is known which uses a push-pull type inverter as shown in FIG. 1. This driver circuit has an E/D-type inverter 2 which includes a depletion-type (D-type) MOS transistor TR2 and an enhancement-type (E-type) MOS transistor TR4 and which inverts an input signal VI, and a push-pull circuit 4 which includes D-type and E-type MOS transistors TR6 and TR8 whose current paths are series-connected between power source terminals VD and VS. The input signal VI is supplied to the gate of the MOS transistors TR4 and TR6, the output signal from the inverter 2 is supplied to the gate of the E-type MOS transistor TR8, and a stray capacitor associated between the power source terminals VD and VS is charged and discharged in accordance with the output signal from the push-pull circuit 4. 
     The driver circuit as described above is advantageous as compared to an E/D-type inverter driver circuit in that a DC current may be made small and in that a relatively large stray capacitor can be charged with a small power consumption since a large charging current is permitted to flow only in the transient state. However, since the output stage of such a driver circuit is formed of an E/D-type push-pull circuit, a DC current will flow through this push-pull circuit. 
     In order to prevent the flow of such a DC current, a static bootstrap buffer circuit has been proposed which uses an E/E-type output circuit, as shown in FIG. 2, and which is capable of raising the output signal of high level to the VD level. The buffer circuit has an inverter 2 which is formed of a D-type MOS transistor TR2 and an E-type MOS transistor TR4 and which inverts the input signal VI, an output circuit 6 formed of E-type MOS transistors TR12 and TR14 whose current paths are series-connected between power source terminals VD and VS, and an E-type MOS transistor TR16 whose current path is connected between the input terminal VI and the gate of the MOS transistor TR12. The output signal from the inverter 2 is supplied to the gate of the MOS transistor TR14, and a capacitor C2 is connected between the gate of the MOS transistor TR12 and a node 8 between the MOS transistors TR12 and TR14. 
     When the input signal VI rises to a logic level &#34;1&#34; in this buffer circuit, the capacitor C2 begins to be charged. The capacitor C2 continues to be charged until the input voltage VI reaches the threshold voltage of the MOS transistor TR4 to turn on the MOS transistor TR4, and the output signal from the inverter 2 is lower than the threshold voltage of the MOS transistor TR14 to turn off the MOS transistor TR14. When the MOS transistor TR14 is turned off, the potential at the node 8 at one end of the capacitor C2 rises to a logic level &#34;1&#34;, and the potential at the other end of the capacitor C2 rises quickly due to the bootstrap action. As a result, a voltage higher than the power source voltage VD is applied to the gate of the MOS transistor TR12 and the power source voltage VD is supplied to the node 8 through the MOS transistor TR12. Thus, the potential at the node 8 rises to the power source voltage VD. On the other hand, when the input signal VI falls to a logic level &#34;0 &#34;, a signal of logic level &#34;1&#34; is generated by the inverter 2 to turn on the MOS transistor TR14 and to discharge the capacitor C2. In this case, the MOS transistor TR12 is turned off, and no DC current flows in the output circuit 6. Accordingly, the power consumption of the buffer circuit is suppressed to the minimum. 
     The push-pull circuit of bootstrap-type as described above is excellent in terms of low power consumption. However, in order to operate this push-pull circuit at a high speed, the switching speed of the inverter acting as a delay circuit must be increased. When the switching speed of the inverter 2 is increased, the bootstrap action may be initiated before the capacitor C2 is sufficiently charged. If the MOS transistor TR14 is turned off in response to the output signal from the inverter 2 before the potential level of the input signal VI reaches a level of (VC - VTH) (VTH is the threshold voltage of the MOS transistor TR16), the charge on the capacitor C2 is discharged through the MOS transistor TR16 since the MOS transistor TR16 is kept on. Then, effective bootstrap action may not be obtained, and therefore, the output signal VO will rise gently. Accordingly, even if the switching speed of the inverter 2 is increased and the timing of the leading edge of an output signal VO is made earlier, the output signal VO will take a long rise time to reach a predetermined high potential level. This lowers the switching speed of the push-pull circuit. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a semiconductor circuit which has a bootstrap circuit capable of operating at a high speed and in a stable manner. 
     According to an aspect of the present invention, there is provided a semiconductor circuit comprising a load MOS transistor and a driver MOS transistor which are series-connected between first and second terminals; bootstrap capacitive means connected between the gate of the load MOS transistor and a node between the load drive MOS transistor and the driver MOS transistor; inverting means for inverting an input signal supplied to a signal input terminal and for supplying an inverted signal to the gate of the driver MOS transistor after a predetermined delay time; and a switching MOS transistor which has a current path connected between the signal input terminal and the gate of the load MOS transistor and which has a threshold voltage greater than that of the driver MOS transistor. 
     According to another aspect of the present invention, there is also provided a semiconductor circuit comprising a load MOS transistor and a driver MOS transistor which are series-connected between first and second terminals; bootstrap capacitive means connected between the gate of the load MOS transistor and a node between the load drive MOS transistor and the driver MOS transistor; inverting means for inverting an input signal supplied to a signal input terminal and for supplying an inverted signal to the gate of the driver MOS transistor after a predetermined delay time; a switching MOS transistor which has a current path connected between the signal input terminal and the gate of the load MOS transistor; and gate voltage applying means for applying a gate voltage lower than a voltage supplied to the first terminal to the gate of the switching MOS transistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a conventional driver circuit for driving a node of large capacitance; 
     FIG. 2 is a circuit diagram of a conventional driver circuit having a bootstrap function; 
     FIG. 3 is a circuit diagram of a driver circuit according to an embodiment of the present invention which properly performs a bootstrap operation; 
     FIG. 4 shows the signal waveform for explaining the mode of operation of the driver circuit shown in FIG. 3; 
     FIG. 5 is a circuit diagram of a driver circuit according to another embodiment of the present invention; 
     FIGS. 6, 7 and 8 show examples of a constant voltage generating circuit used in the driver circuit shown in FIG. 5; 
     FIG. 9 is a circuit diagram of a driver circuit according to still another embodiment of the present invention; and 
     FIG. 10 shows the signal waveform for explaining the mode of operation of the driver circuit shown in FIG. 9. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows a semiconductor circuit according to an embodiment of the present invention. This semiconductor circuit is basically the same as that shown in FIG. 2 except that an n-channel MOS transistor TR18 is used in place of the MOS transistor TR16. The n-channel MOS transistor TR18 has a threshold voltage VTH1 higher than that of E-type MOS transistors TR4, TR12 and TR14. 
     A case will be considered where an input signal VI as indicated by a solid line SL1 in FIG. 4 is applied. When the input signal VI reaches a predetermined level, the E-type MOS transistor TR4 begins to be rendered conductive. At the same time, the output signal from an inverter 2 formed of a D-type MOS transistor TR2 and an E-type MOS transistor TR4 begins to fall from a power source voltage VD as indicated by a broken line BL1. At this time, with an increase in the input signal VI, a capacitor C2 is charged through the MOS transistor TR18 as indicated by a broken line BL2. Then, a predetermined charging time determined by the conduction resistance of the MOS transistor TR18 and the capacitance of the capacitor C2 elapses, and the capacitor C2 is discharged to a predetermined potential level. When the input signal VI reaches the predetermined potential level (VD-VTH1), the MOS transistor TR18 is turned off. Thereafter, when the output signal from the inverter 2 becomes sufficiently small to turn off a MOS transistor TR14 which forms an inverter 6 in cooperation with an E-type MOS transistor TR12, an output voltage VO quickly rises as indicated by a solid line SL2. In this case, the gate voltage of the MOS transistor TR12 quickly rises to a potential level significantly higher than the power source voltage VD due to the bootstrap action, as indicated by the broken line BL2. Then, the MOS transistor TR12 is set in the completely conductive state, and an output voltage of the VD level is obtained. 
     In the embodiment shown in FIG. 3, it is important that the threshold voltage of the MOS transistor TR18 is set to be greater than that of the MOS transistor TR14 so that the MOS transistor TR18 is turned off after the capacitor C2 is charged to a sufficient potential level by the potential rise of the input signal VI and before the MOS transistor TR14 is turned off. The threshold voltage of the MOS transistor TR18 is set to be equal to or smaller than the difference between the power source voltage VD and the voltage level of the input signal VI at a time when the capacitor C2 is charged to the predetermined potential level, and is set to be greater than the difference between the power source voltage VD and the voltage level of the input signal VI at a time when the output voltage from the inverter 2 becomes smaller than the threshold voltage of the MOS transistor TR14. In this manner, after the MOS transistor TR18 is turned off, the MOS transistor TR14 is turned off and the bootstrap operation is properly performed. 
     When forming the MOS transistor TR18, an impurity such as boron is ion-implanted into the channel region in the same manner as the case of the MOS transistors TR4, TR12 and TR14, and then further boron ion-implantation is performed only to the channel region of the MOS transistor TR18, so that the MOS transistor TR18 may have a channel region of higher impurity concentration. Alternatively, boron may be ion-implanted at a higher impurity concentration into the channel region of the MOS transistor TR18 in a separate step from that of doping impurities into the channel regions of the MOS transistors TR4, TR12 and TR14. 
     Referring to FIG. 4, the high level of the input signal VI is indicated lower than the power source voltage VD. This corresponds to the case wherein the load MOS transistor at the output stage of a semiconductor circuit connected to the input stage of the semiconductor circuit shown in FIG. 3 is of E-type. Thus, even if the high level of the input signal VI is lower than the power source voltage VD, a sufficiently effective bootstrap action may be obtained with the semiconductor circuit shown in FIG. 3. 
     When the input voltage VI goes from high to low level, the MOS transistor TR18 is turned on, and the capacitor C2 is discharged through this MOS transistor TR18. 
     FIG. 5 shows a circuit diagram of a semiconductor circuit according to another embodiment of the present invention. This semiconductor circuit is basically the same as that shown in FIG. 3 except that an n-channel MOS transistor TR20 having the same threshold voltage as that of E-type MOS transistors TR4, TR12 and TR14 is used in place of the MOS transistor TR18, and a constant voltage generating circuit 10 is connected to the gate of the MOS transistor TR20. In this semiconductor circuit, since a constant voltage VC lower than a power source voltage VD is applied from the constant voltage generating circuit 10 to the gate of the MOS transistor TR20, the MOS transistor TR20 is turned off in response to the leading edge of an input signal VI at a timing faster than in a case of the circuit shown in FIG. 2. The constant voltage VC is set at such a value that the MOS transistor TR20 may be turned off after a capacitor C2 is charged to a sufficient potential level by an increase in the potential level of the input signal VI and before the MOS transistor TR14 is turned off. In other words, the constant voltage VC is set to be equal to or greater than the sum of the threshold voltage of the MOS transistor TR20 and the voltage level of the input signal VI at a time when the capacitor C2 is charged to a predetermined potential level, and is smaller than the sum of the threshold level of the MOS transistor TR20 and the voltage level of the input signal VI at a time when the output voltage from the inverter 2 becomes smaller than the threshold voltage of the MOS transistor TR14. With this arrangement, similar effects as those obtainable with the semiconductor circuit shown in FIG. 3 are obtained. 
     FIGS. 6, 7 and 8 show examples of the constant voltage generating circuit 10 shown in FIG. 5. Referring to FIG. 6, the constant voltage generating circuit 10 includes two resistors 101 and 102 which are series-connected between the power source terminals VD and VS. The output voltage is obtained from the node between the resistors 101 and 102. Referring to FIG. 7, the constant voltage generating circuit 10 includes a plurality of E-type MOS transistors whose current paths are series-connected between the power source terminals VD and VS. The gate and drain of each MOS transistor are connected to each other. The output voltage is obtained from the node between the E-type MOS transistors. Referring to FIG. 8, the constant voltage generating circuit includes a plurality of D-type MOS transistors whose current paths are series-connected between the power source terminals VD and VS. The gate and source of each MOS transistor are connected to each other. The output voltage is obtained from the node of the D-type MOS transistors. 
     FIG. 9 shows a circuit diagram of a semiconductor circuit according to still another embodiment of the present invention. This semiconductor circuit is basically the same as that shown in FIG. 5 except that the drain of a MOS transistor TR2 and the gate of a MOS transistor TR20 are connected to a power source terminal VD through the current path of a D-type n-channel MOS transistor TR22 which serves as a resistor element. 
     As in the case shown in FIG. 4, when an input signal VI indicated by a solid line SL1 shown in FIG. 10 is applied, an output voltage VO, the output voltage from an inverter 2, and the gate voltage from an E-type MOS transistor TR12 change as indicated by a solid line SL2 and broken lines BL1 and BL2, respectively. When the input signal VI is at a logic level &#34;0&#34;, a power source voltage VD is applied to the gates of the MOS transistors TR14 and TR20. Thereafter, when the input signal VI becomes greater than the threshold voltage of the MOS transistor TR4, the gate voltage of the MOS transistor TR14 decreases as indicated by the broken line BL1 in accordance with the ratio of the sum of the resistances of the MOS transistors TR2 and TR22 to the conduction resistance of the MOS transistor TR14 which decreases with an increase in the input signal VI. Simultaneously, the gate voltage of the MOS transistor TR20 decreases as indicated by a dash-and-dot line DDL in accordance with the ratio of the resistance of the MOS transistor TR22 to the sum of the resistances of the MOS transistors TR2 and TR4. The resistance of the MOS transistor TR22 is set so that, in the conductive state of the MOS transistor TR4, the timing at which the input voltage VI becomes greater than the difference between the gate voltage of the MOS transistor TR20 and a threshold voltage VTH2 of the MOS transistor TR20 is after a capacitor C2 is charged to a sufficiently high potential level and before the output voltage from an inverter 2 becomes smaller than the threshold voltage of the MOS transistor TR14. With this arrangement, a proper bootstrap action may be performed as in the former embodiments. 
     When the input voltage VI goes from high to low level, the MOS transistor TR4 is turned off, and the power source voltage VD is applied to the gate of the MOS transistor TR20. The MOS transistor TR20 is completely turned on, and the capacitor C2 is quickly discharged through the MOS transistor 20. 
     Although the present invention has been described with reference to its particular embodiments, the present invention is not limited to this. For example, it is possible to set the threshold voltage of the switching MOS transistor TR20 to be greater than that of the MOS transistor TR14 in the embodiments shown in FIGS. 5 and 9.