Abstract:
A current-controlling MOS transistor is connected between a power source and an MOS circuit. A control voltage which has a level related to temperature is applied to the gate electrode of the control MOS transistor in order to compensate for current reduction at high temperatures due to the lowering of the mobility of minority carriers. The response time of the MOS circuit is made less dependent on temperature as a result of the current compensation.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates to a semiconductor integrated circuit with high operating speed, and more particularly to a semiconductor integrated circuit wherein the operating speed is not influenced by temperature. 
     II. Description of the Prior Art 
     Generally, the current through an IGFET, such as a metal oxide semiconductor (MOS) transistor, decreases with increased temperature due to the lowering of the mobility of the minority carriers (electrons or holes). Accordingly, the response time in conventional MOS integrated circuits increases with temperature. For instance, the response time at 85° C. is 400 n.sec., as compared to 300 n.sec. at 25° C. due to the fact that as the current through an MOS transistor decreases with temperature, the charge-discharge current at some circuit points decreases and more time is required to charge and discharge it. Accordingly, the operating speed of a conventional device is influenced by temperature as shown in FIG. 1. The solid line and dotted line show response time and current through an MOS transistor, respectively. 
     SUMMARY OF THE INVENTION 
     Therefore, one object of this invention is to provide a semiconductor integrated circuit in which the operating speed is constant independent of temperature. 
     Another object of this invention is to provide a semiconductor integrated circuit in which the operating speed is very high. 
     Yet another object of this invention is to provide a semiconductor integrated circuit in which the reduction of operating speed at high temperatures is compensated. 
     According to this invention, a semiconductor integrated circuit comprises: (a) a first power source; (b) a second power source; (c) an MOS (Metal Oxide Semiconductor) circuit connected between the power sources; (d) a control signal generating means for generating a control voltage corresponding to temperature; (e) a control MOS transistor connected between the first power source and the MOS circuit, the control signal being applied to the gate electrode of the control MOS transistor so as to control the current through the MOS transistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with an accompanying drawing, wherein: 
     FIG. 1 shows a diagram of the characteristics of the current through a conventional MOS transistor and response time of a conventional MOS transistor circuit with respect to temperature. 
     FIG. 2 shows a semiconductor integrated circuit of a preferred embodiment of the present invention. 
     FIGS. 3, 5, 13 and 15 show control signals applied to control MOS transistors. 
     FIG. 4 shows a control signal generating circuit according to the present invention. 
     FIG. 6 shows a diagram of the characteristics of drain current with respect to drain voltage. 
     FIG. 7 shows a control signal generating circuit of another embodiment according to the present invention. 
     FIG. 8 shows a diagram of the characteristics of the current through an MOS transistor circuit, according to the present invention, with respect to temperature. 
     FIGS. 9 to 12, 14, 16, 17 and 18 show semiconductor integrated circuits of other embodiments according to the present ivention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     There will now be discussed a semiconductor integrated circuit according to one embodiment of this invention with reference FIG. 2. 
     A three stage inverter circuit (INV1, INV2 and INV3) is constructed by depletion type MOS transistors 1, 2 and 3 as loads and enhancement type MOS transistors 4, 5 and 6 as driving transistors. Furthermore, enhancement type control MOS transistors 7, 8 and 9 are respectively connected between power source V c  and MOS transistors 1, 2 and 3. A control voltage S 1  (shown in FIG. 3) is generated by control signal generating means 10 and is applied to the gate electrodes of MOS transistors 7, 8 and 9. As indicated in FIG. 3, control voltage S 1  increases with temperature. In this embodiment, N channel type MOS transistors are employed but of course P channel type MOS transistors could be used. 
     One example of control signal generating means 10 is shown in FIG. 4. The control signal generating means generates a control voltage which increases in proportion to temperature as shown in FIG. 3 or increases stepwise as shown in FIG. 5. Depletion type MOS transistor 51 whose gate electrode is connected with its own source electrode is connected between power source V c  and first output terminal O 1 . Two enhancement type MOS transistors 52 and 53 having gate electrodes connected to their drain electrodes respectively are connected between first output terminal O 1  and ground in series. A resistor element 54 made from polysilicon is connected between power source V c  and second output terminal O 2  and an enhancement type MOS transistor 55 whose gate electrode is connected to the first output terminal O 1  is provided between second output terminal O 2  and ground. The output signal from the second output terminal O 2  is amplified by amplifier circuit 70 composed of two amplifier stages each consisting of MOS transistors 56 and 57. The output signal from third output terminal O 3  is transformed by a transforming circuit 80 composed of two inverters INV 1  and INV 2  each consisting of depletion type MOS transistor 58 and enhancement type MOS transistor 59. In this circuit, if the threshold voltage of MOS transistors 52, 53 and 55 is equally Vth, the output voltage V 01  of the first output terminal at room temperature becomes 
     
         V.sub.01 =2Vth+α(α≧0)                   (1) 
    
     where α is the voltage drop caused by the internal resistance (very small) of transistors 52 and 53, and the effective threshold voltage Vth of MOS transistor 55 from output voltage V 01  becomes 
     
         V.sub.g -V.sub.th =(2Vth+α)-Vth=Vth+α          (2) 
    
     Generally, the threshold voltage of an MOS transistor decreases with temperature rise, therefore the threshold voltage of an MOS transistor becomes (Vth-β) at raised temperature T°C. Accordingly, the effective gate voltage in equation (2) is (Vth-β)+α. Namely, the voltage applied to the gate electrode of MOS transistor 55 decreases by β as the mobility of minority carriers decreases, which results in a shift (from A to B as shown in FIG. 6) of the curve which shows characteristics of drain current with respect of drain voltage of MOS transistor 55. On the other hand, the resistor element made from polysilicon has a negative temperature characteristic. Therefore, the load curve shifts from C to D. Accordingly, the output voltage V 02  of the second output terminal O 2  changes from V 1  to V 2  and the control signal as shown in FIG. 3 can be obtained by amplifying the output V 02  by amplifier circuit 70. Furthermore, the step type control signal as shown in FIG. 5 can be obtained by transforming the output signal of amplifier circuit 70 by a transforming circuit 80. 
     Generally, it is well known that polysilicon may have either a positive or negative temperature characteristic with respect to resistor value. Of course, it is possible to employ a polysilicon resistor element which has a positive temperature characteristic, but it is desirable to use a polysilicon resistor element with a negative temperature characteristic so that V 02  changes by a large degree. 
     FIG. 7 shows another embodiment of the control signal generating circuit. A bipolar transistor is used in place of resistor element. 
     In the inverter circuit shown in FIG. 2, MOS transistors 7, 8 and 9 are selected so that the response time and the current through each inverter stage are determined by them and the MOS transistors 1, 2, 3 and 4, 5, 6, and each stage can charge or discharge the load capacitance of following inverter stage. The reduction of current through each inverter stage at high temperature is prevented by changing the voltage applied to the gate electrodes of MOS transistors 7, 8 and 9 corresponding to the temperature. Therefore, it is possible to make the response time relatively constant, independent of temperature as shown in FIG. 8. 
     FIG. 9 shows a second embodiment according to this invention. Depletion type MOS transistors 17, 18 and 19, to which the control signal is applied, are used in place of enhancement type transistors 7, 8 and 9 in FIG. 2. 
     FIG. 10 shows a third embodiment of this invention. Generally a buffer circuit is employed, in a semiconductor integrated circuit using MOS transistors, to drive a circuit point with a large load capacitance. This embodiment employs a buffer circuit in accordance with this invention. The buffer circuit 90 is constructed by inverter stages INV a  and INV b  including depletion type MOS transistors 21 and 23 and enhancement type MOS transistors 22 and 24. Depletion type transistor 25 is controlled by the output of the first inverter stage INV a  and enhancement type MOS transistor 26 is controlled by the output of the second inverter stage INV b . The output terminal of the buffer circuit 90 is connected to circuit point P with a large load capacitance. Enhancement type MOS transistor 28 and depletion type MOS transistor 27 are connected in series between power source V c  and circuit point P. The control signal S 1  and the output of the first inverter stage INV a  are applied to gate electrodes of MOS transistors 28 and 27, respectively. When the input signal is low, MOS transistor 25 is rendered conductive. Any reduction of charge current at circuit point P caused by a temperature rise is prevented by applying control signal S 1  to the gate electrode of MOS transistor 28. In this embodiment, MOS transistor 28 may be a depletion type and it is possible to delete MOS transistor 27. 
     In an integrated circuit, the response time and current consumption depend on the number of circuit points with large load capacitance. Therefore, even if temperature compensation MOS transistors are provided only to the circuit points with large load capacitance, it is still very effective to make the operating speed constant. 
     FIG. 11 shows a fourth embodiment according to this invention. This circuit is similar to that of FIG. 10, with the addition of MOS transistors 36 and 37 connected in series between circuit point P and ground. Gate electrodes of MOS transistors 35 and 36 are connected to output terminals of inverters INV a  and INV b , respectively. This circuit increases the discharging speed of circuit point P as well as charging speed. 
     FIG. 12 shows a fifth embodiment according to this invention. The invention is applied to a CMOS circuit in contrast with circuits using N or P channel type MOS transistors shown in FIGS. 1 to 11. P channel type MOS transistor 43 and N channel type MOS transistor 44 are connected between an inverter circuit consisting of P channel type MOS transistor 41 and N channel type MOS transistor 42 and power sources (V cc , Ground), respectively. Control signals S A  whose voltage decreases with temperature rise and S B  whose voltage increases with temperature rise (as shown in FIG. 13) are applied to gate electrodes of MOS transistors 43 and 44. The control signals S A  and S B  are generated by the control generating circuit shown in FIG. 4. In this embodiment, the current reduction associated with temperature rise is compensated for by applying such control signals to the gate electrodes of MOS transistors 43 and 44 that increase the current therethrough. 
     FIG. 14 shows a sixth embodiment according to this invention. P channel type MOS transistors 43 and 45 are connected in parallel between power source V c  and an inverter circuit similar to that shown in FIG. 12, and control signals SC and SD, shown in FIG. 15, are applied to gate electrodes thereof, respectively. Similarly, N channel type MOS transistors 44 and 46 are connected in parallel between a power source (Ground) and the inverter circuit, and control signals SC and SD are applied to the gate electrodes thereof, respectively. In this embodiment, the channel width of the MOS transistors 44 and 45 is smaller than that of MOS transistors 43 and 46. The control signals SC and SD generated by a control signal generating circuit changes stepwise as shown in FIG. 15. Namely, the control signal SC changes from a high level to a low level and on the other hand the control signal SD changes from a low level to a high level at some temperature point T A . MOS transistors 44 and 45 with small channel width are rendered conductive below temperature T A , which results in a small current flow. MOS transistors 43 and 46, with large channel width, are rendered conductive above temperature T A , which results in large current flow. 
     FIG. 16 shows a seventh embodiment according to this invention applied to an oscillator. The oscillator is composed of three inverter stages each consisting of depletion type MOS transistor 61 and enhancement type MOS transistor 62. Depletion type MOS transistors 63 and 64 are connected between inverter stages. Conventionally, a constant voltage was applied to the gate electrodes of MOS transistors 63 and 64. But, the oscillation frequency of prior art devices dropped due to the current reduction resulting from temperature rise. In this embodiment control signal S 1  is applied to the gate electrodes of MOS transistors 63 and 64 to prevent the reduction of oscillation frequency with temperature rise. It is also possible to raise the oscillation frequency with temperature rise. 
     FIG. 17 shows an eighth embodiment according to this invention. This embodiment is similar to that in FIG. 16 with the addition of enhancement type MOS transistors 65 and 66 connected in parallel with depletion type MOS transistors 63 and 64. Control signal S 1  is applied to the gate electrodes thereof. 
     FIG. 18 shows a ninth embodiment according to this invention. The oscillator is constructed by a CMOS circuit. P channel type MOS transistors 73, 75 and N channel type MOS transistors 74, 76 are coupled and inserted between inverter stages consisting of P channel type MOS transistor 71 and N channel type MOS transistor 72. Control signals SA and SB, shown in FIG. 13, are applied to the gate electrodes of MOS transistors 73 and 75, 74 and 76, respectively. 
     Although this invention has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible of numerous other applications which will be apparent to those skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the claims.