Patent Publication Number: US-2002011889-A1

Title: Semiconductor integral circuit

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a semiconductor integral circuit that operates with a low voltage.  
       [0003] 2. Description of Related Art  
       [0004] In recent years, semiconductor integral circuits that operate with a low voltage are being developed. In this type of circuit, the threshold values of P channel type MOS transistors (PMOS hereafter) and N channel type MOS transistors (NMOS hereafter), which constitute the semiconductor integral circuits, are adjusted so as to achieve low voltage operation.  
       [0005] In other words, the threshold value of each of the PMOS transistors that constitutes a semiconductor integral circuit that operates with a low voltage is set higher than the threshold value of PMOS transistors that constitutes a semiconductor integral circuit that operates with a high voltage. In the case of a PMOS transistor, the threshold value is negative. Therefore, the absolute value of the threshold value of a PMOS that constitutes a semiconductor integral circuit that operates with a low voltage is set smaller than the absolute value of the threshold value of a PMOS that constitutes a semiconductor integral circuit that operates with a high voltage. In the case of an NMOS transistor, the threshold value is positive. Therefore, the threshold value of an NMOS transistor that constitutes a semiconductor integral circuit that operates with a low voltage is set smaller than the threshold value of an NMOS transistor that constitutes a semiconductor integral circuit that operates with a high voltage.  
       [0006] In a semiconductor integral circuit that operates with a low voltage having PMOS transistors and NMOS transistors, the threshold values of the PMOS transistors are large and the threshold values of the NMOS transistors are small. Therefore, in such a semiconductor integral circuit, the leak current of the transistor increases, and the through current of an inverter or the like increases.  
       SUMMARY OF THE INVENTION  
       [0007] Given these problems, in a semiconductor integral circuit that operates with a low voltage having PMOS transistors and NMOS transistors, it is an object of the present invention to reduce the leak current of the transistors and the through current of the inverters.  
       [0008] To achieve the above-stated objectives, a semiconductor integral circuit according to the present invention has an analog circuit which changes linearly the voltage of an input signal and causes the amount of a current flowing through the analog circuit to change in accordance with the change in the voltage of the input signal. This semiconductor integral circuit also has a logic circuit to which an input signal having a first or second voltage is input. This logic circuit outputs an output signal having the first or second voltage in response to the first or second voltage of the input signal. The absolute value of the threshold value of each MOS transistor of the analog circuit is smaller than the absolute value of the threshold value of each MOS transistor of the logic circuit.  
       [0009] According to the present invention, in the semiconductor integral circuit, highly sensitive operation of the analog circuit is guaranteed, and the leak current and through current in the logic circuit can be reduced. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0010]FIG. 1 is a block diagram showing a semiconductor integral circuit according to an embodiment of the present invention.  
     [0011]FIG. 2 is a partial circuit diagram showing an exemplary configuration of an analog circuit and a logic circuit according to the embodiment of the present invention.  
     [0012]FIG. 3( a ) shows an exemplary diode-connected NMOS transistor according to the embodiment of the present invention.  
     [0013]FIG. 3( b ) shows an exemplary diode-connected PMOS transistor according to the embodiment of the present invention.  
     [0014]FIG. 4 shows an exemplary configuration of a charge pump circuit according to the embodiment of the present invention.  
     [0015]FIG. 5 shows an exemplary configuration of a decoder circuit as the logic circuit according to the embodiment of the present invention.  
     [0016]FIG. 6 shows an exemplary configuration of a counter circuit as the logic circuit according to the embodiment of the present invention.  
     [0017]FIG. 7 shows a first method for setting the absolute value of each of the transistors of the analog circuit smaller than the absolute value of each of the transistors of the logic circuit.  
     [0018]FIG. 8 shows a second method for setting the absolute value of each of the transistors of the analog circuit smaller than the absolute value of each of the transistors of the logic circuit.  
     [0019]FIG. 9 shows a third method for setting the absolute value of each of the transistors of the analog circuit smaller than the absolute value of each of the transistors of the logic circuit.  
     [0020]FIG. 10 is a partial cross sectional view for explaining another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0021] In what follows, embodiments of the present invention will be explained with reference to the attached drawings. The following explanation will be focused on parts that are directly related to the present invention. The other parts will not be explained.  
     [0022]FIG. 1 shows a semiconductor integral circuit  100 , and an analog circuit  200  and a logic circuit  300  that are formed in prescribed regions of this semiconductor integral circuit  100 . The analog circuit  200  and the logic circuit  300  are electrically connected to each other. This semiconductor integral circuit  100  is driven by a power source voltage Vcc supplied from a power source and a ground voltage GND supplied from the ground.  
     [0023] In the analog circuit  200 , the voltage of an input signal changes linearly and the amount of a current that flows inside the analog circuit  200  changes in accordance with the change in the voltage of the input signal.  
     [0024] An input signal having the power source voltage Vcc or the ground voltage GND is input to the logic circuit  300 . in response to the voltage of this input signal, the logic circuit  300  outputs an output signal having the power source voltage Vcc or the ground voltage GND.  
     [0025]FIG. 2 shows an exemplary configuration of a circuit that constitutes a portion of the analog circuit  200  and logic circuit  300 .  
     [0026] As shown in FIG. 2, the analog circuit  200  has a differentiating amplifier circuit  210  and an analog output circuit  220  that outputs an output signal in response to a signal output from the differentiating amplifier circuit  210 . The logic circuit  300  has a first inverter  310  which receives the output signal from the analog output circuit  220  and outputs an inverted signal and a second inverter  320  which receives the inverted signal from the first inverter  310  and outputs an inverted signal.  
     [0027] The differentiating amplifier circuit  210  has a first PMOS transistor  211 , a second PMOS transistor  212 , a first NMOS transistor  213 , a second NMOS transistor  214 , and a third NMOS transistor  215 .  
     [0028] The first PMOS transistor  211  is connected between the power source Vcc and the node N 1 . The gate electrodes of the first PMOS transistor  211  are connected to the node N 1  and the gate electrodes of the second PMOS transistor  212 . The second PMOS transistor  212  is connected between the power source Vcc and the node N 2 . The first NMOS transistor  213  is connected between the node N 1  and the node N 3 . An input signal IN is supplied to the gate electrodes of the first NMOS transistor  213 . This input signal IN is an analog signal whose voltage level changes linearly.  
     [0029] The second NMOS transistor  214  is connected between the node N 2  and the node N 3 . The inverted signal /IN of the input signal IN is supplied to the gate electrodes of the second NMOS transistor  214 .  
     [0030] The third NMOS transistor  215  is connected between the node N 3  and the ground GND. A control signal EN is supplied to the gate electrodes of the third NMOS transistor  215 . When this control signal EN becomes a high level signal (power source voltage level) and the third NMOS transistor  215  is turned on, the differentiating amplifier circuit  210  is activated.  
     [0031] The analog output circuit  220  is constituted of a PMOS transistor  221  and an NMOS transistor  222 . The PMOS transistor  221  is connected between the power source Vcc and the output node N 4 . The NMOS transistor  222  is connected between the output node N 4  and the ground GND. The gate electrodes of the PMOS transistor  221  and NMOS transistor  222  are connected to the node N 2 .  
     [0032] The inverter  310  is constituted of a PMOS transistor  311  and an NMOS transistor  312 . The PMOS transistor  311  is connected between the power source Vcc and the output node N 5 . The NMOS transistor  312  is connected between the output node N 5  and the ground GND. The gate electrodes of the PMOS transistor  311  and NMOS transistor  312  are connected to the node N 4 .  
     [0033] The inverter  320  is constituted of a PMOS transistor  321  and an NMOS transistor  322 . The PMOS transistor  321  is connected between the power source Vcc and output node N 6 . The NMOS transistor  322  is connected between the output node N 6  and the ground GND. The gate electrodes of the PMOS Transistor  321  and NMOS transistor  322  are connected to the node N 5 . The output node N 6  is connected to the output terminal OUT of the logic circuit  300 .  
     [0034] The operation of these circuits will now be explained in a comprehensive manner.  
     [0035] First, when the control signal EN is inout to the third NMOS transistor  215 , the third NMOS transistor  215  becomes conductive. When the input signal IN and the inverted signal /IN are input to the first NMOS transistor  213  and second NMOS transistor  214 , respectively, a current flows through the differentiating amplifier circuit  210  in accordance with the voltage level of the input signal IN. The voltage at the node N 2  also changes in accordance with the amount of this current.  
     [0036] When the voltage at this node N 2  exceeds a prescribed level, the analog output circuit  220  outputs a low level (ground voltage level) signal. That is, the voltage at the node N 4  becomes equal to the ground voltage level GND. On the other hand, when the voltage at this node N 2  drops below another prescribed level, the analog output circuit  220  outputs a high level (power source voltage level) signal. That is, the voltage at the node N 2  becomes equal to the power source voltage level Vcc.  
     [0037] The high level (power source voltage level) signal or the low level (ground voltage level) signal output from the analog output circuit  220  is input to the first stage inverter  310  of the logic circuit  300 . The inverter  310  then inverts this input signal and outputs from the output node N 5  the inverted high level (power source voltage level) signal or the inverted low level (ground voltage level) signal.  
     [0038] The inverted high level (power source voltage level) signal or the inverted low level (ground voltage level) signal output from the node N 5  is input to the inverter  320 . The inverter  320  then inverts the input signal and outputs from the output node N 6  the inverted high level (power source voltage level) signal or the inverted low level (ground voltage level) signal.  
     [0039] In the above-provided explanation of the circuits and the operations of the circuits, exemplary analog circuits and logic circuits to which the present invention is applicable are described. Since the circuits used in the explanation are typical ones, it should be easy to understand the operations of these circuits.  
     [0040] In what follows, the characteristics of the present invention will be explained.  
     [0041] In the present invention, the absolute value of the threshold value of each of the MOS transistors that constitute the analog circuit  200  is set smaller than the absolute value of the threshold value of each of the MOS transistors that constitute the logic circuit  300 .  
     [0042] In other words, if the threshold value of each of the PMOS transistors that constitute the analog circuit  200  is denoted by Vtpl, if the threshold value of each of the NMOS transistors that constitute the analog circuit  200  is denoted by Vtnl, if the threshold value of each of the PMOS transistors that constitute the logic circuit  300  is denoted by Vtp 2 , and if the threshold value of each of the NMOS transistors that constitute the logic circuit  300  is denoted by Vtn 2 , then these threshold values satisfy the following inequalities (1).  
       Vtn 1 &lt;Vtn 2, | Vtp 1 |&lt;|Vtp 2|  (1)  
     [0043] In other words, the threshold value of each of the NMOS transistors that constitute the analog circuit  200  is set smaller than that of each of the NMOS transistors that constitute the logic circuit  300 . The threshold value of each of the PMOS transistors that constitute the analog circuit  200  is set larger than that of each of the PMOS transistors that constitute the logic circuit  300 . Since the threshold values of PMOS transistors are negative, the absolute value of the threshold value of each of the PMOS transistors that constitute the analog circuit  200  is set smaller than that of each of the PMOS transistors that constitute the logic circuit  300 .  
     [0044] Moreover, if the power source voltage is denoted by Vcc, then the following inequality (2) is also satisfied in the analog circuit  200 .  
       Vtn 1 +Vtp 1 &lt;Vcc   (2)  
     [0045] In other words, the sum of the threshold value of any of the NMOS transistors and the absolute value of the threshold value of any of the PMOS transistors is smaller than the power source voltage Vcc.  
     [0046] Moreover, the following inequalities (3a), (3b) and (3c) are satisfied in the logic circuit  300 .  
       Vcc&lt;Vtn 2+| Vtp 2|&lt;2 Vcc   (3a)  
       Vtn 2 &lt;Vcc   (3b)  
         51  Vtp 2|&lt; Vcc   (3c)  
     [0047] In other words, in the logic circuit  300 , the sum of the threshold value of any of the NMOS transistors and the absolute value of the threshold value of any of the PMOS transistors is set larger than the power source voltage Vcc and smaller than 2Vcc, that is, twice the power source voltage. Moreover, both the threshold value of any of the NMOS transistors and the absolute value of the threshold value of any of the PMOS transistors are smaller than the power source voltage Vcc.  
     [0048] In this embodiment, the power source voltage Vcc and the ground voltage GND are set to 1.8V and 0V, respectively. The threshold value Vtp 1  of any of the PMOS transistors that constitute the analog circuit  200  is set to a value between 0.5V and −0.7V. The threshold value Vtn 1  of any of the NMOS transistors that constitute the analog circuit  200  is set to a value between 0.5V and 0.7V. The threshold value Vtp 2  of any of the PMOS transistors that constitute the logic circuit  300  is set to a value between −0.7V and −1.5V. The threshold value Vtn 2  of any of the NMOS transistors that constitute the logic circuit  300  is set to a value between 0.7V and 1.5V. All of these threshold values are set so as to satisfy the inequalities (1), (2), (3a), (3b), and (3c).  
     [0049] In practice, the design engineers can determine the threshold value of each of the transistors taking into consideration the relation between the above-provided inequalities and the power source voltage and the margin of circuit operation and the like.  
     [0050] By setting the threshold value of each of the transistors in the above-described manner, not only highly sensitive operation of the analog circuit  200  is guaranteed but also the leak current and the through current in the logic circuit can be reduced.  
     [0051] In other words, as the threshold voltage values of the transistors are made lower, the range of transition of the input signal, that is, the range of voltage variation of the input signal becomes narrow. Therefore, by setting the absolute value of the threshold value of each of the transistors that constitute the analog circuit smaller than the absolute value of the threshold value of each of the transistors that constitute the logic circuit, a circuit configuration capable of responding to a micro change in the input signal is realized.  
     [0052] On the other hand, if transistors having a small threshold value are installed in the logic circuit also as in the case of the conventional circuit, the leak current in each of the transistors increases and the through current in each of the inverters increases. Therefore, the absolute value of the threshold value of each of the transistors that constitute the logic circuit is set larger than the absolute value of the threshold value of each of the transistors that constitute the analog circuit. Hence, the leak current in each of the transistors and the through current in each of the inverters can be reduced inside the logic circuit.  
     [0053] The present invention solves the mutually compromising problems of improving the operation sensitivity of the analog circuit and reducing the leak current and the through current in the logic circuit. Therefore, the power consumption of the semiconductor integral circuit can be reduced while improving the sensitivity of the semiconductor integral circuit.  
     [0054] In many cases, semiconductor integral circuits that operate with a low voltage are driven using cells such as batteries or the like as a power source. Therefore, being able to reduce the power consumption of the semiconductor integral circuit is a very significant advantage. In other words, the present invention extends the life of the cells that drive the semiconductor integral circuits.  
     [0055] In the above-explained embodiment, a differentiating amplifier circuit is used as the analog circuit and an inverter circuit is used as the logic circuit. However, the range of technical applications of the present invention is not limited to these circuits.  
     [0056] Any circuit may be used for the analog circuit provided that the circuit changes linearly the voltage of an input signal and that the circuit causes the amount of the current flowing through the circuit to change in accordance with the change in the voltage of the input signal. Various configurations can be considered for the analog circuit such as a diode-connected MOS transistor, a charge pump circuit, a bias voltage generator circuit, an AD converter, and the like. Moreover, various kinds of specific configurations can be selected for each of these circuits.  
     [0057]FIG. 3( a ) shows an exemplary diode-connected NMOS transistor. FIG. 3( b ) shows an exemplary diode-connected PMOS transistor. In this case, the amount of a current flowing through the transistor changes in accordance with the change in the voltage of the input signal IN. The output is then supplied to the output terminal OUT in accordance with the change in the amount of the current flowing through the transistor.  
     [0058]FIG. 4 shows an exemplary configuration of a charge pump circuit. This charge pump circuit has NMOS transistors  401  and  402 , and a capacitor  403 .  
     [0059] An input signal is supplied to one electrode of the NMOS  401 , and the ground voltage is supplied to the other electrode of the NMOS  401 . The one electrode of the NMOS  401  is connected to the gate electrode of the NMOS  401 . The input signal is also supplied to one electrode of the NMOS  402 , and the other electrode of the NMOS  402  is connected to the capacitor  403  and the output terminal OUT. The other electrode of the NMOS  402  is connected to the gate electrode of the NMOS  402 .  
     [0060] In this charge pump circuit also, the amount of the current that flows through each of the transistors changes in accordance with the change in the voltage of the input signal IN. The voltage is charged in accordance with the change in the amount of the current that flows through each of the transistors and the output is supplied to the output terminal OUT.  
     [0061] Any circuit may be used for the logic circuit  300  provided that an input signal having the power source voltage or the ground voltage is input to the circuit and that the circuit outputs an output signal having the power source voltage or the ground voltage in response to the voltage of the input signal. Various configurations can be considered for the logic circuit, such as an inverter, a NOR circuit, an NAND circuit, a decoder circuit (an exemplary decoder circuit is shown in FIG. 5), a flip flop circuit (an exemplary flip flop circuit is shown in FIG. 6), a counter circuit, an oscillator circuit, and the like. Moreover, various kinds of specific configurations can be selected for each of these circuits.  
     [0062] Methods for setting the threshold values of MOS transistors in the above-described manner will now be explained. In other words, methods for setting the absolute value of each of the transistors of the analog circuit smaller than the absolute value of each of the transistors of the logic circuit will be explained.  
     [0063] First, with reference to FIG. 7, a method for forming two N-type wells in a P-type semiconductor substrate and a P-type well in one of the two N-type wells will be explained. In this case, ions are injected into each well so as to adjust the concentration in the well in forming the well.  
     [0064] To be more specific, boron ions of prescribed concentration are injected into the P-type well P 2  so that the concentration in the P-type well P 2  will be higher than that in the P-type semiconductor substrate P 1 . As a result, the threshold value of an NMOS transistor to be formed later inside the P-type well P 2  will be increased. Thus, NMOS transistors of the analog circuit are formed in the P-type semiconductor substrate P 1 , and NMOS transistors of the logic circuit are formed in the P-type well P 2 .  
     [0065] Moreover, phosphorus ions having a first prescribed concentration are injected into the N-type well N 1  and phosphorus ions having a second prescribed concentration are injected into the N-type well N 2  so that the concentration in the N-type well N 2  will be higher than that in the N-type well N 1 . As a result, the threshold value of a PMOS transistor to be formed later inside the N-type well N 2  will be increased. Thus, PMOS transistors of the analog circuit are formed in the N-type well N 1 , and PMOS transistors of the logic circuit are formed in the N-type well N 2 .  
     [0066]FIG. 8 shows a second method for setting the absolute value of each of the transistors of the analog circuit smaller than the absolute value of each of the transistors of the logic circuit. In this second method, a field oxide film is first formed as shown in FIG. 8, and then ions are injected into an active region AC in which a transistor is to be formed.  
     [0067] To be more specific, after the field oxide film is formed, all the regions but the predetermined active region AC are covered with a resist, and then ions are injected into the active region AC. In this case, the threshold value of an NMOS transistor to be formed later will be decreased if phosphorus ions are injected into a region in which the NMOS transistor is to be formed. The threshold value of the NMOS to be formed later will be increased if boron ions are injected into the region. Similarly, the absolute value of the threshold value of a PMOS to be formed later will be increased if phosphorus ions are injected into a region in which the PMOS transistor is to be formed. The absolute value of the threshold value of the PMOS to be formed later will be decreased if boron ions are injected into the region.  
     [0068]FIG. 9 shows a third method for setting the absolute value of each of the transistors of the analog circuit smaller than the absolute value of each of the transistors of the logic circuit. In this third method, gate electrodes are first formed, and then ions are injected into a predetermined active region from above the gate electrodes.  
     [0069] In these methods, the design engineers can determine what ions to be injected and the concentration of the ions, while taking into consideration the relation between the above equations and the power source voltage, the margin of circuit operation, and the like.  
     [0070] Next, another embodiment of the present invention will be explained with reference to FIG. 10. In the example shown in FIG. 7, the threshold value of each of the MOS transistors to be formed later is adjusted by injecting ions in forming the wells. In contrast, in the embodiment to be shown, the threshold value of each of the MOS transistors, is adjusted by a bias voltage to be supplied to the substrate or well (sometimes wells are collectively referred to as substrates also) in which the MOS transistor is formed. This embodiment will be explained using the analog output circuit  220  in the analog circuit  200  and the inverter  310  in the logic circuit  300 . FIG. 10 is a cross sectional view of these circuits. In FIG. 10, the “N + ” indicate regions in which an N-type concentration is high and the “P + ” indicate regions in which a P-type concentration is high.  
     [0071] The NMOS transistor  222  of the analog output circuit  220  is formed in the P-type semiconductor substrate P 1 , and the PMOS transistor  221  of the analog output circuit  220  is formed in the N-type well N 1 .  
     [0072] The NMOS transistor  312  of the logic circuit  310  is formed in the P-type semiconductor substrate P 2 , and the PMOS transistor  311  of the logic circuit  310  is formed in the N-type well N 2 .  
     [0073] The characteristic of this embodiment is that the P-type well P 2  is biased with a bias voltage VBB that is sufficiently lower than the ground voltage GND, and that the N-type well N 2  is biased with a bias voltage VBB that is sufficiently higher than the power source voltage Vcc.  
     [0074] The P-type semiconductor substrate P 1  is biased with the ground voltage GND and the N-type well N 1  is biased with the power source voltage Vcc. However, the present embodiment is not limited to this configuration.  
     [0075] In other words, any other setup is admissible provided that the bias voltage for the substrate or each well is set to satisfy the following inequalities. Denoting the bias voltages for the P-type semiconductor substrate P 1 , P-type well P 2 , Ntype well N 1 , and N-type well N 2  by VBB 1 , VBB 2 , VPP 1 , and VPP 2 , respectively, the following inequalities are satisfied in the present embodiment.  
     VBB1&gt;GND, VPP1≦Vcc, VBB2≦GND, VPP2≧Vcc  (4)  
     [0076] By setting the bias voltages in this way, the threshold value of each transistor can be easily controlled. That is, since a bias supply circuit is installed to each substrate, the ion injection process in the above-described manufacture process can be simplified.  
     [0077] Such bias voltages VBB 1 , VBB 2 , VPP 1 , and VPP 2  can be supplied using a conventional bias supply circuit. For example, the bias supply circuits disclosed in the Japanese Patent Application Laid-Open No. H2-350, the Japanese Patent Application Laid-Open No. 62-178013, and the Japanese Patent Application Laid-Open No. 61-64148 can be used.  
     [0078] By setting the threshold value of each transistor in this manner, highly sensitive operation is guaranteed for the analog circuit, and the leak current and through current in the logic circuit can be reduced.  
     [0079] According to the present invention, highly sensitive operation is guaranteed for the analog circuit, and the leak current and through current in the logic circuit can be reduced. If the present invention is applied to low voltage operation semiconductor integral circuits that are driven using cells such as batteries or the like as a power source, the life of the cells can be extended.  
     [0080] So far, the present invention has been explained using embodiments. However, the range of technical applications of the present invention is not limited to these embodiments. Other variations and modifications of the above-described embodiments should be evident to those skilled in the art. Accordingly, it is intended that such alterations and modifications be included within the scope and spirit of the present invention as defined by the following claims.