Abstract:
A semiconductor device can output a reference voltage for an arbitrary potential and can detect the voltage of each cell in a battery including multiple cells very precisely. The device includes a depletion-type MOSFET  21  and an enhancement type MOSFET  22 , and has a floating structure that isolates depletion-type MOSFET  21  and enhancement type MOSFET  22  from a ground terminal. The depletion-type MOSFET  21  and enhancement type MOSFET  22  are connected in series to each other, wherein the depletion-type MOSFET  21  is connected to high-potential-side terminal and the enhancement type MOSFET  22  is connected to low-potential-side terminal. The semiconductor device having the configuration described above is disposed in a voltage detecting circuit section in a control IC for a battery including multiple cells.

Description:
BACKGROUND 
     The present invention relates to a semiconductor device. Specifically, the invention relates to a semiconductor device for outputting a reference voltage. 
     According to the prior art, a reference voltage circuit is used for feeding a reference voltage to all the control circuits in an integrated circuit (hereinafter referred to as an “IC”). Therefore, the reference voltage circuit is required to always output a certain voltage not adversely affected by temperature variations nor by power supply voltage variations.  FIG. 6  is a cross sectional view of a semiconductor device that constitutes a conventional MOS reference voltage circuit. The semiconductor device shown in  FIG. 6  is manufactured employing p-type substrate  1 . In the surface portion of p-type substrate  1 , p-type well layer  73  is formed. Depletion-type MOSFET  101  and enhancement-type MOSFET  102  are formed in the surface portion of p-type well player  73 . Depletion-type MOSFET  101  and enhancement-type MOSFET  102  are spaced apart from each other by field oxide film  17  (See, for example, Unexamined Laid Open Japanese Patent Application Publication No. 2003-31678). 
     In depletion-type MOSFET  101 , n + -type drain layer  5  and n + -type source layer  6  are formed in a first surface portion of p-type well player  73  such that n + -type drain layer  5  and n + -type source layer  6  are spaced apart from each other. In the first surface portion of p-type well player  73 , n − -type depletion layer  7  is formed such that n − -type depletion layer  7  is in contact with n + -type drain layer  5  and n + -type source layer  6 . Gate electrode  10  is formed above n − -type depletion layer  7  with gate oxide film  9  interposed between n − -type depletion layer  7  and gate electrode  10 . 
     In enhancement-type MOSFET  102 , n + -type drain layer  11  and n + -type source layer  12  are formed in a second surface portion of p-type well player  73  such that n + -type drain layer  11  and n + -type source layer  12  are spaced apart from each other. In the second surface portion of p-type well player  73 , p-type channel layer  13  is formed such that p-type channel layer  13  is in contact with n + -type drain layer  11  and n + -type source layer  12 . Gate electrode  16  is formed above p-type channel layer  13  with gate oxide film  15  interposed between p-type channel layer  13  and gate electrode  16 . In a third surface portion of p-type well player  73 , p + -type pickup layer  74  is formed. Pickup layer  74  is spaced apart from enhancement-type MOSFET  102  by field oxide film  19 . 
     High-potential power supply terminal Vcc is connected electrically to n + -type drain layer  5  in depletion-type MOSFET  101 . Output terminal Vref that outputs a reference voltage is connected electrically to n + -type source layer  6  and gate electrode  10  in depletion-type MOSFET  101  and to n + -type drain layer  11  and gate electrode  16  in enhancement-type MOSFET  102 . Ground terminal GND is connected electrically to n + -type source layer  12  in enhancement-type MOSFET  102  and p + -type pickup layer  74 . The MOS reference voltage circuit as described above makes it possible to detect the cell voltage of a lithium ion battery including, for example, one battery cell very precisely. 
     Now the configuration of a voltage detecting circuit, which employs the conventional semiconductor device for a MOS reference voltage circuit, will be described below.  FIG. 7  is a block circuit diagram describing the configuration of a voltage detecting circuit that uses the conventional semiconductor device for the MOS reference voltage circuit thereof. In  FIG. 7 , voltage detecting circuit  110  includes high resistance R 1 , resistance R 2 , and voltage detecting circuit section  112 . Voltage detecting circuit section  112  includes comparator  114  and MOS reference voltage circuit  113 . The reference voltage outputted from MOS reference voltage circuit  113  is applied to the reference voltage side of comparator  114 . The voltage obtained by dividing the output voltage from a lithium ion battery, including lithium battery cells  111  connected in series, with resistance R 1  and resistance R 2  is applied to the input potential side of comparator  114 . 
     As described above, a high voltage is divided by resistance to a lower voltage and the lower voltage is compared with a reference voltage to detect the high voltage. Alternatively, a high voltage is divided by a differential amplifier circuit to a lower voltage and the lower voltage is compared with a reference voltage to detect the high voltage. 
     However, a large voltage difference is caused between a battery voltage and a reference voltage level, to which the battery voltage is lowered, in the voltage detecting circuit section in a charging control IC for a battery including many cells. The large voltage difference makes it hard to detect the battery voltage with high precision. Since only one reference voltage circuit is included, it is impossible to detect the voltage of every cell. 
     In view of the foregoing, it would be desirable to obviate the problem described above, and to provide a semiconductor device that facilitates outputting a reference voltage for an arbitrary potential. It would further be desirable to provide a semiconductor device that facilitates detecting the voltage of every cell in the battery with high precision. 
     SUMMARY OF THE INVENTION 
     The invention provides a semiconductor device that obviates the above-described problem, facilitates outputting a reference voltage for an arbitrary potential, and further facilitates detecting the voltage of every cell in the battery with high precision. 
     In one preferred embodiment of the invention, a semiconductor device is provided that includes a substrate of a first conductivity type, a first well layer of a second conductivity type disposed in the surface portion of the substrate, a second well layer of the first conductivity type disposed in the surface portion of the first well layer, a third well layer of the first conductivity type disposed in the surface portion of the first well layer, the third well layer being spaced apart from the second well layer, a depletion-type MOSFET disposed in the second well layer; and an enhancement-type MOSFET disposed in the third well layer. 
     Further, the invention provides a semiconductor device including a substrate of a first conductivity type, a buried layer of a second conductivity type on the substrate, an epitaxial layer of the first conductivity type on the buried layer, a first well layer of the second conductivity type disposed in the surface portion of the epitaxial layer, a second well layer of the first conductivity type disposed in the surface portion of the first well layer, a third well layer of the first conductivity type disposed in the surface portion of the first well layer, the third well layer being spaced apart from the second well layer, a depletion-type MOSFET disposed in the second well layer; and an enhancement-type MOSFET disposed in the third well layer. 
     In a further preferred embodiment, in the above semiconductor devices include, the depletion-type MOSFET a first drain layer of the second conductivity type disposed in the surface portion of the second well layer, a first source layer of the second conductivity type disposed in the surface portion of the second well layer, the first source layer being spaced apart from the first drain layer, a depletion layer of the second conductivity type disposed in the surface portion of the second well layer, the depletion layer being in contact with the first drain layer and the first source layer, a first pickup layer of the first conductivity type disposed in the surface portion of the second well layer, and a first gate electrode above the depletion layer with a first gate oxide film interposed between the depletion layer and the first gate electrode. 
     Still further, in the above semiconductor devices, the enhancement-type MOSFET includes a second drain layer of the second conductivity type disposed in the surface portion of the third well layer, a second source layer of the second conductivity type disposed in the surface portion of the third well layer, the second source layer being spaced apart from the second drain layer, a channel layer of the first conductivity type disposed in the surface portion of the third well layer, the channel layer being in contact with the second drain layer and the second source layer, a second pickup layer of the first conductivity type disposed in the surface portion of the third well layer, and a second gate electrode above the channel layer with a second gate oxide film interposed between the channel layer and the second gate electrode. 
     The semiconductor devices of the present invention further preferably include an output terminal connected electrically to the first gate electrode, the first source layer, the second gate electrode and the second drain layer, a high-potential-side terminal connected electrically to the first drain layer; and a low-potential-side terminal connected electrically to the first pickup layer, the second source layer and the second pickup layer. 
     The semiconductor device of the present invention feeds a reference voltage to the reference-potential-side of a comparator that compares the voltage of each cell in a battery including a plurality of the cell with the reference voltage. 
     The semiconductor device according to the invention has a floating structure, in which the depletion-type MOSFET and the enhancement-type MOSFET are isolated from the ground terminal. Therefore, the semiconductor device used for a reference voltage circuit facilitates detecting the voltage of each cell in the battery including a plurality of the cell. Since each cell voltage is compared with a reference voltage, it is effective to divide the voltage of each cell by low resistance. Therefore, the error caused by voltage drop is reduced and voltage detection can be conducted very precisely. 
     The semiconductor device according to the invention can output a reference voltage for an arbitrary potential. The semiconductor device according to the invention can detect the voltage of every cell constituting a battery very precisely. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in greater detail with reference to certain preferred embodiments thereof and the accompanying figures, wherein: 
         FIG. 1  is a cross sectional view of a semiconductor device according to a first embodiment of the invention; 
         FIG. 2  is a circuit diagram showing the circuit configuration of the semiconductor device according to the first embodiment of the invention; 
         FIG. 3  is a block circuit diagram describing the configuration of a voltage detecting circuit that uses the semiconductor device according to the first embodiment of the invention; 
         FIG. 4  is a cross sectional view of a semiconductor device according to a second embodiment of the invention; 
         FIG. 5  is a cross sectional view of a semiconductor device according to a third embodiment of the invention; 
         FIG. 6  is a cross sectional view of a semiconductor device that constitutes a conventional MOS reference voltage circuit; and 
         FIG. 7  is a block circuit diagram describing the configuration of a voltage detecting circuit that uses the conventional semiconductor device. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a cross sectional view of a semiconductor device according to a first embodiment of the invention.  FIG. 2  is a circuit diagram showing the circuit configuration of the semiconductor device according to the first embodiment. 
     Referring now to  FIG. 1 , n-type well layer  2  is formed in the surface portion of p-type substrate  1 . The n-type well layer  2  works as a floating layer. In the surface portion of n-type well player  2 , p-type well layer  3  and p-type well layer  4  are formed such that p-type well layer  3  and p-type well layer  4  are spaced apart from each other. Depletion-type NMOSFET  21  is formed in the surface portion of p-type well player  3 . Enhancement-type NMOSFET  22  is formed in the surface portion of p-type well player  4 . 
     In depletion-type NMOSFET  21 , n + -type drain layer  5  and n + -type source layer  6  are formed in the surface portion of p-type well layer  3  such that n + -type drain layer  5  and n + -type source layer  6  are spaced apart from each other. In the surface portion of p-type well layer  3 , n − -type depletion layer  7  is formed such that n − -type depletion layer  7  is in contact with n + -type drain layer  5  and n + -type source layer  6 . An impurity such as phosphorus (P 31 ) is doped in n − -type depletion layer  7 . In the surface portion of p-type well layer  3 , p + -type pickup layer  8  is also formed. Gate electrode  10  is formed above n − -type depletion layer  7  with gate oxide film  9  interposed between n − -type depletion layer  7  and gate electrode  10 . For example, gate oxide film  9  is 170 Å in thickness. 
     In enhancement-type NMOSFET  22 , n + -type drain layer  11  and n + -type source layer  12  are formed in the surface portion of p-type well layer  4  such that n + -type drain layer  11  and n + -type source layer  12  are spaced apart from each other. In the surface portion of p-type well layer  4 , p − -type channel layer  13  is formed such that p − -type channel layer  13  is in contact with n + -type drain layer  11  and n + -type source layer  12 . In the surface portion of p-type well layer  4 , p + -type pickup layer  14  is also formed. Gate electrode  16  is formed above p − -type channel layer  13  with gate oxide film  15  interposed between p − -type channel layer is  13  and gate electrode  16 . For example, gate oxide film  15  is 170 Å in thickness. 
     Field oxide film  17  is formed in the surface portion of n-type well layer  2  such that field oxide film  17  spaces apart depletion-type NMOSFET  21  and enhancement-type NMOSFET  22  from each other. Field oxide film  18  isolates depletion-type NMOSFET  21  from the other devices not shown. Field oxide film  19  isolates enhancement-type NMOSFET  22  from the other devices not shown. 
     Output terminal Vref is connected electrically to n + -type source layer  6  and gate electrode  10  in depletion-type NMOSFET  21  and to n + -type drain layer  11  and gate electrode  16  in enhancement-type NMOSFET  22 . High-potential-side terminal VH is connected electrically to n + -type drain layer  5  in depletion-type NMOSFET  21 . Low-potential-side terminal VL is connected electrically to p + -type pickup layer  8  in depletion-type NMOSFET  21  and to n + -type source layer  12  and p + -type pickup layer  14  in enhancement-type NMOSFET  22 . 
     Now the method for manufacturing a MOS reference voltage circuit according to the first embodiment of the invention will be described below. First, n-type well layer  2  is formed in the surface portion of p-type substrate  1 . Then, field oxide films  17 ,  18  and  19  are formed. In the surface portion of n-type well layer  2 , p-type well layers  3  and  4  are formed. Then, n − -type depletion layer  7  is formed in the surface portion of p-type well layer  3 . Depletion layer  7  is doped, for example, with phosphorus (P 31 ). Then, gate oxide film  9  of, for example, 170□ in thickness is formed on n − -type depletion layer  7 . Further, gate electrode  10  is deposited on gate oxide film  9 . 
     In p-type well layer  4 , p − -type channel layer  13  is formed. Then, gate oxide film  15  of, for example, 170 Å in thickness is formed on p − -type channel layer  13 . Further, gate electrode  16  is deposited on gate oxide film  15 . 
     Shielding masks are formed on the portions of p-type well layers  3  and  4 , in which any n + -type layer will not be formed. Then, n + -type drain layers  5 ,  11  and n + -type source layers  6 ,  12  are formed by implanting n-type impurity ions over gate electrode  10 ,  16  and field oxide films  17 ,  18 ,  19 . Shielding masks are formed on the portions of p-type well layers  3  and  4 , in which any p + -type layer will not be formed. Then, p + -type pickup layers  8  and  14  are formed by implanting p-type impurity ions over gate electrodes  10 ,  16  and field oxide films  17 ,  18 ,  19 . Then, output terminal Vref is connected electrically to n + -type source layer  6  and gate electrode  10  in depletion-type NMOSFET  21  and to n + -type drain layer  11  and gate electrode  16  in enhancement-type NMOSFET  22 . High-potential-side terminal VH is connected electrically to n + -type drain layer  5  in depletion-type NMOSFET  21 . Low-potential-side terminal VL is connected electrically to p + -type pickup layer  8  in depletion-type NMOSFET  21  and to n + -type source layer  12  and p + -type pickup layer  14  in enhancement-type NMOSFET  22 . In  FIG. 2 , depletion-type NMOSFET  31  and enhancement-type NMOSFET  32  are shown. 
       FIG. 3  is a block circuit diagram describing the configuration of a voltage detecting circuit that uses the semiconductor device according to the first embodiment of the invention. As shown in  FIG. 3 , voltage detecting circuit section  42  in voltage detecting circuit  40  includes comparators  44  connected to respective lithium battery cells  41 , and MOS reference voltage circuits  43  which feed reference voltages to respective comparators  44 . MOS reference voltage circuit  43  is configured by the semiconductor device shown in  FIGS. 1 and 2 . 
     If the cell voltage of each lithium battery cell  41  is 4.0 V, the high-potential-side voltage of the battery, which includes four lithium battery cells  41  as shown in  FIG. 3 , will be 16 V. MOS reference voltage circuit  43  according to the first embodiment is connected to the reference-potential-side of each lithium battery cell  41 . Therefore, it is effective to divide the voltage difference of 4.0 V and to feed the divided voltage difference to the input-potential-side of each comparator  44 . 
     Since comparator  44  is disposed for every lithium battery cell  41  in the MOS reference voltage circuit according to the first embodiment, the voltage of every lithium battery cell  41  is detectable. When the battery includes four lithium battery cells, the error caused by the resistance for dividing the high-voltage cell potential and for obtaining a low voltage is suppressed to be ¼ the error caused in the conventional voltage detecting circuit including one comparator. Therefore, the voltage of every cell in the battery including many battery cells is detected very precisely according to the first embodiment of the invention. 
     In detail, when the battery includes four lithium battery cells  41 , the voltage for over-charge detection is different by the magnitude of several tens mV from maker to maker according to the prior art. Further, for trimming the detected charging voltage finely, it is necessary for voltage dividing resistance R 1  (cf.  FIG. 7 ) to be 16 MΩ to 20 MΩ. In contrast, for dividing the voltage of each cell according to the invention, it is enough for the voltage dividing resistance to be 4 MΩ to 5 MΩ. Therefore, the error caused by the voltage dividing resistance according to the invention is ¼ the error caused according to the prior art. 
     As described above, the precision, with which the voltage of the battery including many cells is detected, is improved and the safety of battery charging is improved. According to the first embodiment, the circuit for detecting the voltages of the respective cells included in a battery can be configured on one chip. 
       FIG. 4  is a cross sectional view of a semiconductor device according to a second embodiment of the invention. The semiconductor device according to the second embodiment is different from the semiconductor device according to the first embodiment in that gate oxide films  51  and  52  thereof are around 300□ in thickness. Generally, the recommended operating voltage per the thickness of a gate oxide film in the MOSFET is from 3.0 MV/cm to 3.3 MV/cm. Therefore, the gate oxide film is 300 Å in thickness for sustaining the breakdown voltage of around 10 V. 
     The semiconductor device according to the second embodiment facilitates detecting a voltage very precisely when it is required for the semiconductor device to exhibit a breakdown voltage of around 10 V. 
       FIG. 5  is a cross sectional view of a semiconductor device according to a third embodiment of the invention. The semiconductor device according to the third embodiment is different from the semiconductor devices according to the first and second embodiments in that the semiconductor device according to the third embodiment is manufactured using an epitaxial substrate. As shown in  FIG. 5 , the epitaxial substrate includes n-type buried layer  71  on p-type substrate  1 , and p-type epitaxial layer  72  laminated on n-type buried layer  71 . Epitaxial layer  72  works as a floating layer. In the surface portion of p-type epitaxial layer  72 , p-type well layer  73  is formed. In the surface portion of p-type well layer  73 , depletion-type NMOSFET  101  and enhancement-type NMOSFET  102  are formed such that depletion-type NMOSFET  101  and enhancement-type NMOSFET  102  are spaced apart from each other. 
     By making the potential of p-type epitaxial layer  72  float, the semiconductor device according to the third embodiment obtains the effects similar to the effects which the semiconductor devices according to the first and second embodiments exhibit. 
     As described above, the semiconductor device according to the invention is very useful for a reference voltage circuit. Especially, the semiconductor device according to the invention is suitable for a voltage detecting circuit for detecting the voltage of a battery such as a lithium ion battery. 
     This application is based on, and claims priority to, Japanese Patent Application No: 2007-238924, filed on Sep. 14, 2007. The disclosure of the priority application, in its entirety, including the drawings, claims, and the specification thereof, is incorporated herein by reference.