1.  Field of the Invention
The present invention generally relates to a reference voltage source circuit used in analog circuits, etc.,  and in particular to a reference voltage source circuit that is able to operate with a low voltage. 
2.  Description of the Related Art
According to the conventional reference voltage source circuits employing MOS transistors such as described in, for example, Japanese Patent Publication No.  04-65546 (citation 1), the difference between a threshold voltage of a depletion type transistor and a threshold voltage of an enhancement type transistor, both transistors formed by changing substrates or the channel doping concentration, is provided as a reference voltage. 
As for the related art described in Japanese Laid-Open Patent Application No.  2001-284464 (citation 2), a voltage Proportional-To-Absolute-Temperature (PTAT) is provided as the reference voltage by using a weak inversion region of gates of MOS transistors instead of bipolar transistors.  Using the weak inversion region of gates of MOS transistors means to make the transistor operate in the vicinity of the threshold voltage, which inverts the gate.  Usually, transistors operate in a strong inversion region. 
According to the citation 1, since conductivity and its temperature characteristics of the transistors with different substrates or different channel dopant concentrations vary, it is difficult to realize a reference voltage source circuit with a desired temperature characteristic.  In addition, there is a disadvantage in that such transistors are susceptible to variation in the manufacturing process since the amount of channel dopant in transistors must be controlled individually. 
In order to adjust the temperature characteristic of the conductivity, a separate current bias circuit is necessary as described in reference F (R. A. Blauschild et al, “A New NMOS Temperature—Stable Voltage Reference” IEEE Journal of Solid-State Circuits, Vol.  SC-13, No.  6, pp.  767-773, December 1978) in the citation 2. 
According to the related art described in the citation 2, the following disadvantages are present since a weak inversion region of gates is used. 
a) In order to cause the weak inversion state in the gate of a MOS transistor, a minute electric current bias circuit is necessary.  According to reference B (E. Vittoz and J. Fellrath, “CMOS Analog Integrated Circuits Based on Weak Inversion Operation” IEEE Journal of Solid-State Circuits, Vol.  SC-12, No.  3, pp.  224-231, June 1997) cited in the citation 2, in order to keep the MOS transistor in the weak inversion region, the drain current must fulfill the following relation:I≦((n−1)/e2)SμCoxUT2where n is a slope factor, S is ratio between effective channel width W and effective channel length L (Weff/Leff), μ is mobility of carriers in the channel, and Cox is capacitance of oxide film per unit area. 
In particular, when the parameters are set as follows: n=1.7, S=1, μ=750 cm2/Vs, Cox=45 nF/cm2, UT=26 mV as in reference E (U.S. Pat.  No.  4,327,320, April 1982, “REFERENCE VOLTAGE SOURCE”, Oguey) cited in the citation 2, the drain current at room temperature must be under 2 nA, which is extremely difficult to realize. 
b) Further, in the case of operating the transistor with a minute drain current such as 2 nA as mentioned above, the operation of the transistor is susceptible to leakage current of the parasitic diode between drain and substrate and problems due to the parasitic diodes may occur.  For example, in page 268 of reference D (Oguey et al., “ MOS Voltage Reference Based on Polysilicon Gate Work Function Difference”, IEEE Journal of Solid-State Circuits, Vol.  SC-15, No.  3, June 1980) cited in the citation 2, it is mentioned that at temperatures above 80 degrees Celsius, deviations are generated due to the leakage current. 
Therefore, according to the citation 2, as shown in FIG.  1 (corresponding to FIG.  22 in citation 2), a first voltage source circuit having negative temperature coefficients and comprises MOS transistors 101, 102 having semiconductor gates of a different conduction type, and a second voltage source circuit having positive temperature coefficients and comprises MOS transistors 103, 104 having semiconductor gates of the same conduction type but of different impurity concentrations, are combined so as to provide a reference voltage source circuit that can be dispensed with a minute current bias circuit by making use of a strong inversion region.  The reference voltage source circuit uses MOS transistors that can operate stably at temperatures above 80 degrees Celsius.  Accordingly, the reference voltage source circuit using MOS transistors having a desired temperature characteristic is realized. 
However, according to the citation 2, the output voltage of the first voltage source circuit amounts to approximately 1 V as opposed to the output voltage of the second voltage source circuit that only ranges from several tens of mV to one hundred and tens of mV. In order to configure a reference voltage source circuit with a desired temperature characteristic, these two output voltages are summed at some proportion.  Therefore, in the reference voltage source circuit shown in FIG.  1, more than 1 V power supply voltage (operating voltage) Vcc is necessary.  When actually tested with a production prototype of the circuit, approximately 1.2 V was the lowest necessary operating voltage.  This is because, as shown in FIG.  1, a source follower transistor M5, which needs a voltage of several mV to start operating, is provided between the power supply voltage (Vcc) terminal and the terminal V1 of the second voltage source circuit, which terminal V1 has the output voltage of approximately 1 V, and the sum of the operation starting voltage of the source follower transistor 5 and the output voltage of the terminal V1 becomes the lowest necessary power supply voltage Vcc. 
In the meanwhile, along with the recent increase in the popularity of portable equipment, the requests for LSIs performing battery operations are increasingly varied.  Especially, reducing the value of the operating voltage (power supply voltage) so as to drastically extend the service life of batteries is strongly requested for the systems driven by a single battery.  The lowest necessary operating voltage of 1.2 V of the circuit in FIG.  1 is not a high voltage at all, however, the lowest necessary operating voltage Vcc lower than 0.9 V is requested.  Inventions described in other than the citation 2 also cannot realize such a reference voltage source circuit operating with such low operating voltage.