Bandgap voltage generators are generally used to create a voltage which is equal to the bandgap potential of silicon devices at 0.degree. Kelvin. There are several basic techniques used to generate the bandgap voltage, which is approximately 1.2 volts.
In one technique, equal currents are passed through two diodes of different sizes; in another different currents are passed through different equal sized diodes. In both cases the voltage across each diode is a function of the current density, equal to the current passed by the diode divided by its area, which is larger in one diode than the other. The two diodes will have different voltage drops, as defined by the exponential diode law. The voltage difference between the two diode drops has a positive temperature coefficient, and can be scaled to offset the approximate -2.0 mv/.degree.C. temperature coefficient of the absolute diode voltage drop itself. A circuit which does this produces the 1.2 volt bandgap voltage independent of temperature.
A wide variety of circuits have been published to perform this function, many of them employing operational amplifiers (for example, as described in the article "CMOS Voltage Preferences Using Lateral Bipolar Transistors" by M. Degrauwe, IEEE JSSC, Vol. SC-20, No. 6, December 1985, p. 1151). In low power applications the current consumed in the various stages of an operational amplifier and in the operational amplifier bias chain is a disadvantage.
Other circuits have been proposed which require no operational amplifier and the only currents flowing are those through the bipolar devices (see the article "MOS Transistors Operated in CMOS Technology", by E. Vittoz et al, IEEE JSSC, Vol. SC-18, June 1983, P. 273). Those circuits require transistors connected in common emitter configuration.
A lateral bipolar transistor in a typical CMOS process could be used in common emitter configuration but these devices have poor performance. Bipolar devices with reasonable performance which can be integrated in CMOS circuits without special processing steps consist of a vertical structure comprised of a substrate, and well and source/drain diffusions for the collector, base and emitter respectively and can only be employed in common collector configurations. Until the present invention therefore a bandgap voltage generator requiring no operational amplifier could not be provided using the common collector vertical bipolar devices.