Any electrical circuit that uses a transistor as an amplifier needs to bias the transistor at some operating point so that the device is in the active region. The current through the device or the voltage applied to the transistor device will determine this operating or bias point. Over temperature, the bias point also needs to be maintained by a stable bias voltage, so that the design characteristics will also remain within the specified design limits. For example, a stable oscillator bias circuit is necessary to provide a constant or regulated oscillator voltage output over temperature.
Conventional biasing circuits employing only bipolar transistors to generate a relatively temperature stable bias voltage for a bipolar amplifier are generally well known in the prior art. Nevertheless, because the voltage of V.sub.BE across the base-emitter junction of a bipolar transistor has a negative temperature coefficient, other voltages derived from this voltage inevitably will also have the same negative characteristic. Hence, the output voltage of the amplifier such as in an oscillator or voltage generator will still be varied over temperature. To combat the varying output voltages, more complicated methods such as bandgap voltage reference and zener circuits have been utilized. However, the method utilizing the zener breakdown voltage of a PN junction as a reference is limited in accuracy to its positive temperature coefficient.
A more accurate reference voltage may be achieved by using the bandgap voltage reference circuits to provide a regulated output in a bipolar implementation. However, due to higher density and lower power consumption, MOS is the technology of choice in today's integrated circuits (IC). However, MOS circuits have the inherent problem of being unable to provide a precise control of outputs. In addition, the threshold voltage V.sub.T of a P-MOS transistor, which is the equivalent of V.sub.BE in bipolar technology, has a positive temperature coefficient from which to derive voltages. (Likewise, an N-MOS transistor has a negative temperature coefficient.) Accordingly, the output voltage in MOS circuits can change drastically due to temperature or process variations.
It would therefore be desirable to provide a merged or composite bipolar/MOS circuit which combines the advantages of bipolar and MOS technologies together. Hence, bandgap voltage reference circuits have been accomplished using both bipolar and MOS circuits. The major defects of these prior art circuits are that complicated circuits are often required and which in turn require a larger area to implement the circuit in an integrated device.
Therefore, it is a desire to merge or arrange bipolar transistors and MOSFET transistors in a common semi-conductor substrate in order to form a simple integrated circuit which can be manufactured at a relatively low cost but yet provides a much improved constant output performance over temperature.