CMOS and Bipolar are two of the pioneering technologies of the electronics field. Components fabricated with the CMOS technology dissipate lower power, have smaller noise margins, and are physically smaller. On the other hand, components fabricated with the bipolar technology operate at higher speeds, switch faster, and offer good noise performance. By combining the two, scientists have created the BiCMOS technology that offers a combination of advantages from both processes. For instance, BiCMOS offers higher speeds compared to that of CMOS, and lower power dissipation compared to that of bipolar. However, the penalty comes in the form of added process complexity and it adds to the cost. Both CMOS and bipolar issues need optimization of impurities, and this increase in process complexity results in higher costs compared to that of conventional CMOS.
Scientists have worked out the optimum approach to fabricate high performance BiCMOS devices. They have found it best to start with a baseline CMOS process and add the bipolar process steps. This produces an optimum BiCMOS process flow, emphasizes reliability and process simplicity, while maintaining compatibility with the CMOS technology.
There are several advantages of the BiCMOS technology. The higher impedance of the CMOS circuitry facilitates the analog amplifier input design, while bipolar transistors define the rest. BiCMOS can stand wide temperature variations and process variations, which make this technology more economical. BiCMOS devices can source and sink much higher load currents because of the MOS part, while it handles higher speeds because of the bipolar part. BiCMOS can drive high capacitance loads with lower cycle times. As the source and drain can be interchanged, BiCMOS demonstrates bidirectional capabilities, which makes it suitable for IO intensive applications.
BiCMOS technology has its drawbacks as well. The fabrication complexity is higher because both CMOS and bipolar technologies are involved. This increases the cost of fabrication also. However, as BiCMOS devices have higher density, the amount of lithography required is lower.
BiCMOS technology is versatile for several applications. Its higher speed makes it suitable for AND functions of high density. It easily replaces devices formed with earlier technologies such as CMOS, ECL, and bipolar, for instance, in some cases BiCMOS has higher speed performance compared to that from bipolar. A single chip with the BiCMOS technology can span the analog-digital boundary. Their high impedance input makes BiCMOS a very good candidate for applications such as sample and hold, adders, mixers, ADCs, DACs.
STMicroelectronics integrates RF, analog, and digital parts on a single chip. Their BiCMOS SiGe technology reduces the number of external components drastically, while optimizing the power consumed by the chip. The advantages of the integration are significant as earlier, only more expensive technologies were able to achieve this level of performance.
As ST explains, the Heterojunction Bipolar Transistor (HBT) of BiCMOS has a much higher cut-off frequency compared to bulk CMOS. To attain such frequencies, the bulk CMOS designs need to use far smaller process nodes. This forces design compromises leading to overall lower performances and higher costs. Therefore, the BiCMOS technology offers a better cost profile compared to other alternatives.