In integrated circuit technologies, BiCMOS, also called BiMOS, refers to the integration of bipolar junction transistors and CMOS technology into a single device. This technology has commercial application in amplifier and discrete component logic design. More recently it has become the technology of choice for power electronics products such as voltage regulators.
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History
Historically, integrating bipolar and MOS transistors into a single device proved difficult and uneconomical. For this reason, until now most integrated circuits have used one or the other, according to application requirements. Particularly, bipolar transistors offer high speed, high gain, and low output resistance, whereas CMOS technology offers high input resistance, which translates to simple, low-power logic gates. For years designers of circuits utilizing discrete components have realized the advantages of integrating the two technologies; however, lacking implementation in integrated circuits, application was restricted to fairly simple designs. In the 1990s, modern IC fabrication technologies began to make BiCMOS a reality. This technology rapidly found application in amplifiers and analog power management circuits, and has some advantages in digital logic. BiCMOS circuits use the characteristics of each type of transistor most appropriately. Generally this means that high current circuits use MOSFETs for efficient control, and portions of specialized very high performance circuits use bipolar devices. Examples of this include RF oscillators, bandgap-based references and low noise circuits. The Pentium, Pentium Pro, and SuperSPARC microprocessors also used BiCMOS.
Advantages
MOS circuits are ideally suited for use in logic (digital) applications because of their low current consumption. Bipolar devices are critical for creating accurate voltage references and when very low noise is required. Since the early 1990's power MOS devices (power FETs) are also used extensively in BiCMOS ICs for switching or regulating high currents as this can be done easily with low complexity control circuits. The major mixed-signal semiconductor suppliers all have power BiMOS processes for these applications. Freescale's SMARTMOS process is an example of a commercially successful BiCMOS process which uses the characteristics of the different transistors very effectively. Other companies' BiCMOS process include Texas Instruments' LBC process, and Maxim's BCDMOS.
Difficulties
BiCMOS as a fabrication process is not currently as commercially viable for some applications such as microprocessors as either BJT or CMOS fabrication. Unfortunately, many of the improvements to CMOS fabrication, for example, do not transfer directly to BiCMOS fabrication. An inherent difficulty arises from the fact that fine tuning of both the BJT and MOS components is impossible without adding many extra fabrication steps, and consequently increasing the cost. Finally, in the area of high performance logic, BiCMOS may never offer the (relatively) low power consumption of CMOS alone. ...
