BiCMOS
|
|
In integrated circuit design, BiCMOS refers to the integration of bipolar junction transistors and CMOS technology into a single device. This technology is currently under active research, and appears to have potential in the near future for commercial application in amplifier and discrete component logic design.
| Contents |
History
Until recently, 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 other other, according to design criteria of the application. 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. Designers of circuits utilizing discrete components have for years realized the advantages of integrating the two technologies; however, lacking implementation in integrated circuits, application was restricted to fairly simple designs.
In the late 1990s, modern IC fabrication technologies began to make BiCMOS a reality. This technology rapidly found application in amplifiers, and appears to have some advantages in digital logic. At this time, the level of integration possible in CMOS circuits is still not possible. This is not a serious concern for amplifiers. It does, however, restrict BiCMOS to small to medium scale integration in digital logic circuits.
Advantages
As a simple example of a BiCMOS circuit, consider a two-stage amplifier which uses a MOS transistor in the first stage and a BJT in the second. It should be clear that this provides the high input resistance of the MOS and the low output resistance of a BJT amplifier. What is not so obvious is that in certain configurations, particularly the cascode, these characteristics may be obtained while maintaining a low capacitance, nearly as low as the BJT alone. This translates to high bandwidth (for amplifiers) and high switching speed (for logic circuits).
Difficulties
BiCMOS as a fabrication process is not nearly as mature 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.