Silicon Gate CMOS Linear Technology

Historically, MOS technology has been the domain of the digital designer. Analog designers might use MOS transistors for the input stage of a high input impedance operational amplifier or use discrete MOS transistors in a linear circuit, but bipolar technology ruled the linear integrated circuit world. CMOS technology has changed this.

The incredible growth of the CMOS market has caused rapid development of CMOS technology which has led to advanced CMOS linear devices. There are a number of factors that will cause the CMOS linear market to continue to grow rapidly and become a very important part of linear technology. The proliferation of digital circuitry (digital displays, digital computers, etc.) in analog applications, the large increase in chip density, demands for smaller integrated circuits, and the rapid increase in demand for accurate, low power devices will lead to a tremendous number of CMOS linear applications.

This article discusses silicon gate CMOS technology and the advantages and disadvantages of the CMOS devices in order for the design engineer to fully understand the role ALD products can play in linear design.

Since most linear design is accomplished with bipolar junction transistors, an examination of CMOS and Bipolar Junction Technology (BJT) and the merits of each will best explain the advantages of each technology. Then, a comparison of metal and silicon gate CMOS technology will further identify silicon gate CMOS transistors’ unique role. CMOS technology is “simpler” than BJT technology in that the BJT’s three dimensional parameters like base depth, base thickness and base and collector doping do not need to be considered.

Since the three-dimensional bipolar parameters are more difficult to control, CMOS technology leads to a better controlled process with less variation in crucial device parameters. In addition, recent advances in processing techniques and equipment are more applicable to CMOS ICs. This means it is likely CMOS technology will advance more quickly than BJT technology.

Finally, as the number of chips required per function and the chip count per device goes down, the need for monolithic analog and digital circuitry will increase. CMOS linear devices use the same fabrication process as CMOS digital devices, making integration of analog and digital devices simple. Bipolar junction transistor technology needs to combine with CMOS technology, making fabrication cumbersome, complex and expensive.
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