CMOS (complementary metal oxide semiconductor) technology continues to be the dominant technology for fabricating integrated circuits (ICs or chips). This dominance will likely continue for the next 25 years and perhaps even longer. Why? CMOS technology is reliable, manufacturable, low power, low cost, and, perhaps most importantly, scalable. The fact that silicon integrated circuit technology is scalable was observed and described in 1965 by Intel founder Gordon Moore. His observations are now referred to as Moore's law and state that the number of devices on a chip will double every 18 to 24 months. While originally not specific to CMOS, Moore's law has been fulfilled over the years by scaling down the feature size in CMOS technology. Whereas the gate lengths of early CMOS transistors were in the micrometer range (long-channel devices) the feature sizes of current CMOS devices are in the nanometer range (short-channel devices).
To encompass both the long- and short-channel CMOS technologies in this book, a two-path approach to custom CMOS integrated circuit design is adopted. Design techniques are developed for both and then compared. This comparison gives readers deep insight into the circuit design process. While the square-law equations used to describe MOSFET operation that students learn in an introductory course in microelectronics can be used for analog design in a long-channel CMOS process they are not useful when designing in short-channel, or nanometer, CMOS technology. The behavior of the devices in a nanometer CMOS process is quite complex. Simple equations to describe the devices' behavior are not possible. Rather electrical plots are used to estimate biasing points and operating behavior. It is still useful, however, for the student to use mathematical rigor when learning circuit analysis and design and, hence, the reason for the two-path approach. Hand calculations can be performed using a long-channel CMOS technology with the results then used to describe how to design in a nano-CMOS process.