0.3. The Primary Focus: The MOSFET and CMOS Integrated Circuits
The Metal-Oxide-Silicon Field-Effect-Transistor (MOSFET) is the main subject of this text, since it is already the prevailing device in microprocessors and memory circuits. In addition, the MOSFET is increasingly used in areas as diverse as mainframe computers and power electronics. The MOSFET’s advantages over other types of devices are its mature fabrication technology, its successful scaling characteristics and the combination of complementary MOSFETs yielding CMOS circuits.
The fabrication process of silicon devices has evolved over the last 25 years into a mature, reproducible and reliable integrated circuit manufacturing technology. While the focus in this text is on individual devices, one must realize that the manufacturability of millions of such devices on a single substrate is a minimum requirement in today’s industry. Silicon has evolved as the material of choice for such devices, for a large part because of its stable oxide, silicon dioxide (SiO2), which is used as an insulator, as a surface passivation layer and as a superior gate dielectric.
The scaling of MOSFETs started in the seventies. Since then, the initial 10 micron gatelength of the devices was gradually reduced by about a factor two every five years, while in 2000 MOSFETs with a 0.18 micron gatelength were manufactured on a large scale. This scaling is expected to continue well into the 21st century, as devices with a gatelength smaller than 30 nm have already been demonstrated. While the size reduction is a minimum condition when scaling MOSFETs, successful scaling also requires the reduction of all the other dimensions of the device so that the device indeed delivers superior performance. Devices with record gate lengths are typically not fully scaled, so that several years go by until the large-scale production of such device takes place.
The combination of complementary MOSFETs in logic circuits also called CMOS circuits has the unique advantage that carriers only flow through the devices when the logic circuit changes its logic state. Therefore, there is no associated power dissipation if the logic state must not be changed. The use of CMOS circuits immediately reduces the overall power dissipation by a factor ten, since less that one out of ten gates of a large logic circuit switch at any given time.