Earlier writeups discuss how MOSFETs work and the digital circuit components
node provides much information about how MOSFETs can perform digital logic
. This writeup focuses on the MOSFET's place in the electrical engineering
world and in our daily lives.
The MOSFET is the workhorse transistor of integrated circuits. While its analog properties are inferior to those of the bipolar junction transistor (BJT), its properties as a digital switch are superior. The microprocessors in our computers employ millions of MOSFET switches that perform arithmetic, store data, etc.
The basic function of a MOSFET in a digital circuit is to switch back and forth between a conducting state in which current is allowed to flow and an insulating state in which current is blocked. This switching between the two digital states (corresponding to binary 0 and binary 1) is controlled by the MOSFET gates. The clock speed of digital circuits is limited by how fast MOSFETs can switch back and forth. This switching corresponds physically to MOSFETs in conducting states charging or discharging the gate capacitors of other MOSFETs (and other parasitic capacitances).
There are many ways to make MOSFETs switch faster. Historically, the most important has been to make the gate length (i.e. the silicon channel length between the source and drain) smaller. As of 2002, this critical length is about 0.1 microns in state-of-the-art devices. One micron is one-thousandth of a millimeter. Such small devices are very difficult to fabricate and have required tremendous university and industry research. The fabrication of integrated circuits is a fascinating topic that is beyond the scope of this writeup. The fundamental obstacle to fabricating smaller MOSFETs is that photolithography cannot define structures smaller than 0.1 microns. New techniques--extreme ultraviolet lithography for instance--to replace photolithography are being investigated.
While new techniques are needed to fabricate smaller MOSFETs, the structure of MOSFETs must evolve as well. The most fundamental problem with today's structure is that as it shrinks to smaller than 0.1 microns, it cannot reach the insulating state--the switch never turns off. One solution to this problem is to fabricate circuits on silicon on insulator (SOI) wafers. Extremely small MOSFETs will require more exotic structures, such as that of the FINFET developed by the silicon device group at UC Berkeley.