on the capacitor, acting as a gate. Beneath the top plate is an insulating layer—which is normally silicon dioxide, sometimes doped with nitrogen—and below that is the “bulk” silicon crystal.
He showed an illustration of the basic features of the transistor superimposed on a transmission electron micrograph to indicate the scale used in building transistors today. Small dots visible in the silicon substrate represented individual atoms of the transistor. This device, still in development, showed a separation between the source and the drain, called the “channel length,” which was only about 30 nanometers, or billionths of a meter. The smallest transistors in production today have channels about twice that long. Along the channel of the research transistor were only about 80 columns of silicon atoms, again about half as many as in products being made today. Given these tiny dimensions, said Dr. Doering, “we are rapidly approaching a scale where it is feasible and appropriate to talk about transistor structure in terms of counting the number of atoms.”
When a positive (for NMOS) voltage is placed on the gate, some electrons are attracted out of the substrate into a relatively thin layer near the surface called an “inversion layer.” This creates a conductive path between the source and the drain. If another voltage is then applied between the source and drain, a current is pulled through the device; when the gate voltage is off, there is (ideally) no current for any drain to source voltage. In this way, the transistor acts as an on-off control switch.
He suggested that one way to summarize the history of the industry was to track the continual diminution of the transistors and the wires that connect them into integrated circuits—a process that has been under way since the integrated circuit (IC) was invented in 1958. The IC feature sizes in 1962 were about a millimeter–25 microns, a micron equaling one-millionth of a meter. Today the feature sizes are described in nanometers, or billionths of a meter. We have currently reached interconnect and transistor feature sizes of about 130 nm and 70 nm, respectively, he said. IC makers hope to continue along this path within the next decade or so, toward feature sizes approaching 10 nm. Such infinitesimally small sizes mean that engineers are working with a small number of atoms, something that will soon present problems associated with quantum effects.
A parallel and related trend, he said, is one that economists are familiar with: the growth in integrated circuit sales. Once the cyclicality of the semiconductor industry is “smoothed” on a plot of prices against time, the annual growth of sales is seen to be roughly 15 percent over the long term. He said that total sales were