AN INTRODUCTION TO NANOCOMPUTING - Bio-Inspired and Nanoscale Integrated Computing - page 7

1,000,000,000

100,000,000

10,000,000

1,000,000

100,000

10,000

1,000

100
1970 1975 1980 1985 1990 1995 2000 2005

Year

Figure 1.4. The number of transistors found in commercial processors. Note that
the y-axis is logarithmic. This exponential trend is mostly due to the decreasing
size of transistors. It is expected that very soon transistors will become as small as
physically possible, which motivates the exploration of other devices that may be
smaller and more powerful. Data acquired from [2].

transistors has decreased so much that in today's tiny transistors, electrons
regularly tunnel between the gate and the wire. (Fig. 1.6). Since electrons and
charge cannot be controlled as easily at the nanometer scale, the transistor
behaves less and less like an ideal switch.
Tunneling has become part of a larger tradeoff between performance and
power consumption. The size of transistors has reached the point where tradi-
tional models of transistors cannot be applied without a detailed understanding of
nonideal characteristics [3]. There are many reasons that electrons can uninten-
tionally leak across the wire, even when the gate tries to block current.
Furthermore, the smaller the wires become, the more difficult it becomes for
electrons to move through wires; that is, thinner wires have greater resistance.
Because of this, even more power is required to push electrons through the wires
quickly. Most processors today are limited to about 4GHz, largely because power
requirements beyond this speed are too costly and generate too much heat for a
processor to function properly.
Many creative solutions have kept transistors useful despite these limitations.
For example, by placing the appropriate stress or strain on the crystalline