Conventional Nanoscale Devices
In conventional semiconductors, the carrier velocity is limited to about 47 in/sec (107 cm/sec), which results in limitations of current density and turn-on time, or frequency response. Because of the need for faster devices (it takes less time for a carrier to cross a smaller device) and the desire to squeeze more devices onto a single chip, e.g. a processor chip for computers, significant advances have been made since the 1970s in fabricating smaller devices.
Additional advantages have been found for operating devices in the nanoscale regime. The carrier velocity limit of 47 in/sec (107 cm/sec) is set in large-scale semi-conductors by collisions with crystalline defects, photons, etc., and can be characterized by a mean free path between collisions: the longer the mean free path, the fewer collisions and the higher the carrier velocity. It has been found that in nanoscale devices, the device itself can be considerably shorter than the mean free path, in which case a carrier injected at a high velocity, say 48 in/sec (108 cm/sec), never suffers any collisions and, therefore, does not slow down. This phenomenon is termed ballistic transport and allows semiconductors to operate far faster than was possible before, reaching frequencies of 200 Ghz.
There are complexities associated with the manufacturing of conventional nanoscale devices that appear to be major obstacles on the way to producing large quantities of these types of devices. This difficulty in manufacturing, coupled with the fact that ballistic transport devices perform best at low signal levels (as opposed to conventional electronics that operate well at high power levels) suggests that these new devices will not become widely available for use in computer chips in the near future.
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