NANO-ELECTRONICS AND TRANSISTORS

Nano-electronics and Transistors

Nano-electronics and Transistors

Introduction

“The smaller we are, the better we perform.” That is the siren song of quantum transistors, in which electrons skip on and off quantum dots or tunnel through barriers thought impenetrable in the world of classical physics. (LaVan, 2003) Nothing stands between these devices and everincreasing density and performance. Manufacturing processes keep on shrinking their feature sizes, even down to atomic-scale dimensions, and switching frequencies approaching a terahertz are foreseeable because only a handful of electrons are needed to operate the devices. So great is the allure of quantum devices that several types are under development. One variant is the double-electron-layer tunneling transistor (Deltt) built by researchers at Sandia National Laboratories, in Albuquerque, N.M. Another avenue is to boost the performance of conventional transistors by teaming them with resonant tunnel diodes, quantum devices similar to the Deltts. So far, most resonant tunneling devices (both Deltts and diodes) have utilized indium phosphide or gallium arsenide processes. But engineers are busy building silicon-based devices as well. Another sort of quantum device shows great promise for nonvolatile memory. Called the singleelectron transistor, or sometimes the quantum dot transistor, it is under development by research groups worldwide. A single-electron memory cell—the nanocrystal device pioneered by Sandip Tiwari, now at Cornell University, Ithaca, N.Y.—is of silicon, operates at room temperature, and should prove to have faster read and write times than conventional nonvolatile memories. (Goicoechea, 2007)

Quantum cellular automata are a fourth type of device. Automata are cells that contain four quantum dots arranged in a square. An extra electron resides on each dot of one diagonal or the other, determining if the cell stores logic 1 or a 0. The cells perform the necessary logic functions by interacting with neighboring cells. The dots can be metal but arrangements of molecules are also possible. Though quantum transistors are a novelty today, they will be needed once the classical field-effect transistor (FET) can be made no smaller—an event even now on engineers' radar screens. “Already in research labs around the world the last generation of bulk CMOS is being explored,” wrote Hon-Sum Philip Wong in the April 1999 Proceedings of the IEEE. What will eventually stop CMOS technology in its tracks is not the inability to shrink its physical size further, but the dire effect of quantum phenomena on the ever-tinier transistor's operation. (Frist, 2005) In nanoscale FETs, tunneling through ultrathin oxides and extremely narrow channels leaks an unacceptable amount of current. And the minuscule number of dopant atoms in the channel varies enough from one transistor to the next to wreak unavoidable havoc with operating margins.

Electrons in a Well

Perhaps the most striking quantum effect in transistor-like devices is tunneling. The term alludes to a particle plunging through a barrier that would be impenetrable in the classical world. The mechanism is basic to the Deltt being developed at Sandia. (Das, 2007) Though Deltt developers are still in the early stages of exploration, they are pinning their hopes for high speed on performance ...