Nanoelectronic Devices With Precise Atomic-Scale Structures

Field-effect transistors with nanometer dimensions are under development.

Since its invention in 1948, the transistor has revolutionized everyday life. The electronics revolution is based on miniaturization of transistors; smaller transistors are faster, and denser circuitry has more functionality. Transistors in the present generation of integrated-circuit chips have sizes of ≈0.18 µm, and the electronics industry has completed development of 0.13-µm transistors, which will enter production within the next few years. Industry researchers are now working to reduce transistor sizes below 0.1 µm — a thousandth of the width of a human hair. However, studies indicate that the miniaturization of silicon transistors will soon reach its limit.

For further progress in microelectronics, it is necessary to turn to nanotechnology. Rather than continuing to miniaturize transistors to a point where they become unreliable, nanotechnology offers the new approach of building devices on the atomic scale. One vision for the next generation of miniature electronic circuitry is that of atomicchain electronics; according to this vision, each device is composed of atoms aligned on top of a substrate surface in a regular pattern. The Atomic Chain Electronics Project (ACEP) — part of a nanotechnology group at Ames Research Center — has been developing the theory for understanding atomicchain devices, and a patent for atomic-chain electronics has been filed and is now pending.

The use of dopants is critical to the functionality of transistors. Dopants are impurities intentionally added to the semiconducting transistor channel to raise or lower the device switch-on voltage. Typically, a macroscopic transistor (one with a size of the order of 10–6 m) contains several thousand dopant atoms that, on the scale of the overall device, appear to be smeared out as a dopant “jelly,” as illustrated in the top part of Figure 1. Because of the large number of dopant atoms, the precise location of each dopant atom is not very important to the functioning of the transistor. However, when the size of a transistor is reduced to the range of 10–8 to 10–7 m, as illustrated in the middle part of Figure 1, the number of dopant atoms is less than about 100, in which case the position of each dopant atom does matter. Current manufacturing techniques do not provide the means to control the locations of the few dopant atoms precisely, and as a result, small variations in functioning occur among transistors. Such variations are fatal when millions or billions of transistors are integrated in a computer chip, because the variations can cascade from one device to the next, eventually giving rise to a malfunction. A solution of this aspect of the miniaturization problem, devised by the ACEP, is to create all the device structures with atomic chains laid out in a regular precise pattern by anchoring atoms to a substrate, as illustrated in the bottom part of Figure 1.

A second critical aspect of the functioning of a transistor is gain: the output of the transistor should be a magnified version of its input. ACEP research has led to the conclusion that in designing atomic-chain transistors to produce gain, one should exploit the field effect, which is the only mechanism verified experimentally so far at the atomic scale. Semiconductors conduct current only when a gate voltage is (is not) applied — field effect. Metals always conduct current. A field-effect transistor uses a semiconductor for the channel and a metal for electrodes, and this is how it achieves large gain. Thus, it is necessary to devise chains with semiconductor and metal properties. First, the ACEP team tackled a simple problem theoretically: If one can arrange silicon atoms along a line floating in air, is this chain semiconducting? The answer is surprising: although bulk or thin-film silicon is semiconducting, an isolated chain of silicon atoms is always metallic. Fortunately, ACEP research also leads to the finding that a magnesium chain is semiconducting, even though bulk magnesium is metallic.