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Monday, March 5, 2012

The single atom transistor


By designing a single atom transistor this year, researchers from the University of New South Wales and their colleagues have jumped forward almost a decade over the predicted time for the development of such a device.  Why is this important?  Not only are single atom transistors the end stage of extreme miniaturization, but they are also critical for the development of quantum computers.

The scientists covered a pure silicon wafer with a layer of hydrogen.  They then used a scanning electron microscope to remove individual hydrogen atoms from specific sites along that silicon chip.  Next, they exposed the wafer to phosphorous, which binds to the silicon only where the hydrogen had previously been removed.  In this way, single atoms of phosphorous were placed exactly where needed along the silicon chip.

 Schematic of the single-atom transistor made by Simmons and co-workers.

A single phosphorus atom (blue sphere) is placed with atomic precision on the surface of a silicon crystal (yellow spheres) between the metallic source (S) and drain (D) electrodes, which are formed by phosphorus wires that are multiple atoms wide. Electric charge flows (red arrows) from the source to the drain through the phosphorus atom when an appropriate voltage is applied across the gate electrodes (G). This schematic is not to scale: there are several tens of rows of silicon atoms between the phosphorus atom and the source and drain electrodes, and more than 100 rows of silicon atoms between the phosphorus atom and the gate electrodes.

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