What Don Quixote thought were giants turned out to be windmills, and what researchers five years ago thought was giant piezoresistance appears, at least for now, to have been just as illusory. Unfortunately, that’s bad news for everyone who hoped to take advantage of this exciting property of miniscule silicon wires. But it is a very nice example of the scientific process at work.
What is piezoresistance anyway? And what good is it?
Normal-sized piezoresistance (or, more formally, “the piezoresistive effect”) is a change in the electrical resistivity of a material that results from mechanical stress — such as stretching or compression.
Piezoresistance was first discovered in the 1950s, and it occurs only in semiconductors. Unlike the considerably higher resistivity of insulators and considerably lower resistivity of conductors, the intermediate resistivity of semiconductors is sensitive to the tiny changes in the atomic structure of a material that occur when it is stretched or compressed.
There are lots of applications of the piezoresistive effect — again, this is normal-sized piezoresistance. Many commercial pressure sensors are based on the effect; they use the change in resistivity to measure the force on the sensor. At the forefront of research, the piezoresistive effect can provide an electronic measurement of nanoscale motion, such as that of a cantilever; this can be used both on man-made nanoscale systems and biological structures. It has also been found that mechanical stress can improve the performance of transistors via the same phenomenon.
The ephemeral promise of giant piezoresistance
Given all that piezoresistance can already do, there was significant excitement in 2006, when researchers at UC Berkeley’s Lawrence Berkeley National Laboratory published measurements of a piezoresistive effect nearly 40 times larger than it is in normal, bulk silicon. Continue reading