Piezoresistive effect
This article includes a list of general references, but it lacks sufficient corresponding inline citations. (March 2013) |
The piezoresistive effect is a change in the
History
The change of electrical resistance in metal devices due to an applied mechanical load was first discovered in 1856 by
Mechanism
In conducting and semi-conducting materials, changes in inter-atomic spacing resulting from strain affect the
where
- ∂ρ = Change in resistivity
- ρ = Original resistivity
- ε = Strain
are constant.
Piezoresistivity in metals
Usually the resistance change in metals is mostly due to the change of geometry resulting from applied mechanical stress. However, even though the piezoresistive effect is small in those cases it is often not negligible. In cases where it is, it can be calculated using the simple resistance equation derived from Ohm's law;
where
- Conductor length [m]
- A Cross-sectional area of the current flow [m2][2]: p.207
Some metals display piezoresistivity that is much larger than the resistance change due to geometry. In platinum alloys, for instance, piezoresistivity is more than a factor of two larger, combining with the geometry effects to give a strain gauge sensitivity of up to more than three times as large than due to geometry effects alone. Pure nickel's piezoresistivity is -13 times larger, completely dwarfing and even reversing the sign of the geometry-induced resistance change.
Piezoresistive effect in bulk semiconductors
The piezoresistive effect of semiconductor materials can be several orders of magnitudes larger than the geometrical effect and is present in materials like germanium, polycrystalline silicon, amorphous silicon, silicon carbide, and single crystal silicon. Hence, semiconductor strain gauges with a very high coefficient of sensitivity can be built. For precision measurements they are more difficult to handle than metal strain gauges, because semiconductor strain gauges are generally sensitive to environmental conditions (especially temperature).
For silicon,
Giant piezoresistance in metal-silicon hybrid structures
A giant piezoresistive effect – where the piezoresistive coefficient exceeds the bulk value – was reported for a microfabricated silicon-aluminium hybrid structure.[3] The effect has been applied to silicon-based sensor technologies.[4]
Giant piezoresistive effect in silicon nanostructures
The longitudinal piezoresistive coefficient of top-down fabricated silicon nanowires was measured to be 60% larger than in bulk silicon.[5][6] In 2006, giant piezoresistance[7] was reported in bottom-up fabricated silicon nanowires – a >30 increase in the longitudinal piezoresistive coefficient compared to bulk silicon was reported. The suggestion of a giant piezoresistance in nanostructures has since stimulated much effort into a physical understanding of the effect not only in silicon [8][9][10][11][12][13][14] but also in other functional materials. [15]
Piezoresistive silicon devices
The piezoresistive effect of semiconductors has been used for sensor devices employing all kinds of semiconductor materials such as germanium, polycrystalline silicon, amorphous silicon, and single crystal silicon. Since silicon is today the material of choice for integrated digital and analog circuits the use of piezoresistive silicon devices has been of great interest. It enables the easy integration of stress sensors with Bipolar and CMOS circuits.
This has enabled a wide range of products using the piezoresistive effect. Many commercial devices such as
Piezoresistors
Piezoresistors are resistors made from a piezoresistive material and are usually used for measurement of mechanical
Fabrication
Piezoresistors can be fabricated using wide variety of piezoresistive materials. The simplest form of piezoresistive silicon sensors are diffused resistors. Piezoresistors consist of a simple two contact diffused n- or p-wells within a p- or n-substrate. As the typical square resistances of these devices are in the range of several hundred ohms, additional p+ or n+ plus diffusions are a potential method to facilitate ohmic contacts to the device.
Schematic cross-section of the basic elements of a silicon n-well piezoresistor.
Physics of operation
For typical stress values in the MPa range the stress dependent voltage drop along the resistor Vr, can be considered to be linear. A piezoresistor aligned with the x-axis as shown in the figure may be described by
where , I, , , and denote the stress free resistance, the applied current, the transverse and longitudinal piezoresistive coefficients, and the three tensile stress components, respectively. The piezoresistive coefficients vary significantly with the sensor orientation with respect to the crystallographic axes and with the doping profile. Despite the fairly large stress sensitivity of simple resistors, they are preferably used in more complex configurations eliminating certain cross sensitivities and drawbacks. Piezoresistors have the disadvantage of being highly sensitive to temperature changes while featuring comparatively small relative stress dependent signal amplitude changes.
Other piezoresistive devices
In silicon the piezoresistive effect is used in
The electrically-conductive packaging material
See also
- Piezoelectricity
- Electrical resistance
References
- PMID 20198118.
- ISBN 0131472860. Retrieved March 3, 2013.
- S2CID 42265969.
- .
- ISSN 1057-7157.
- ISSN 0924-4247.
- S2CID 17694712.
- ^ Allain, P. (November 8, 2012). Étude des propriétés électro-thermo-mécaniques de nanofils en silicium pour leur intégration dans les microsystèmes (Doctoral) (in French). Université Paris Sud. Retrieved October 31, 2021.
- ISSN 1084-6999.
- PMID 18698439.
- S2CID 12201580.
- S2CID 24747354.
- S2CID 119238891.
- S2CID 119189299.
- PMID 35304454.
- Kanda, Yozo (1991). "Piezoresistance effect of silicon". Sensors and Actuators A: Physical. 28 (2). Elsevier BV: 83–91. ISSN 0924-4247.
- S. Middelhoek and S. A. Audet, Silicon Sensors, Delft, The Netherlands: Delft University Press, 1994.
- A. L. Window, Strain Gauge Technology, 2nd ed, London, England: Elsevier Applied Science, 1992.
- Smith, Charles S. (April 1, 1954). "Piezoresistance Effect in Germanium and Silicon". Physical Review. 94 (1). American Physical Society (APS): 42–49. ISSN 0031-899X.
- S. M. Sze, Semiconductor Sensors, New York: Wiley, 1994.