Piezoresistance

What Is Piezoresistance?

Piezoresistance is the property of a material to change its electrical resistance in response to applied mechanical stress. Unlike the piezoelectric effect, which generates a surface charge, piezoresistance is a bulk resistivity change driven by stress-induced shifts in the electronic band structure of the material. The effect is particularly pronounced in semiconductors such as silicon and germanium, where it can be 50 to 100 times larger than the geometric contribution observed in metallic strain gauges. Piezoresistance was first characterized systematically by C.S. Smith in 1954, and the subsequent development of silicon microfabrication gave it a central role in microelectromechanical systems (MEMS) sensing.

Physical Mechanism and Gauge Factor

In metals, resistance changes under strain arise almost entirely from the change in conductor geometry: elongation reduces cross-sectional area and increases length. In semiconductors, this geometric effect is minor compared to the change in resistivity itself. Applied stress alters the curvature of the conduction and valence bands, shifting carrier effective masses and scattering rates in a direction-dependent way. These band-structure changes produce resistivity shifts that can be positive or negative depending on crystal orientation and the direction of stress. The sensitivity of a piezoresistive element is quantified by the gauge factor (GF), defined as the fractional change in resistance divided by applied strain. Metal foil gauges typically achieve gauge factors of 2 to 4, while doped silicon piezoresistors routinely reach 100 to 200, a range that translates directly into higher signal-to-noise in precision measurement. A comprehensive review of semiconductor piezoresistance for microsystems surveys gauge factors and their dependence on doping, crystallographic orientation, and temperature across a wide range of silicon processing conditions.

Materials and Anisotropy

Silicon's piezoresistive coefficients depend strongly on crystal orientation and doping type. The (100) crystal plane with p-type boron doping is commonly chosen for pressure-sensing diaphragms because it yields large transverse and longitudinal piezoresistive coefficients in the [110] direction. N-type doping gives a different anisotropy pattern and is preferred in some accelerometer designs. Germanium was studied alongside silicon in the 1950s but fell out of favor for most MEMS applications as silicon processing became the industrial standard. Polycrystalline silicon offers reduced piezoresistive coefficients compared to single-crystal material but can be deposited as a thin film over an oxidized surface, enabling integration on non-silicon substrates. Silicon carbide is pursued for high-temperature applications such as combustion monitoring, where conventional silicon piezoresistors lose sensitivity above approximately 150°C.

MEMS Integration

The standard MEMS piezoresistive sensor consists of a thin silicon diaphragm etched by bulk micromachining, with doped piezoresistors implanted or diffused at the locations of maximum stress, typically at the diaphragm edges. When pressure deforms the diaphragm, the piezoresistors change resistance in a pattern that reflects the stress field. Connecting four matched elements in a Wheatstone bridge configuration provides differential output, cancels common-mode temperature drift, and doubles the electrical sensitivity compared to a single element. Advances in high-performance MEMS pressure sensors describe how modern designs achieve full-scale pressures from below 1 kPa to several hundred kPa while maintaining linearity errors below 0.1% of full scale. The same fabrication approach underlies piezoresistive accelerometers, where a proof mass suspended by beams replaces the diaphragm, and inertial force replaces differential pressure as the driving stimulus. The IEEE Standards Association has published testing standards for MEMS inertial sensors that specify calibration and performance verification procedures applicable to piezoresistive designs.

Applications

Piezoresistance has applications in a wide range of fields, including: