Piezoresistive/piezoresistivity effect

In bulk material, the resistivity is independent of crystal orientation because silicon is cubic. However, if the carriers are constrained to travel in a very thin sheet, eg, in an inversion layer, the mobility, and thus the resistivity, become anisotropic (18). Mobility is also sensitive to both hydrostatic pressure and uniaxial tension and compression, which gives rise to a substantial piezoresistive effect. Because of crystal symmetry, however, there is no piezoelectric effect. The resistivity gradually decreases as hydrostatic pressure is increased, and then abrupdy drops several orders of magnitude at ca 11 GPa (160,000 psi), where a phase transformation occurs and silicon becomes a metal (35). The longitudinal piezoresistive coefficient varies with the direction of stress, the impurity concentration, and the temperature. At about 25°C, given stress in a (100) direction and resistivities of a few hundredths of an O-cm, the coefficient values are 500—600 m2/N (50—60 cm2/dyn). 

Semiconductor strain gauges have a much larger piezoresistance effect leading to gauge factors of between 100 and 175 for P type material and between -100 and -140 for N type material. These consequently are much more sensitive to changes in strain than the metal resistance types. On the other hand, they are affected to a greater extent by variations in temperature. 

Fig. 7.3.1 shows the principle of the piezoresistive sensor. Diffused resistors (gages) are formed on the thin-walled section called the diaphragm. An applied pressure is detected via the piezoresistive effect, which is the change in electrical resistance when a stress is applied to the diaphragm. The sensitivity is determined by the material, diameter, and thickness of the diaphragm. The thin-film piezoresistive sensor offers low sensitivity because the piezoresistive coefficient of thin-film silicon is less than one-third of that of single-crystal silicon. 

The sensitivity of a piezoresistive pressure sensor depends on the piezoresistive coefficient. Silicon crystal face selection and gage layout on the crystal face are important because of the anisotropy of the piezoresistive effect. Silicon (100) and (110) are often used with P-type diffused resistors to achieve a desired sensitivity. The next consideration is the thermal stress effect originating from the silicon crystal face. Fig. 7.3.5 shows the stress-distribution maps for silicon (100) and silicon (110) by the finite element method (FEM). 

Piezoresistive sensors. To measure the pressure, the resistance change to stress (the piezoresistance effect) may be employed. When silicon is stressed, the resulting strain breaks the cubic symmetry of the underlying crystal structure. The band structure of silicon is very sensitive to its crystal structure and, as a result, the consequent modification causes changes in the resistivity of the material (holes in the case of p " material). This change is... 

The piezoresistive effect is a change in the electrical resistivity of a semiconductor or metal when mechanical strain is applied. Polymers that incorporate conductive fillers in the form of micro/nanoparticles or nanotubes can act as an insulator, semiconductor or conductor material, depending on the amount of the conductive fillers. Fig. 14.1 shows the evolution of logarithm of resistivity p with volume fraction of conductive fillers (f) for a nonconductive polymer matrix charged with conductive fillers. The resistivity-volume fraction plot can be divided into two distinctive zones ... 

Acceleration Accelerometer, gyroscopes Suspended spring, capacitance, resonant piezoresistive effect, diaphragm... 

Impacted force (static and inertial) Load cells, elastic elements, force gauge, balances, strain gauge, bending elements Hooke s law, transforming force to displacement by an elastic element, piezoresistive effect, capacitive effect, direct or inverse magnetostrictive and magnetoelastic effects... 

He R, Yang P (2006) Giant piezoresistance effect in silicon nanowires. Nat Nanotechnol 1 42-46 Hong K-H, Kim J, Lee S-H, Shin JK (2008) Strain-driven electronic band structure modulation of Si nanowires. Nano Lett 8(5) 1335-1340... 

The piezoresistive effect is a phenomenon occurring when stress is applied to a crystal causing strain inside the crystal and changing the resistivity, according to the equation below... 

Silicon is a well-known single crystalline semiconductor material, which is widely used in integrated circuits (ICs). Silicon possesses excellent mechanical properties, which surpass stainless steel in yield strength and hardness with good mechanical stability. Silicon also has a high piezoresistance effect, whose resistivity changes when mechanical loads or stresses are applied to it. This makes silicon... 

Semiconductors. Semiconductor strain gauges are also available. These operate on the same piezoresistive effect that applies to metal strain gauges. This effect incorporates the change in electrical resistivity of a material due to an applied stress. The piezoresistive coefficients of germanium and silicon can be extremely high, with gauge factors up to 175 as compared to 2-5 for metallic wires. 

Other mechanical sensors rely on the piezoresistive effect, which represents the variation of resistance of a given material when subjected to strain (Fraden, 2010). The change in resistance results from the mechanical deformation of the material and is the physical principle underlying the operation of many force and pressure sensors. 

Riedel, R., Toma, L., Janssen, E., Nuffer, J., Melz, T., Hanselka, H. (2010b). Piezoresistive effect in SiOC ceramics for integrated pressure seusors. Journal of the American Ceramic Society, 93(4),920-924. doi 10.1111/j.1551-2916.2009.03496.x. 

Dynamic explosion detectors use a piezoresistive pressure sensor installed behind the large-area, gas-tight, welded membrane. To ensure optimum pressure transference from the membrane to the active sensor element, the space between the membrane and the sensor is filled with a special, highly elastic oil. The construc tion is such that the dynamic explosion detec tor can withstand overpressures of 10 bar without any damage or effect on its setup characteristic. The operational range is adjustable between 0 and 5 bar abs. Dynamic explo-... 

The principle of the resistance strain gauge 2 0 is that the electrical resistance of a conductor will change when it is stretched or compressed due to the consequent variation in its physical dimensions. There is an additional effect called the piezoresistance which is the relation between the resistivity p of the material and the mechanical strain. The resistance R of a conductor of area of cross-section A and length x is given by ... 

It is also interesting to briefly consider online measurements of variables different from temperature [5], Since pressure is defined as the normal force per unit area exerted by a fluid on a surface, the relevant measurements are usually based on the effects deriving from deformation of a proper device. The most common pressure sensors are piezoresistive sensors or strain gages, which exploit the change in electric resistance of a stressed material, and the capacitive sensors, which exploit the deformation of an element of a capacitor. Both these sensors can guarantee an accuracy better than 0.1 percent of the full scale, even if strain gages are temperature sensitive. 

Use was made of a piezoresistive strain gauge array to measure the stress distribution on the surface of the die. A beam bending apparatus was used to study the importance of the thermoviscoelastic properties of the molding compound. The strain gauge allowed for the study of the effects of thermal shock testing. 

The principle of operation of transducers is based on the conservation of either linear (i.e., Coriolis effect) or angular momentum, making a transducer well suited for micromachined rate-sensing gyros. One or more linearly or rotationally vibrating probe masses are required, for which the input motion stimulus and the output signal can be accomplished by various physical effects (electrostatic, electromagnetic, piezoresistive, etc.). Usually the drive motion is resonant, so the detection motion can also be resonant or the two natural frequencies are separated by a certain frequency shift. Drive and detection motion can be excited by inplane motions or by a mixture of in-plane and out-of-plane motions. 

The silicon piezoresistive sensor detects distortion. Therefore diffused resistor (gage) layout and diaphragm geometry design have a profound effect on the sensor characteristics. We utilize simulation technology for pressure-sensing element design. 

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