When a high value of the gauge factor is required, a semiconductor strain gauge is preferred. We know that the general equation for the gauge factor or the sensitivity of a strain gauge is given as,
From the above relation, G = (dR/R)/(dL/L), a conclusion can be drawn that a relatively high change in resistance will result in a high value of the gauge factor. In metallic resistance strain gauges, the change in resistance occurs mainly due to the change in dimensions and the change in resistivity is almost negligible. Hence the above equation reduces to,
Usually, Poisson's ratio v = 0.3 (for most of the metals working in the elastic limit).
The gauge factor can be improved, if the change in resistivity is brought into effect. Unlike metallic gauges, in semiconductor strain gauges, when the strain has applied the change in resistance occurs mainly due to the piezo-resistive effect.
The change in resistivity due to strain is known as a piezo-resistive effect. Hence, for semiconductor strain gauges, the term (dρ/ρ)/(dL/L) comes into effect in the gauge factor equation.
Where,- π = Piezo-resistive co-efficient
- E = Young's modulus of elasticity of the material.
As the resistance of the material is mostly affected by its resistivity characteristic, the piezo-resistive effect results in a relatively high gauge factor or high sensitivity.
Therefore, the gauge factor (G) of a semiconductor strain gauge is 50 times that of the resistance (wire) strain gauge. Gauge factors of the order of 100 to 150 are also achieved by suitable designs. Hence, the change in dimensions due to strain is considered negligible in such cases.
Construction of Semiconductor Strain Gauge :
The design of a semiconductor strain gauge is simple, formed by a semiconductor wafer or filaments. The semiconductor materials like silicon and germanium are used for the strain gauge construction. A typical semiconductor strain gauge with length 2 to 10mm and thickness of 0.05mm is bonded on a suitable insulating base such as Teflon as shown below.
Two leads made up of gold are connected to the semiconductor wafer and are brought out for electrical connectivity. The other ends of the two leads are connected to a measuring instrument (wheat stone bridge) which determines the strain applied.
The semiconductor wafers can be of two types. They are Negative or N-type semiconductor wafers and Positive or P-type semiconductor wafers. For a tensile strain, the resistance of semiconductor gauge increases for P-type while it decreases for N-type.
Working of Semiconductor Strain Gauge :
The principle of semiconductor strain gauge is based on the piezo-resistive effect i.e., the variation in resistance due to variation in resistivity. We know that the resistivity of a material depends upon length, area of cross-section, temperature, and kind of material.
Whenever the substance to which a semiconductor filament (i.e., a typical semiconductor strain gauge) bonded is subjected to a force (tensile or compressive). Due to this force, deformation occurs in the substance changing its length and cross-section (i.e., it is strained). This in turn also causes a strain on the semiconductor strain gauge bonded on that substance.
Due to this, there will be a change in the resistance of the semiconductor strain gauge due to change in its resistivity. By measuring this change in resistance of the semiconductor material the strain applied can be measured. A Wheatstone bridge can be used for the measurement of resistance.
A high degree of accuracy in the measurement can be obtained due to a relatively high change in resistance. A semiconductor strain gauge provided with temperature compensation will have capable of measuring micro strains. A semiconductor material of resistance 350Ω and dimension 1×0.5×0.005 inch have a gauge factor of 130 ± 10%.
