The use of semiconductor material, notably silicon, for strain gauge (SG) application has increased over the past few years. There are presently several disadvantages to these devices compared to the metal variety, but numerous advantages for their use.
As in the case of the metal SGs, the basic effect is a change of resistance with strain. In the case of a semiconductor, the resistivity also changes with strain, along with the physical dimensions. This is due to changes in electron and hole mobility with changes in crystal structure as strain is applied. The net result is a much larger gauge factor than is possible with metal gauges.
The semiconductor device gauge factor (GF) is still given by Equation (5.13)
However, the value of the semiconductor gauge factor varies between -50 and -200. Thus, resistance changes will be factors of from 25 to 100 times those available with metal SGs. It must also be noted, however, that these devices are highly nonlinear in resistance versus strain. In other words, the gauge factor is not a constant as the strain takes place. Thus, the gauge factor may be -150 with no strain, but drop (nonlinearly) to -50 at 5000 mm/m. The resistance change will be nonlinear with respect to strain. To use the semiconductor strain gauge to measure strain, we must have a curve or table of values of gauge factor versus resistance.
The semiconductor strain gauge physically appears as a band or strip of material with electrical connection, as shown in Figure 5.19. The gauge is either bonded directly onto the test element or, if encapsulated, is attached by the encapsulation material. These SGs also appear as 1C assemblies in configurations used for other measurements.
FIGURE 5.19 Typical semiconductor strain gauge structure..
The signal conditioning is still typically a bridge circuit with temperature compensation. An added problem is the need for linearization of the output because the basic resistance versus strain characteristic is nonlinear.
a. Contrast the resistance change produced by a 150-mm/m strain in a metal gauge with GF = 2.13 with
b. A semiconductor SG with GF = -151. Nominal resistances are both 120 W.
From the basic equation
a. We find for the metal gauge SG
DR = (120W)(2.13)(0.15 X 10-3)
DR = 0.038 W
b. For the semiconductor gauge, the change is
DR = (120W)(-151)(0.15 X 10-3)
DR = -2.72 W
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