Semiconductor strain gauge uses the property of piezo resistivity possessed by doped silicon or germanium. When such materials are stressed due to load imposed on them causing their deformation, their resistivity changes. This change in resistivity (resistance) is used as a measure of strain. These gauges are produced in the form of wafers of thickness 150 μm. The material used may be Silicon or Germanium, in which, exact amount of impurities, such as Boron or Phosphorous have been added to import certain desirable characteristics. This process is called as doping and the crystals are known as doped crystals.
Construction and Working of Semiconductor Strain Gauge
A typical semiconductor strain gauge consists of semiconductor materials and leads enclosed in a protective casing. It is bonded on suitable insulating material such as Teflon. These types of gauges have negative temperature coefficient resistance, which permits very large change in length as compared to other strain gauges, such as wire and foil type gauges. Electrodes are made up of gold wires. They are generally used as one arm of wheatstone bridge.
Fig. 1: Semiconductor Type Bonded Strain Gauges
Advantages of Semiconductor Strain Gauge
- Semiconductor strain gauge has high gauge factor (100 to 200) as compared to metallic wires having gauge factor 2. Due to high gauge factor, very small strains or deformations can be measured accurately.
- Low hysteresis.
- More life under fatigue loading.
- Very compact in length from 0.7 to 7 mm, so occupies less space.
Disadvantages of Semiconductor Strain Gauge
- It is very sensitive to change in temperature, so it is difficult to obtain reliable reading.
- It is more expensive and difficult to attach to the object under test.
- It has non-linear characteristic relationship of output voltage with strain.
- Not suitable for high strain measurements.
- It can be easily broken as compared to its equivalent wire or foil type strain gauge.
Applications of Semiconductor Strain Gauge
Use of semiconductor strain gauges is preferred in apprcations, where high sensitivity is required.
DIFFERENCE BETWEEN SEMICONDUCTOR GAUGES AND METAL GAUGES
| Comparative point | Semiconductor Gauges | Metal Gauges |
| Gauge factor | Very high. | Comparatively small. |
| Size | Very small. | Large except foil gauges. |
| Linearity | Poor. | Very good. |
| Temperature stability | Poor. | Excellent. |
| Elastic strain range | High. | Low. |
| Fatigue life | High. | Low. |
| protection from strong fluctuating light | Essential due to photosensitivity of semiconductor. | Not required. |
