A STUDY OF SEMICONDUCTOR STRAIN GAUGES

Pickthorne, Brian (1987). A STUDY OF SEMICONDUCTOR STRAIN GAUGES. BPhil thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f937

Abstract

Piezoresistance is the change in the electrical resistance of a material due to the application of a strain and is employed for measuring strain. The effect in semiconductors was reported in 1954 by Smith and it was mainly in America, during the following decade, that the investigation and development of semiconductor strain gauges took place. Recent literature in this sphere is somewhat scant.

This investigation examines the theoretical basis of piezoresistance in semiconductors, explores the manufacture and properties of semiconductor sprain gauges and reviews factors which influence their properties, such as dopant level and type. In comparison to metal foil or wire gauzes it is shown that the gauge factor of semiconductor strain gauges is high. However, at elevated temperatures they display the following undesirable features to a greater extent than metal foil or wire strain gauges:
(a) T.C.R. : temperature coefficient of resistance
(b) T.C.G.F. : temperature coefficient of gauge factor
(c) Apparent strain induced by differential thermal expansion between the gauge and substrate
(d) A non-linear strain/resistance characteristic which varies with temperature.

Investigations were undertaken regarding the behaviour, use and testing of typical semiconductor strain gauges and recommendations are made based upon the findings. This includes the bonding of semiconductor strain gauges, gauge non-linearity and gauge energization circuits. Studies of bridge circuits were also carried out and a special bridge/amplifier system, which was devised for use with semiconductor strain gauges, is described and assessed. Investigations of the thermal characteristics are then reported and suggestions made concerning the compensation of these effects. Following this, details of the photosensitivity of semiconductor strain gauges are presented. A photoconductive effect was detected, and the photo - transient response of typical semiconductor strain gauges was found to be much more marked than their static response.

In a final section two significant potential applications of semiconductor strain gauges are outlined:
First, the use of semiconductor strain gauges in nuclear radiation fields was explored. The results indicate that such an application is feasible. The second application is the use of semiconductor strain gauges, and the special bridge/amplifier unit, for condition monitoring of automotive engine sub-units. It was again found that this application proved feasible, within the particular contexts explored.

Conclusions are finally drawn concerning the relative properties of semiconductor and other electrical strain gauges. Consideration is given to possible improvements to semiconductor strain gauges, and suggestions are made concerning future developments and further investigations arising as a result of this study.