Silicon strain gages for cryostatic temperature use

Abstract

Doped silicon piezo-resistive strain gages have been used to monitor torsional stress waves at temperatures down to 44 K. These gages had the commonly employed dopant level of about 1.3 X 10^(18) acceptors/cm^3. At temperatures below approximately 38 K, the gage resistance increases as electrons freeze out of the conduction band, and the resistivity becomes too high to make use of the piezo-resistive effect. At approximately 1 X 10^(19) acceptors/cm^3, the silicon loses its semiconductor character and becomes a pure resistor with piezo-resistive properties. The room temperature gage constant decreases with an increase in doping level, but data on the gage constant down to 4.2 K were not available. This note reports the use of heavily doped gages at temperatures down to 4.2 K.