The heterode strain sensor: An evaporated heterojunction device

A new type of strain sensor has been demonstrated in the laboratory. The basic device is a p-n heterojunction diode that is fabricated by vacuum evaporation techniques directly onto a flexible substrate. Its two important properties are 1) its mechanical input characteristics are determined by the flexible substrate used in fabricating the heterojunction and 2) it functions as a low-output-impedance voltage source modulated by the substrate surface strain. The heterojunction combination that has been most commonly used is CdSe on Se. Typical devices exhibit a voltage/strain sensitivity of 500 volts per unit strain, with a differential resistance of 100 ohms, when biased at 5 mA. This corresponds to a "gauge factor" of 1000 for the diode strain sensor. Thus far, the substrates used have ranged in mechanical properties from 2-mil-thick plastic to 40 mil microscope slides but the most useful substrate appears to be anodized Al. This device is still under active experimental study. Thus the characteristics cited in this paper are preliminary in nature, and represent the current research status of this strain sensor.