A new model of the effect of mechanical stress on the saturation current of bipolar transistors

The saturation current of bipolar transistors is modified by mechanical stress through the piezojunction effect. This effect can be used in mechanical sensors, but is unwanted in electronic circuits stressed by packaging. A new model of the effect has been developed which is practical in sensor and circuit design. It is similar to the widespread model of the piezoresistive effect, and also consists of a polynomial series with the symmetry of the silicon crystal. The model has been verified experimentally by characterising transistors under bending stress, which also yielded values for many of the model coefficients. The set of first-order coefficients is completed by using the sensitivity to hydrostatic pressure. The piezojunction effect can be explained by changes in both the valence and the conduction bands, whereas the piezoresistive effect is determined by only one band, depending on the doping type. (C) 2002 Elsevier Science B.V. All rights reserved.