Anisotropy of the piezojunction effect in silicon transistors

Mechanical stress influences the saturation current of a bipolar transistor by means of the piezojunction effect. This effect can therefore serve to detect stress in micromachined sensors. It has recently been modelled for the relatively low stress levels occurring in those sensors. With the use of this model, the first-order stress sensitivity of the most common transistors can be analysed. It appears that the sensitivity of vertical transistors in both {100} and {111} wafers is isotropic with respect to the stress orientation. However, the sensitivity of lateral transistors is strongly anisotropic on these wafers. This fact is visualised by a number of polar plots.