The Orientation Dependence of the Piezojunction Effect in Bipolar Transistors

Mechanical stress by packaging and processing can considerably change the saturation current of bipolar transistors. This effect was usually modelled through the stress-induced change in the intrinsic carrier concentration, which depends on the orientation of the stress. This change has now been calculated for silicon for different uniaxial stresses up to 200 MPa. It has been found to vary parabolically with stress, especially for the <100> directions. Measurements show a similar variation of the collector current. These measurements were done on both pnp and npn transistors, for different orientations of current and stress. They also show that the change in the minority mobility cannot be neglected for the stress range considered. It strongly depends on the minority type and the direction of its current through the base.