Some effects of localized stress on silicon planar transistors

When local compressive stress is applied to the emitter surface of a silicon planar transistor, increases in collector and base currents are observed. From the relationship between these currents and the base-to-emitter junction potential, it is inferred that changes in minority-carrier current occur in the stressed material. It is well known that the energy gap of a semiconductor is affected by mechanical stress, and the effects described here are consistent with a decrease in energy gap of a few tenths of an electron volt at compressive stresses of the order of 10¹¹dynes cm^-2. It is shown that the influence of stress on the current gain of a transistor depends on the strain distribution in the material. When the stress is applied over a relatively large area using a blunt stressing element, the increased minority-carrier current causes a rise in common-emitter current gain at low currents. In the case of the stress pattern produced by a sharper stressing element, a decrease in current gain is observed over a wide current range. It is proposed that this decrease results from a degradation in emitter efficiency caused by the stress concentration in the emitter region.