The Piezojunction Effect in Silicon, its Consequences and Applications for Integrated Circuits and Sensors

Abstract

This thesis describes an investigation of the piezojunction effect in silicon. The aim of this investigation is twofold. First, to propose some techniques to reduce the mechanical-stress-induced inaccuracy and long-term instability of many analogue circuits such as bandgap references and monolithic temperature transducers. Second, to apply the piezojunction effect to new mechanical sensor structures. The piezojunction effect changes the bipolar-transistor saturation current. This stress-induced change is mainly caused by the change in the conductivity of the minority-charge carriers. The piezojunction effect can be modeled by a polynomial approximation with a set of experimental constants, which are called the piezojunction coefficients. The magnitude of the piezojunction effect is determined according to the stress orientation and main carrier-flow direction through the bipolar-transistor base, both related to the silicon crystal axis. It has been found that the piezojunction effect hardly depends on the current density, as far the transistor is not operated in the high-injection level. Therefore, the voltage which is proportional to absolute temperature voltage VPTAT is much less stress sensitive than the base-emitter voltage VBE. The knowledge of the piezo-effects on device level has been used to predict and suggest methods to reduce their negative influence on the performance of important basic analogue circuits such as translinear circuits, temperature transducers and bandgap voltage references. A new stress-sensing element based on the piezojunction effect has been designed and tested. This stress-sensing element consists of two orthogonal L-PNP transistor pairs operated as a current mirror, which maximises the piezojunction effect and reduces the temperature cross-sensitivity. It has been verified that the linearity, gauge factor and temperature coefficient of the gauge factor are approximately the same as those of the sensors based on the piezoresistive effect. The predictable temperature-dependent base-emitter voltage can be used to compensate for the temperature coefficient of the gauge factor. For further reduction of the sensor area, a single lateral transistor with four split collectors has been designed to implement a stress-sensing device. In conclusion, it appears that the piezojunction effect is very suited to be used for low-power and miniaturised mechanical-stress sensors