Stress-sensitive differential amplifiers using piezoresistive effects of MOSFETs and their application to three-axial accelerometers

A novel detection transducer, which can be applied to silicon integrated accelerometers and various mechanical sensors, is proposed. Mechanical quantities such as acceleration and pressure, etc., are translated to voltage signals with a 'stress-sensitive differential amplifier' (SSDA) using the piezoresistive effects of p-MOSFETs. Signal amplification and adjustment of the sensitivity are realized with the SSDA, and removal of cross-axis errors is also performed for accelerometers. The operation of the SSDA is analysed theoretically, and its application to an integrated three-axial accelerometer is presented as an effective example. The characteristics of SSDAs have been evaluated with the fabricated three-axial accelerometers. A Z-axis sensitivity of 11 mV g^-1 is obtained even at a very small bias current (0.5 μA). By changing the operating point of the SSDA, the sensitivity at a sufficient bias current (above 10 μA) is adjustable from 25 to 55 mV g^-1 in the Z-axis, and from 3.0 to 6.5 mV g^-1 in the X- and Y-axes, respectively. Observed temperature drift of the sensitivity at a rate of -2000 ppm °C^-1 can be compensated by the adjustability of the sensitivity. The SSDA is considered to be of possible use in an elemental detection circuit for many types of integrated mechanical sensors due to its many advantages.