Piezoresistive effect in inversion layers on silicon

The piezoresistive effect has been observed in the metal-oxide-silicon field effect transistor (MOSFET). Hall measurements show that the piezoresistive effect in the inversion layer of the device is caused by modulation of carrier mobility. Stress induced variations in surface hole mobility as high as ± 10% in p channel enhancement mode devices have been observed. As the device parameters are a function of carrier mobility, any change in mobility produces a corresponding change in conductance and transconductance of the device. Hole mobility in the device is found to vary linearly with applied stress. The MOSFET therefore functions as a transducer, responding linearly to static, or dynamic stress. It is also to be noted that the device has three terminals and functions as an "active" transducer (it has gain). The output impedance of the device is variable and can be designed to have a value from several hundred ohms to a few megaohms by adjustment of device dimensions in the photomasking process and by adjustment of the gate voltage. The MOSFET transducer is therefore compatible with either high- or low-input impedance circuitry and can be fabricated in integrated circuit configuration.