The piezoresistive behavior of integrated resistors and of piezo MOS transistors on silicon substrates is known, as disclosed in F. Fruett and G. C. M. Meijer, titled “The Piezojunction Effect In Silicon Integrated Circuits And Sensors,” by Kluwer Academic Publishers. Sensitivity to stress is ordinarily investigated through measurements performed with special devices, designed ad hoc, like traditional resistor and transistor devices placed in a circular configuration, as shown in FIGS. 1 and 2, respectively.
Theoretical and experimental results on different diffused structures on silicon have been analyzed to provide design guidance for calculating and minimizing the sensitivity of traditional analog circuits to mechanical stress that is inevitably induced at many steps of any fabrication process of integrated circuits (ICs), and during the packaging processes. The packaging processes include die attachment and encapsulation. The induced stress can affect the behavior of both analog and digital circuits leading to malfunctioning of the devices.
A straightforward analog structure based on an NMOS differential pair is shown in FIG. 3. The drain current variation for parallel and orthogonal channel orientations with respect to the crystallographic axis of silicon has been measured. The effect of a load applied onto the (100) plane of the silicon crystal is illustrated in the figure. Reference is directed to the article by R. C. Jaeger et al., titled CMOS Stress Sensors on (100) Silicon, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 35, NO. 1, JANUARY 2000 85 m. An analog amplifier is used to manage the signal coming from the sensor.
Though effective, the proposed solid state sensor circuits have non-negligible current absorption which make them hardly useful for close and far field EM coupling excitation of sensors intended to be permanently embedded in stress structures. Moreover, as disclosed in R. C. Jaeger et al., the alternative use of intrinsically low power absorption CMOS structures has the drawback of the marked influence of stress induced in the semiconductor crystal on carrier mobility. This interferes with a correct measurement of stress-induced drain current variations. Laborious correction algorithms would be required. A further drawback is the limited signal-to-noise ratio.
On a different account, important real-world applications of compression induced stress measurements in reinforced concrete structures may require transducer sensitivity from 30 to 500 kg/cm2 and beyond with a good linearity of response. Generally, solid state pressure sensors are based on the use of MEMS structures (micro-machined silicon sensing elements) that make them relatively bulky and expensive.
Use of piezo MOSFETs as compressive stress detectors in devices containing near or far field powering and communication circuits to be embedded in concrete pillars has been proposed and is the subject of growing literature. Unfortunately, a reliable quantitative measure is hardly achievable and utility of these solid state permanently embedded devices is generally in providing an indication of an intervening shift from a previously recorded value gathered from the same compressive stress detector, when stimulated by a powering and reading external exciter-recorder instrument. This IT architecture may require a large database for storing information about the numerous sensors deployed for monitoring a large structure, like a building or a bridge.