What is the working principle of Piezo resistive pressure sensor?

(The main sensor which used in wireless strain gauge)

 

Answer:

A piezoresistive sensor is made from semiconductor material in which a p-type region has been diffused into an n-type base. The resistance of this varies greatly when the sensor is compressed or stretched. This is frequently used as a strain gauge, where it produces a significantly higher gauge factor than that given by metal wire or foil gauges. Also, measurement uncertainty can be reduced down to ±0.1%. It is also used in semiconductor-diaphragm pressure sensors and in semiconductor accelerometers.

It should also be mentioned that the term "piezoresistive sensor" is sometimes used to describe all types of strain gauge, including metal types. However, this is incorrect since only about 10% of the output from a metal strain gauge is generated by piezoresistive effects, with the remainder arising out of the dimensional cross-section change in the wire or foil. Proper piezoelectric strain gauges, which are alternatively known as semiconductor strain gauges, produce most (about 90%) of their output through piezoresistive effects, and only a small proportion of the output is due to dimensional changes in the sensor.

Step-by-step explanation

Temperature Coefficient of Sensitivity

For a typical piezoresistive sensor, the output voltage is proportional to π44 of the resistor material. According to §6.1, π44 is a function of doping level. π44 and the temperature coefficient of π44 (i.e., the temperature coefficient of piezoresistance, TCπ) are also a function of the doping level. TCπ is usually negative in sign.

For example, for a pressure transducer with voltage supply Vs, the output is

where c is a constant related to the diaphragm structure of the sensor. Therefore, for a constant supply voltage, the temperature coefficient of sensitivity (TCS) of the pressure transducer is

For example, if TCπ=-0.2%, the sensitivity of the pressure transducer will be reduced by about 10% if the temperature is raised by 50°C. As the TCS is significant, the compensation for TCS has to be considered for many applications.

A widely used compensation scheme for TCS is to use a constant current supply instead of a constant voltage supply. If the supply current for a piezoresistive pressure transducer is IS, the output voltage becomes

where RB is the bridge resistance. From Eq. , we have

where the temperature coefficient of resistance (TCR) is also a function of doping level. As TCR has a positive temperature coefficient (i.e., TCR>0), the effect of TCR is opposite to that of TCπ. The curves in Fig. show TCπ and TCR as functions of doping level.

From Fig. TCπ and TCR have the same value (but with opposite signs) at two critical doping levels: NC1≅2×1018/cm3 and NC2≅5×1020 /cm3. Therefore, if the doping level of the resistors is controlled to be equal to one of the two critical doping levels, the TCS of the pressure transducer will be zero due to the cancellation of TCR and TCπ.

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