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handbook of modern sensors

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536 18 Sensor Materials and Technologies<br />

(A)<br />

(B)<br />

Fig. 18.2. Temperature coefficient as function <strong>of</strong> doping (A) and piezoresistive sensitivity <strong>of</strong><br />

silicon (B).<br />

is the temperature coefficient and R 20 is the resistance at the calibrating point (T 0 =<br />

20 ◦ C). Figure 18.2A shows that the temperature sensitivity <strong>of</strong> PS is substantially<br />

higher than that <strong>of</strong> SCS and can be controlled by doping. It is interesting to note that<br />

at a specific doping concentration, the resistance becomes insensitive to temperature<br />

variations (point Z).<br />

For the development <strong>of</strong> <strong>sensors</strong> for pressure, force, or acceleration, it is critical to<br />

know the strain sensitivity <strong>of</strong> PS resistors expressed through the gauge factor. Figure<br />

18.2B shows curves <strong>of</strong> the relative resistance change <strong>of</strong> boron-doped PS resistors,<br />

referenced to the resistance value R 0 under no-stress conditions, as a function <strong>of</strong><br />

longitudinal strain ε 1 . The parameter varies with the implantation dose. It can be<br />

seen that the resistance decreases with compression and increases under tension. It<br />

should be noted that the gauge factor (a slope <strong>of</strong> the line in Fig. 18.2B) is temperature<br />

dependent. PS resistors are capable <strong>of</strong> realizing at least as high a level <strong>of</strong> long-term<br />

stability as any that can be expected from resistors in SCS, because surface effects<br />

play only a secondary role in device characteristics.<br />

18.1.2 Plastics<br />

Plastics are synthetic materials made from chemical raw materials called monomers.<br />

A monomer (one chemical unit) such as ethylene is reacted with other monomer<br />

molecules to form long chains <strong>of</strong> repeating ethylene units, forming the polymer<br />

polyethylene. In a similar manner, polystyrene is formed from styrene monomers.<br />

The polymers consist <strong>of</strong> carbon atoms in combination with other elements. Polymer

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