tailieunhanh - Silicon Carbide Materials Processing and Applications in Electronic Devices Part 12

Tham khảo tài liệu 'silicon carbide materials processing and applications in electronic devices part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Recent Developments on Silicon Carbide Thin Films for Piezoresistive Sensors Applications 375 When subjected to a mechanical stress the electrical resistance of the resistors change leading to a variation of the output voltage according to the following relationship AVụ R3 AR3_R4 AR4 16 Vs R1 AR1 R3 AR3 R2 AR2 R AR4 v Whereas the four resistors have the same nominal resistance value R1 R2 R3 R4 and that under mechanical stress the resistances R2 and R3 increases their values in AR the resistances R1 and R4 decreases their values in -AR. Therefore the equation 16 can be simplified to AVout R AR R-AR AR 17 Vs 2R 2R R Given this the sensitivity of a piezoresistive pressure sensor is determined by S R Af AVo i 18 where AP is change in pressure. Whereas for a piezoresistive accelerometer the sensitivity is defined as the electrical output per unit of applied acceleration S 1 i 1 19 R g Vs g v where g is the acceleration of gravity. 3. When and why to use SiC films in piezoresistive sensors As shown in the previous section in recent years many researchers have been reported on the piezoresistive characterization of different SiC polytypes aiming the applicability of these materials in sensors. When comparing these studies it is observed that for a same SiC polytype a dispersion of different values can be obtained for piezoresistive coefficient GF and TCR Okoije 2002 . It is known that the SiC has about 200 polytypes with different physical properties. This is one of the difficulties in characterizing the piezoresistivity in SiC. Moreover studies show that maximum value of GF for SiC at room temperature is between 30 at 49 while for the monocrystalline p-type Si is 140 see Table 1 . However all studies published until now have demonstrated the potential of the 6H-SiC and 3C-SiC polytypes besides a-SiC for the development of piezoresistive sensors for high temperature application. Given this it is important to evaluate when it is advantageous to use SiC in .