As for polysilicon, the p-n junction isolation is avoided, so that the devices can work at higher temperatures. Moreover, polysilicon based devices have the advantages of low cost, facile processing and good thermal stability, compared to homogeneous silicon based devices. Thus, the piezoresistive properties and electromechanical sensors based on the material have been investigated successively for over 20 years [8-11]. Many efforts have been spent on optimizing the film structure by improving fabrication technologies. The most popular technology is chemical vapor deposition, including APCVD, LPCVD, PECVD, etc. Subsequently, by the metal-induced lateral crystallization (MILC) technique, the GF was enhanced to be 60 [12]. However, the sensor chips based on MILC technique could suffer the contamination from the metal layer.

LPCVD is a stable and mature CVD method with advantages of low cost, good product uniformity, IC process compatibility, etc., so the PSNFs studied here were prepared by LPCVD to ensure the performance and uniformity of samples.With the above review, it can be seen that it is necessary to investigate the piezoresistive properties of polysilicon and built up the theoretical model. The experimental results reported by other researchers indicated that the GF of polysilicon common films (PSCFs, film thickness �� 200 nm) reaches the maximum as the doping concentration is at the level of 1019 cm-3, and then decreases drastically as doping concentrations are increased further [9, 13-15].

Based on this phenomenon, the existing piezoresistive theories of polysilicon were established during 1980s~1990s and used to predict the Cilengitide process steps for the optimization of device performance. In the early models proposed by Mikoshiba [16], Erskine [17] and Germer [18], the contribution of GBs to piezoresistive effect was neglected, thereby resulting in the discrepancy between experimental data and theoretical results at low doping levels. To tackle this issue, Schubert et al. took the piezoresistive effect of depletion region barriers (DRBs) arising from carrier trapping at GBs into account and established a theoretical model for calculating GFs [14]. Thereafter, French et al. suggested that the piezoresistive effect of p-type polysilicon is not only due to the shift in heavy and light hole band minima relative to each other, but also due to the warpage of two sub-bands [15].

Moreover, the barrier effect of GBs was introduced into the model, achieving the good agreement with the experimental data. Noticeably, it was considered in these models that the PRCs of GBs and DRBs are much lower than that of grain neutral regions. Based on this viewpoint, since the PRC of grain neutral regions (bulk Si) falls off rapidly at high doping concentrations [19], it has been considered that the GF of polysilicon could be degraded sharply with increasing doping concentrations.