It results in that the research works have been mainly focused on PSCFs and scarcely involving PSNFs.However, our previous experimental results indicated that boron-doped PSNFs exhibit higher piezoresistive sensitivity (GF �� 30) at high doping concentrations than PSCFs (GF is only 20~25) at the same doping levels [20, 21]. Interestingly, as the doping level exceeds 2��1020 cm-3, the GF of PSNFs increases with elevating doping concentration [22]. This could not be explained reasonably by the existing piezoresistive model of polysilicon. Additionally, by adjusting doping concentration, PSNFs present good temperature stability (the temperature coefficient of resistance (TCR) is less than 10-4/��C, one order of magnitude lower than that of PSCFs; the temperature coefficient of GF (TCGF) is less than 10-3/��C, at least twice lower than that of PSCFs) [23, 24].

Moreover, since the signs of TCR and TCGF in PSNFs could be contrary by adjusting technological parameters, the temperature self-compensation of sensors may be achieved. These unique physical properties of heavily doped PSNFs make the material potential for the development of low cost, high temperature stability and miniature volume piezoresistive sensors. Consequently, in order to analyze the piezoresistive properties of highly doped PSNFs, the tunneling effect was introduced and considered as the dominant transport mechanism of carriers traversing GBs in our previous work [20, 22]. The theoretical prediction of GF versus doping concentration gives better agreement with the experimental data than the existing models [21].

Significantly, the research work by He et al. showed that silicon nanowires possess giant PRCs [2]. The interpretation of this phenomenon was given by Rowe, and the origin of the giant piezoresistance was considered to be the stress-induced modulation of the surface DRB width [3]. It seems to be relevant to the enhanced GF of PSNFs. However, for highly doped PSNFs here, the DRB width is reduced greatly so that the contribution of DRBs could be neglected and the tunneling effect becomes dominant. It will be demonstrated further in the model calculation later.In our previous research work, the dependence of the GF on film thickness indicated that highly doped PSNFs with the thickness of ~80 nm present the highest GF of 34 and the lowest TCR and TCGF.

Therefore, the film thickness was selected to be 80 nm in this paper. Nevertheless, the nano-scale Brefeldin_A thickness is not the direct origin of the enhanced GF. In fact, the reduction of film thickness causes the contraction of grain sizes, which could increase the proportion of GB barriers to grain neutral regions and enhance the influence of the tunneling effect. Namely, the film microstructure (including grain size, GB width, trap density, etc.