Performance Dependence on Width-to-Length Ratio of Si Cap/SiGe Channel MOSFETs

This paper measures the n- and p-MOSFETs fabricated through 65-nm high-k/metal gate CMOSFET process flow. The [110] channels of the Si cap on SiGe with different width (W) and length (L) ratios were compared with Si-only channels. The results show that a high W-L ratio in the [110] n-channel can alleviate the degradation of biaxial compressive stress. Meanwhile, a low W-L ratio in the p-channel can improve the performance; however, the ratio should at least be below two in this paper. The dominance of the longitudinal or transverse configurations successfully explains this phenomenon because of the reliance of the different levels of piezoresistance coefficient on the channel orientation. The threshold voltage shifts versus the W-L ratio in the p-channel is in agreement with the result. The result is verified by a quantitative current comparison at a high bias in the n-/p-channels between the strained SiGe and Si-only channels, which shows that an extreme W-L ratio in the original biaxially strained SiGe channel can result in longitudinal or transverse strain, thereby leading to different levels of performance degradation/improvement.