Device asymmetric current is a common soft failure type in real nano-probing cases. Generally speaking, it is not easy to find out the root cause. LDD shadowing, Poly depletion are well known failure mechanisms causing asymmetric current due to an equivalent high resistance at defect location. Based on the defect serious condition, even some hard defect like channel AA pitting will show the similar I-V feature with previous 2 soft fail cases. This paper will introduce another type of asymmetric current feature – poly bottom undercut which I-V curve still keeps ideal linear and saturation area, but the asymmetric current difference is obvious.