Detecting Unintended Schottky Junctions and Their Impact on Tunnel FET Characteristics

We demonstrate in this paper a fast and simple method for evidencing and classifying the ambipolar response of tunneling-based field effect transistors in pull-down (nFET-like) and pull-up (pFET-like) modes. This technique enables to unequivocally determine whether carrier injection on either side of the device occurs via band-to-band-tunneling or single carrier tunneling through a Schottky barrier. It was applied to Silicon On Insulator (SOI) and SiGeOI tunnel FETs, which were fabricated to be nominally identical, yet showed a discrepancy of several orders of magnitude in ON-state current. The electrostatic analysis of their respective ambipolar signature revealed that the high-drive-current devices were in fact operating like Schottky barrier FETs in the pull-up mode due to a silicidation defect occurring only on the n-doped side. These new findings bring about a reassessment of previously published results in terms of on current--subthreshold swing tradeoff perspectives for nanowire SiGe pTFETs. On the other hand, the resulting unintended asymmetrical device geometry suggests a possible route to fabricating Schottky barrier FETs with reduced parasitic leakage.