Negative Differential Resistance in Negative Capacitance FETs

We report the investigation of negative differential resistance (NDR) in negative capacitance (NC) germanium (Ge) pFETs. The NDR in NC transistors is attributed to the coupling of drain voltage to the internal gate voltage V-int via the gate-to-drain capacitance. It is demonstrated that NDR strongly depends on the matching between the NC induced by ferroelectric capacitance C-FE and the positive capacitance associated with the underlying transistor capacitance C-MOS. For the non-hysteretic devices, NDR gets pronounced with an increased thickness of ferroelectric film t(fe) and V-GS. This is attributed to the fact that the drain coupling factor is improved with an increased t(fe) and V-GS, leading to the better matching between C-FE and C-MOS. For the hysteretic NC transistors, however, NDR is only obtained at the lower V-GS, but not observed at higher V-GS.