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 <inline-formula> <tex-math notation="LaTeX">${V}_{\text {int}}$ </tex-math></inline-formula> via the gate-to-drain capacitance. It is demonstrated that NDR strongly depends on the matching between the NC induced by ferroelectric capacitance <inline-formula> <tex-math notation="LaTeX">${C}_{\text {FE}}$ </tex-math></inline-formula> and the positive capacitance associated with the underlying transistor capacitance <inline-formula> <tex-math notation="LaTeX">${C}_{\text {MOS}}$ </tex-math></inline-formula>. For the non-hysteretic devices, NDR gets pronounced with an increased thickness of ferroelectric film <inline-formula> <tex-math notation="LaTeX">${t}_{\text {fe}}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${V}_{\text {GS}}$ </tex-math></inline-formula>. This is attributed to the fact that the drain coupling factor is improved with an increased <inline-formula> <tex-math notation="LaTeX">${t}_{\text {fe}}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${V}_{\text {GS}}$ </tex-math></inline-formula>, leading to the better matching between <inline-formula> <tex-math notation="LaTeX">${C}_{\text {FE}}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${C}_{\text {MOS}}$ </tex-math></inline-formula>. For the hysteretic NC transistors, however, NDR is only obtained at the lower <inline-formula> <tex-math notation="LaTeX">${V}_{\text {GS}}$ </tex-math></inline-formula>, but not observed at higher <inline-formula> <tex-math notation="LaTeX">${V}_{\text {GS}}$ </tex-math></inline-formula>.