Nonideality of Negative Capacitance Ge Field-Effect Transistors Without Internal Metal Gate
We demonstrate negative capacitance (NC) Ge field-effect transistors (FETs) utilizing the TaN/HfZrO<sub><italic>x</italic></sub>/SiO<sub>2</sub> gate stack. Typical characteristics of NC transistors, including the sub-60 mV/decade subthreshold swing (SS), the gate capacitance <inline-formula> <tex-math notation="LaTeX">${C} _{G}$ </tex-math></inline-formula> peak, and the negative differential resistance effect are achieved in NC Ge FETs without internal metal gate. Significant <inline-formula> <tex-math notation="LaTeX">${C} _{\textit {G}}$ </tex-math></inline-formula> peak, as the evidence of NC effect is obtained in NC transistors at the frequency up to MHz. Ferroelectric Ge FETs with counterclockwise <inline-formula> <tex-math notation="LaTeX">${I} _{\text {DS}} - {V} _{\text {GS}}$ </tex-math></inline-formula> loops due to trapping/detrapping, opposite hysteresis to the NC switching are also observed. The NC transistor has much steeper SS compared to the device dominated by the trapping/detrapping process. Statistical results show that only 10% of devices are dominated by the NC effect, and the density of defects in ferroelectric needs to be reduced to improve the yield of NC transistors.