Nonideality of Negative Capacitance Ge Field-Effect Transistors Without Internal Metal Gate

Abstract

We demonstrate negative capacitance (NC) Ge field-effect transistors (FETs) utilizing the TaN/HfZrO x /SiO2 gate stack. Typical characteristics of NC transistors, including the sub-60 mV/decade subthreshold swing (SS), the gate capacitance ${C} _{G}$ peak, and the negative differential resistance effect are achieved in NC Ge FETs without internal metal gate. Significant ${C} _{\textit {G}}$ peak, as the evidence of NC effect is obtained in NC transistors at the frequency up to MHz. Ferroelectric Ge FETs with counterclockwise ${I} _{\text {DS}} - {V} _{\text {GS}}$ 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.