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/HfZrOx/SiO2 gate stack. Typical characteristics of NC transistors, including the sub-60mV/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-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-DS -V-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.