Subthreshold Behavior of MFMIS and MFIS Transistors caused by Ferroelectric Polarization Switching

Ferroelectric polarization switching shows various effects on the performance of metal-ferroelectric-metal-insulator (MFMIS) FETs and metal-ferroelectric-insulator-semiconductor (MFIS) FETs. In a MFMIS FET, polarization switching causes a sudden charge change in the floating gate, which can improve the subthreshold swing (SS) of the FET while the ferroelectric capacitor shows a positive capacitance. The improvement has been proven by experimentally connecting ferroelectric capacitors with different sizes to a normal MOSET. A modified analytic SS equation was developed to account for these effects. In the MFIS FET, the polarization switching causes a transient negative capacitance (NC) effect, which can also improve SS. Sub-thermal SS with a minimum of 8 mV/dec and an average of 30 mV/dec for 2.5 decades was observed. However, the transient NC is susceptible to trap charging. With the increasing cycling sweeping measurements of the MFIS FET, the drain current range with sub-thermal SS degrades and finally vanishes. Trap charging effects are supposed to cause such phenomenon. The polarization effect on these two transistors is discussed.