Improvement in Electrical Characteristics of ZnSnO/Si Bilayer TFET by W/Al₂O₃ Gate Stack

We have examined impacts of gate insulator (Al
₂
O
₃
or HfO
₂
) and gate electrode (TiN or W) on electrical performance of ZnSnO/Si bilayer tunneling field-effect transistors (TFETs). It is found from the capacitance-voltage (C-V) characteristics that the W gate is effective to reduce the counter-clockwise hysteresis. Additionally, the optimal temperature of post-metallization annealing (PMA) is different for each gate stack, and this optimization is critically important for the high ON-state current (ION) and steep ON/OFF switching. The W/Al
₂
O
₃
/ZnSnO/Si bilayer TFET provides the hysteresis-free steep ON/OFF switching with the minimum sub-threshold swing (SS) of 65.4 mV/dec. and average SS of 72.0 mV/dec. in a gate-voltage swing of 0.3 V, which are 19% and 18% lower than the control TiN/Al
₂
O
₃
/ZnSnO/Si bilayer TFET. When the gate stack is replaced by W/HfO
₂
/Al
₂
O
₃
, the TFET exhibits less steep ON/OFF switching in spite of thinning capacitance equivalent thickness (CET) from 5.9 and 2.3 nm. These results indicate the importance of improvement in the gate stack quality on the sub-threshold characteristics of the bilayer TFETs.