Strain and Materials Engineering for the I-MOS Transistor With an Elevated Impact-Ionization Region

An impact-ionization MOS (I-MOS) transistor with an elevated impact-ionization region (I-region) or the L-shaped I-MOS (LI-MOS) transistor has been proposed as a promising candidate among various I-MOS structures for enhanced performance through strain and materials engineering. The elevated I-region allows for the incorporation of novel materials to induce strain and reduction in the bandgap to increase the impact- ionization activity. In addition, the LI-MOS structure is more compact and compatible with conventional CMOS processes. In this paper, we discuss and explore the relationship and impact of strain and bandgap on the generation of impact-ionization carriers. Si n-channel I-MOS transistors with Si raised source/drain (RSD), Si_1-yC_y RSD, and Si_1-xGe_x RSD were studied and explored through simulations and experiments. An excellent subthreshold swing of sub-5 mV/dec at room temperature is demonstrated for the three I-MOS transistor structures. Compared to an unstrained I-MOS with Si RSD, strain-engineered I-MOS with Si_0.99C_0.01 RSD exhibits a twofold enhancement in both ON-state current and maximum transconductance at a gate length of 60 nm. For materials- or bandgap-engineered I-MOS with Sio.75Ge_0.25 RSD, a greater enhancement of approximately three times is observed. In addition, a lower breakdown voltage and enhanced breakdown characteristics are achieved with both strain- and materials-engineered I-MOS transistors.