While a wide bandgap material with poor mobility can saturate the output current, we demonstrate a way to achieve clear current saturation in the output characteristics using narrow-bandgap, high mobility graphitic-channels (Fig.4b,4c) without hurting the mobility. Using gate engineering alone, we preserve the intrinsic narrow bandgap but locally cascade them along the channel. This filters intermediate conduction and valence bands and widens the gap in the tranmission (Fig.3) without sacrificing mobility. A widen transmission gap delays the onset of band-to-band tunneling, which normally plagues devices with a narrow bandgap channel. Results are verified using an optimized fully atomistic non-equilibrium Green's Function (NEGF) solver with complex 3-D Poisson. A graphitic channel is used as a template but is one of many possible narrow-bandgap materials with high mobility. Without hurting mobility, the improved current saturation is expected to enhance gain for radio frequency (RF) and potentially digital switching applications by significantly decreasing output conductance(g_(ds)). Graphene's bandstructure imposes a fundamental trade-off between intrinsic bandgap opening and mobility. Various common Group IV and III-IV semiconductors also see this trend (Fig. 1a). Narrow bandgap channels with higher mobility typically lack current saturation in the output characteristics due to source-drain leakage currents. A consequence of poor current saturation or high output conductance(g_(ds)) is poor gain as seen in voltage transfer curve (VTC) in Fig.1b. Gain can be expressed as the ratio g_m/g_(ds), where g_m is the intrinsic transconductance and related to mobility. Our aim is to preserve mobility and g_m while decreasing g_(ds) solely using gate engineering.
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