Modeling and characterization of quantization, polysilicon depletion, and direct tunneling effects in MOSFETs with ultrathin oxides

The electrical characteristics (C-V and I-V) of n(+)- and p(+)-polysilicon-gated ultrathin-oxide capacitors and FETs were studied extensively to determine oxide thickness and to evaluate tunneling current. A quantum-mechanical model was developed to help understand finite inversion layer width, threshold voltage shift, and polysilicon gate depletion effects. It allows a consistent determination of the physical oxide thickness based on an excellent agreement between the measured and modeled C-V curves. With a chip standby power of less than or equal to 0.1 W per chip, direct tunneling current can be tolerated down to an oxide thickness of 15-20 Angstrom. However, transconductance reduction due to polysilicon depletion and finite inversion layer width effects becomes more severe for thinner oxides. The quantum-mechanical model predicts higher threshold voltage than the classical model, and the difference increases with the electric field strength at the silicon/oxide interface.