Measurements and modeling of MOSFET I-V characteristics with polysilicon depletion effect

Abstract
The authors study the degradation of MOSFET current-voltage (V-I) characteristics as a function of polysilicon gate concentration (Np ), oxide thickness (tox) and substrate impurity concentration (ND) using measured and modeled results. Experimentally it is found that for MOSFETs with thin gate oxide (tox≈70 Å) and high substrate concentration (ND ≈1.6×1017 cm-3) the reduction in the drain current IDS can be as large as 10% to 20% for devices with insufficiently doped polysilicon gate (5×1018 ⩽Np⩽1.6×1019 cm-3). Theoretically it is shown that the drain current degradation becomes more pronounced as Np decreases, tox decreases, or ND, increases. A modified Pao-Sah model that takes into account the polysilicon depletion effect and an accurate gate-field-dependent mobility model are used to compute I-V characteristics for various values of Np, tox, and ND. Good agreement between experimental and modeled results is observed over a wide range of devices