Optimum Conditions of Body Effect Factor and Substrate Bias in Variable Threshold Voltage MOSFETs

The effects of body effect factor (γ) and substrate bias ( V bs ) in a variable threshold voltage metal oxide semiconductor field effect transistor (VTMOS) have been systematically examined by device simulation. The characteristics of a VTMOS are significantly affected by the value of γ and the V bs difference (Δ V bs ) between the active mode and the standby mode. Optimal γ and Δ V bs to obtain higher on-current in the active mode and lower off-current in the standby mode are derived. When off-current in the active mode is limited, a larger Δ V bs and smaller γ are preferable to obtain a higher drive current. When γ is fixed, |Δ V bs | should be as large as the breakdown and leakage current permits. When Δ V bs is fixed for some reason, such as breakdown, the optimum γ depends on the relationship between Δ V bs and V dd : γ should be larger when a large |Δ V bs | can be applied, while it should be smaller when the |Δ V bs | is small. The scalability of VTMOS is also discussed and it is found that channel engineering is strongly required in a scaled VTMOS. These results will greatly help in designing ultra-low power VTMOS VLSIs, and the VTMOS could be expected to survive in the 50 nm generation depending on the scaling scenario and applications.