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

Abstract

The effects of body effect factor (∞ ) and substrate bias (Vbs) 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∞ and the Vbs difference (1Vbs) between the active mode and the standby mode. Optimal ∞ and1Vbs to obtain higher on-current in the active mode and lower off-current in the standby mode are derived. When offcurrent in the active mode is limited, a larger 1Vbs and smaller ∞ are preferable to obtain a higher drive current. When ∞ is fixed,j1Vbsj should be as large as the breakdown and leakage current permits. When1Vbs is fixed for some reason, such as breakdown, the optimum∞ depends on the relationship between 1Vbs and Vdd: ∞ should be larger when a largej1Vbsj can be applied, while it should be smaller when thej1Vbsj 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.