H. Koura et al., Optimum conditions of body effect factor and substrate bias in variable threshold voltage MOSFETs, JPN J A P 1, 39(4B), 2000, pp. 2312-2317
The effects of body effect factor (gamma) and substrate bias (V-bs) in a va
riable threshold voltage metal oxide semiconductor field effect transistor
(VTMOS) have been systematically examined by device simulation. The charact
eristics of a VTMOS are significantly affected by the value of gamma and th
e V-bs difference (Delta V-bs) between the active mode and the standby mode
. Optimal gamma and Delta V-bs to Obtain higher on-current in the active mo
de and lower off-current in the standby mode are derived. When off-current
in the active mode is limited, a larger Delta V-bs and smaller gamma are pr
eferable to obtain a higher drive current. When gamma is fixed, \Delta V-bs
\ should be as large as the breakdown and leakage current permits. When Del
ta V-bs is fixed for some reason, such as breakdown, the optimum gamma depe
nds on the relationship between Delta V-bs and V-dd: gamma should be larger
when a large \Delta V-bs\ can be applied, while it should be smaller when
the \Delta V-bs\ is small. The scalability of VTMOS is also discussed and i
t 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 dependin
g on the scaling scenario and applications.