DC Modeling of Composite MOS Transistors

Mixed-signal circuit design on sea-of-gates arrays requires the use of composite MOSTs, combinations of in-series and in-parallel connected unit MOSTs. To avoid an increase in circuit simulation complexity these are in general replaced by artificial single MOSTs. The analysis in this paper shows that a straightforward replacement will lead to incorrect results. Series MOSTs (in-series connected unit MOSTs) are essentially different from single MOSTs due to the presence of diffusion areas interrupting the channel at regular distances. The influence of lateral diffusion, charge sharing, and series resistance needs to be reconsidered. The theoretical results are confirmed by measurements on an experimental IC. Parameter decks of existing MOST models for circuit level simulation can be modified easily to reflect the length dependences of composite MOST parameters.