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In this paper we present a design methodology for the ballistic FET (BFET) which illuminates many topics currently under discussion such as the injection velocity, the role of the contacts, and the meaning of the capacitance and mobility. Both InGaAs/InAlAs in the 20-30 nm gate length range, and Si/SiO/sub 2/ FETs in the 10-20 nm range are simulated using Monte Carlo techniques, where the InGaAs FETs are almost purely ballistic, whereas the Si FETs were not. Expressions for the BFET drain current are derived in the extreme quantum limit and for zero thickness gate insulator, where the importance of the degeneracy capacitance is emphasized. An ultimate speed limit for the BFET is derived.