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Some new understandings on the triode-like behavior and the "punchthrough mode" operation of a short-channel MOSFET are obtained based on a two-dimensional computer simulation. The drain current of a MOSFET is essentially space-charge-limited, showing a shift from a pentode-like behavior to a triode-like behavior with decreasing (source-drain distance/gate oxide thickness) ratios which serve to manifest the relative modulations by the gate and the drain upon the channel conductivity. In a short-channel MOSFET, the triode-like behavior is more accentuated by a "punchthrough effect" due to a lowering of the potential barrier at the source by the drain field. The current thus resulted includes a gate-uncontrollable component flowing deep in the bulk, which is termed "punchthrough current" to distinguish it from the gate-controllable component characterizing the triode-like behavior. Optimized conditions for a device for reducing the "punchthrough current," are also described in detail.