On the punchthrough phenomenon in submicron MOS transistors

s the channel length of MOS transistors reduces to the submicron dimension, the punchthrough becomes more of a surface-initiated and gate-controlled phenomenon, A surface diffusion current (I-sdif) originates from the injection of minority carriers from the source junction due to the combined effect of drain-induced-barrier-lowering (DIBL) and surface-band-bending (Delta phi(so)). The DIBL effect increases rapidly with decreasing channel length, In addition, the extracted Delta phi(so) from the punchthrough current indicates that surface space charges at the source edge shift from the accumulation/depletion mode for long submicron devices (approximate to 0.62 mu m) to the strong-inversion mode for deep submicron devices (approximate to 0.12 mu m). In general, I-sdif dominates over the low drain bias range and eventually converts to the bulk space-charge-limited current (I-scl) as the drain bias increases and the source/drain depletion regions connect, The drain bias for this conversion to occur strongly depends on the channel dimension, Only intermediate submicron devices (approximate to 0.37 mu m) in this study clearly show both the surface and bulk (space-charge-limited) punchthrough components, For long submicron devices, I-sdif essentially dominates, while for deep submicron devices, it converts rapidly to I-scl over the drain bias range investigated, A semi-empirical closed form equation is proposed to describe both I-sdif and I-scl and their merging over the entire range of drain bias, The punchthrough current simulated from this equation shows an excellent agreement with the experimental data.