Observation of double-hump substrate current in funnel-shape transistors

n-channel transistors with a funnel-shape (FS) channel region were fabricated with thin gate oxide (21 nm) and short-channel length (1 µm) to study the effects of channel shapes on hot-electron effects. Two interesting phenomena are observed. First, the double-hump substrate current phenomenon is found when operating with wider channel close to drain side (wide-drain mode), while the narrow-drain mode shows the usual single-peak substrate current characteristics. Second, an enhanced gate current injection is found in the wide-drain mode, which is surprising as substrate current is actually lower in this mode. The finding is interesting as it suggests that floating-gate FS-tranistors with short-channel length and thin gate oxide are more efficient in programming when operating in wide-drain mode. This contradicts the previous SIMOS EPROM device that utilizes funnel-shape channel region operating in narrow-drain mode. The discrepancy is ascribed to the occurrence of double-hump effect in substrate current and associated enhanced gate current injection in FS-transistors when channel length and gate oxide are scaled down.