The influence of gate edge shape on the degradation in hot-carrier stressing of n-channel transistors

The hot-carrier properties of planar and graded gate structures (upturning of the gate edge in the gate overlap region) of n-MOS transistors were examined. It was found that the type of degradation suffered by each type of device depends on the shape of the gate edge. This is interpreted in terms of the degree of gate control of the gate over the region in which the damage takes place in the different devices. The nongraded gate (NGG) devices degrade chiefly by a V/sub t/ shift, whereas the graded gate (GG) devices show a pronounced transconductance decay, with practically no V/sub t/ shift. It is suggested that the damage is situated in the gate overlap region, and that the different degradations result from a weaker field control of the gate over the degraded region leading to a series resistance type of effect in the case of the GG structure. This is supported by two-dimensional simulations.<
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