The flatband shift induced by 25hyphen;keV electronhyphen;beam radiation in unbiased MOS structures depends in great part on the type and concentration of the dopant in the silicon. The flatband shifts fornhyphen;type samples are less pronounced than those forphyphen;type samples, and in general are in the opposite direction. The radiationhyphen;induced interface states, however, are independent of the dopant. The difference between the flatband shifts is shown to be best explained by a model based on the assumption that the interface states above midgap are acceptorhyphen;type states, and those below are donorhyphen;type states. A quantitative analysis made with this model indicates that the radiationhyphen;induced fixed oxide charge also is independent of the dopant. The dopant dependence of the flatband shifts can thus be attributed entirely to the charging effects of the interface states. By use of the same model, the apparent discrepancy between the shifts in the threshold voltage and in the flatband voltage of a MOSFET can also be readily resolved. A simple midgap method is implemented to obtain the fixed oxide charge without using interfacehyphen;state information; the results agree well with those obtained by the more conventional flatband method, which must take the interfacehyphen;state effects into account.
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