Radiation-induced space-charge buildup in MOS structures

An analysis of a simple model for radiation-induced space-charge buildup in the SiO
2
layers of MOS structures has been carried out. The model assumes that hole-electron pairs are created in the SiO
2
by the radiation and that some of the electrons thus created drift out of the SiO
2
layer under the action of an applied potential across the oxide, V
G
, while the corresponding holes become trapped. The diffusion of electrons is assumed to be negligible. The analysis predicts 1) a dependence of charge buildup on radiation dose
D
, approximately of the form (
1 - e^{-\beta D}
); 2) a linear dependence of the charge buildup on V
G
, for both polarities of V
G
; and 3) the dependence of the charge buildup on the total dose absorbed and not on the rate at which the dose was received. Experimental observations on the SiO
2
layers found in commercial MOS-FET's show good general agreement with the predictions of the analysis. To obtain quantitative agreement, however, it was necessary to assume that the mobility-lifetime product for electrons in the oxide is much lower at the SiO
2
-Si interface than at the SiO
2
- metal interface. Other discrepancies were observed but they can be explained as the result of oversimplifications employed in the analysis. In particular, it was necessary to postulate that under some circumstances diffusion of electrons out of the oxide was important and that, in addition to holes, a small number of electrons may be trapped in the oxide in some cases. The space charge was found to accumulate within ≤ 200 Å of the cathode-oxide interface.