Surface Effects of Space Radiation on Silicon Devices

Experimental study indicates that a significant contribution to the radiation-sensitivity of certain contemporary silicon planar transistors is caused by space-charge modifications in the oxide surface region. Certain silicon planar transistors, which are inherently passivated and protected by a silicon dioxide surface layer, have been found to realize appreciable degradation after exposure to space-type ionizing radiation. The model of radiation surface-effects proposed in the literature claims the radiation-induced ionization of the ambient gas of the transistor to be the major cause of transistor radiation-sensitivity. Contrary to this model, silicon planar transistors irradiated in an ultra-high vacuum environment realize comparable changes in electrical charactw. istics to those silicon planar transistors irradiated in gaseous ambients. Since the degradation occurs at radiation doses as low as 104 rads and is dependent upon the polarity and magnitude of electrical bias, it has been determined to be a surfa. ce effect. In order to study the effects of radiation on the silicon-silicon dioxide interface, a metal-oxide-semiconductor (MOS) structure was used. From the capacitance versus voltage characteristics of irradiated MOS transistors it has been confirmed that a major contribution to the radiation-surface degradation in silicon planar devices is the buildup of positive space charge in the silicon dioxide films covering the silicon-device surfaces. This space charge is mobile and able to drift by motion in electric fields. This positive charge can be accumulated at certain areas of the surface causing the silicon surf ace to have a tendency to go n type.