Surface radiation damage in planar processed silicon detectors is caused by radiation generated holes being trapped in the silicon dioxide layers on the detector wafer. We have studied charge trapping in thick (field) oxide layers on detector wafers by irradiating FOXFET biased strip detectors and MOS test capacitors. Special emphasis was put on studying how a negative bias voltage across the oxide during irradiation affects hole trapping. In addition to FOXFET biased detectors, negatively biased field oxide layers may exist on the n-side of double-sided strip detectors with field plate based n-strip separation. The results indicate that charge trapping occurred both close to the Si-SiO[sub 2] interface and in the bulk of the oxide. The charge trapped in the bulk was found to modify the electric field in the oxide in a way that leads to saturation in the amount of charge trapped in the bulk when the flatband/threshold voltage shift equals the voltage applied over the oxide during irradiation. After irradiation only charge trapped close to the interface is annealed by electrons tunneling to the oxide from the n-type bulk. (orig.).