By means of capacitance-voltage technique the positive charging of 9 nm thick silicon dioxide after both positive and negative bias Fowler-Nordheim injection has been evaluated. Measurement results allow to distinguish between two types of positive charge depending on their response to gate voltage variation, i.e. trapped holes (TH) and anomalous positive charge (APC). These two types of positive charges also show a different location in the oxide. More precisely, TH are characteristic traps of the SiO2/Si interface and can be found only after negative bias stress, while APC are present in both cathodic and anodic oxide regions independently on the stress polarity.