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The effective space charge density Neff is the average density of carriers over the depletion layer in a semiconductor diode and is measured from the capacitance-voltage curve extrapolated to full depletion Vd. Full semiconductor modeling has been performed for PIN diodes made of materials with a large density of generation-recombination (g-r) centers, such as irradiated or semi-insulating semiconductors. The results show that this extrapolation method can give incorrect values for the introduction rate of charged traps and g-r centers with large irradiation fluence. This is because the introduction of midgap g-r centers moves the Fermi level toward midgap which allows existing traps to change their ionization state. We propose an alternative approach to evaluate the effective density from the C-V characteristics without the need to evaluate the depletion voltage.