A simple method to evaluate the mobile charge effect on the transition capacitance of an asymmetrical PN junction

A simple method to evaluate the effect of the mobile carriers on the transition capacitance of the asymmetrical junctions at very low voltages is proposed. The method takes into account that the presence of both electrons and holes at the depletion region boundaries is equivalent with an ionic charge decrease. Quantitatively the method considers that this decrease amounts to a shrink of the depletion region width (w). With the new approach, the built-in voltage in the Schottky formula, should be replaced by: V_bi=2Φ_F+(1.4+2.5)kT/q where Φ_F is the Fermi potential of the low-doped side of the junction