Depletion-Layer Capacitance of <i>p</i>+<i>n</i> Step Junctions

Abstract

It is shown that when the contribution of holes in a p+n junction to the space charge in the n region adjacent to the doping step is considered, the capacitance C as function of applied voltage Va is given by C−2=[2(ND+NA)/qεNDNA](Va+Vx).Here NA and ND are acceptor and donor concentrations and Vx, called the offset voltage, has the classical value of built-in voltage minus 2kT/q for nearly symmetrical junctions, independent of applied voltage. For large doping ratios NA/ND, the offset voltage is nearly independent of built-in voltage but increases nearly logarithmically with applied voltage. For forward bias the present treatment gives capacitance values considerably larger than those resulting from the widely accepted treatment which neglects the charge of holes on the n side.