P-N-JUNCTION CAPACITANCES .2. THE NEUTRAL CAPACITANCE

The depletion layer of a pn junction, especially at high forward bias, is far from depleted of mobile carriers. The overlapping part of these carrier charges gives rise to an extra component in the total junction capacitance, the so-called neutral capacitance. Kerr and Berz reported that it plays an essential role in the high-frequency behavior of bipolar transistors. Simple expressions for this neutral capacitance are presented for abrupt and linear junctions. The agreement with computer simulations is satisfactory. The principal conclusion for device modeling is that the contribution of the neutral capacitance to the total junction capacitance can in practice be neglected. This contradiction with the findings of Kerr and Berz can be attributed to differences in the definition of the depletion region.