Simple phenomenological modeling of transition-region capacitance of forward-biased p-n junction diodes and transistor diodes

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Abstract
Phenomenological modeling used here yields simple expressions for the capacitance C(V) of the transition (or space‐charge) region of a forward‐biased p‐n junction diode or transistor diode. Predictions are in general accord with results of previous detailed analytic modeling and of numerical solution of the finite‐difference counterparts of the standard (Shockley) equations underlying p‐n junction theory. For the capacitance treated, two components exist: the dielectric capacitance CD, from changes in majority carrier concentration near the edges of the space‐charge region, and the component CF from free hole and electron accumulation in the space‐charge region. The modeling involves picturing the space‐charge region as the i (intrinsic) region of an n‐i‐p structure for a space‐charge region markedly wider than the extrinsic Debye lengths at its edges. This i region is excited in the sense that the forward bias creates hole and electron densities orders of magnitude larger than those in equilibrium. The recent Shirts–Gordon modeling of the space‐charge region using a dielectric response function is contrasted with the more conventional Schottky–Shockley modeling. Various uses of expressions for C(V) are discussed.