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

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.