Analysis of Surface Charge Effects and Edge Fringing Capacitance in Planar GaAs and GaN Schottky Barrier Diodes

In this article, by means of a 2-D ensemble Monte Carlo simulator, the Schottky barrier diodes (SBDs) with realistic geometries based on GaAs and GaN are studied as promising devices for increasing the high-frequency performance- and power-handling capability of frequency mixers and multipliers. The nonlinearity of the capacitance-voltage (C-V) characteristic is the most important parameter for optimizing the performance of SBDs as frequency multipliers. The small size of the diodes used for ultrahigh-frequency applications makes the value of its intrinsic capacitance to deviate from the ideal one due to fringing effects. We have observed that the value of the edge capacitance well into reverse bias does not depend on the applied voltage. We define an edge-effect parameter β, which, interestingly, is affected by the presence or absence of surface charges at the semiconductor-dielectric interface σ. Two physical models have been considered: a fixed σ related to a surface potential Vs constant surface-charge model (CCM) and a self-consistent model in which the local value of σ is dynamically evaluated depending on the surrounding electron density self-consistent surface-charge model (SCCM). Using the CCM, we obtain that β depends on the depth of the depletion region W
_s
created by the surface charges, nearly irrespectively of the epilayer doping or semiconductor type. The more realistic SCCM indicates that, at low frequencies, when the surface charges are able to follow the variations of the applied voltage, the value of β approaches the one obtained without surface charges, while the high-frequency value (the significant one) is smaller.