In this paper we present a novel physics based analytic model for C- V charactoristics of planar schottky doide which is made in GaAs MESFET process and used in GaAs MMIC.The model focuses on small size

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pays much attention to the influnce of side-wall capacitance and under-gate channel series resistance on the C-V chazacteristics of planar schottky doide. Those influnces affect the characteristcs of planar schottky doide notably and are neglected or paid little attention to in other model. In the paper a calculation example of presented model and a comparison between presented model and some other models are presented. The results reveal that the parameter involved in presented model is easy to be determined and the calculation result fits the measured result well.