GaN 基发光二极管( LED) 是典型的PN 结型器件，本文以GaN-LED 为例，系统阐述了PN 结在交流小信号下的电容 特性; 首次区分、定义了‘电压区间'，并阐明了不同‘电压区间'对电容起贡献的机制.指出: 小电压下，PN 结电容主要由耗尽层 电容贡献，随电压几乎不发生变化; 过渡区，PN 结电容主要由扩散电容贡献，随电压近e 指数增加.这些特性与经典Shockley 理论一致.大电压下，PN 结电容变为负值.精确分析负电容特性后，总结出了负电容随电压和频率的精确变化关系式.负电容的 实验结果难以被经典Shockley 的扩散电容理论解释，但为Hess 教授的《Advanced Theory of Semiconductor Devices》提供了有力 的实验基础，将为半导体器件物理的发展起到推动作用.

GaN-based light-emitting diode ( LED) is a typical PN junction device. In this paper，GaN-LED is taken as an example to illustrate the capacitance characteristics of PN junction device under AC small signal. We have precisely pointed out the physical mechanism contributing to the total capacitance in different forward bias voltage intervals. Under a small forward bias voltage，the total capacitance of the PN junction is mainly contributed by the depletion layer capacitance. With the increase of the voltage，the total capacitance of the PN junction is mainly contributed by the diffusion capacitance. It increases with the voltage near an e-index. These characteristics are consistent with the classic Shockley theory that PN junction capacitance consists of depletion layer capacitance and diffusion capacitance. At large voltages，the PN junction capacitance becomes negative，which is defined as negative capacitance ( NC) . After accurately analyzing the NC characteristics，the empirical formula of the NC depending on voltage and frequency is summarized. The experimental results of NC cannot be explained by the classical Shockley's theory，but it provides a strong experimental foundation for Professor Hess's“Advanced Theory of Semiconductor Devices”，which will promote the development of semiconductor device physics.