GaN发光二极管表观电容极值分析

Analysis of the Apparent Capacitance Extremum of GaN Light-emitting Diode

利用正向交流(ac)小信号方法对GaN发光二极管的电容-电压特性进行测量,可以观察到GaN发光二极管中的负电容现象。正向偏压越大,测试频率越低,负电容现象越明显。测量到的负电容现象是表象,不存在负电容;提出GaN发光二极管p-n结的结电容在特定的正向电压范围内等效于可变电容。分析可变电容对正向交流小信号响应得到:特定参数的可变电容使结电容电流相位落后于交流小信号电压相位π/2,使得在测量中表现为负电容。发现表观电容-正向电压曲线的极值点与理论模型相吻合,证明了该理论模型的正确性。

The forward current-voltage characteristic and forward capacitance-voltage characteristic measurings are the most important methods to study the forward electrical characteristic of GaN light-emitting diodes. We can use the forward alternating current （ae） small signal method to measure the capacitance-voltage characteristic of the GaN light-emitting diodes. Some values of GaN light-emitting diodes parameters can be deduced from capacitance-voltage characteristic. The negative capacitance phenomenon of GaN light-emitting diode can be observed by using this method. The negative capacitance phenomenon appears more noticeable when the bias voltage is lager and the frequencies of forward alternating current （ac） small signal are lower. In this paper, we propose that the phenomenon of measured negative capacitance is as the superficial phenomenon, and there no negative capacitance exists in fact. We propose that the p-n junction capacitance of GaN lightemitting diode is equivalent to a variable capacitance in a certain range of forward voltage. By analysis the responds of variable capacitance to the forward alternating current small signal, we find that the variable capacitance for specific parameter can make the current phase of junction capacitance behind the voltage phase of alternating current small signal, lead the variable capacitance behave as a negative capacitance on measuring. The behavior of a GaN light-emitting diode can be modeled by the equivalent electrical circuit which has a capacitance with a series resistance. There is an extreme point of apparent capacitance-forward voltage curve when use the equivalent circuit for measurement. We analyze the extreme point with the theoretical model of the variable capacitance for specific parameter. The result of theory analysis tallies with the experiment. That proved the accuracy of this theory model. The research on negative capacitance phenomenon will be valuable for study the electrical characteristics of GaN light-emitting diodes, will be valuable for the knowledge improvement of the characteristic and parameter relevant the p-n junction internal structure of GaN lightemitting diodes.