GaN发光二极管表观电阻极值分析

Analysis of Apparent Resistance Extremum in GaN LED

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。利用LED串联等效电路对表观电容和表观电阻进行了测量,表观电阻-正向电压曲线出现了一个极值点。通过对相关文献分析,提出负电容现象的根本原因是在较高的正偏下微分电容dQ/dU<0;推论出pn结的微分电容先随正向偏压的增大而急剧增大,当出现复合发光后随正向偏压的增大而减小,直到随正向偏压的增大而出现负值;正向偏置电压较大时,结电导电流的变化率根据I-V特性曲线取极大值,此时微分电容由于强复合效应已快速变小,表观电阻有极大值;得到了表观电阻极大值表达式。表观电阻与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 electricity characteristic of GaN light-emitting diodes. We can use the forward alternating current （ac） small signal method to measure the capacitance-voltage characteristic of the GaN light-emitting diodes. Some value 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. Measuring the apparent capacitance and resistance by the serial equivalent circuit of LED for measurement, there is an extremum point in apparent resistor-voltage curve. Propose that the basic reason of negative capacitance is that the differential capacitance less than zero by analyzing relation literature. Infering that the differential capacitance of pn junction will grow by leaps and bounds follow the voltage of forward bias grows ; the differential capacitance of pn junction will lower till less than zero when the forward bias voltage is higher than the threshold voltage of light-emitting diodes begin emitting light., The current change rate of junction conductor will be max base on current-voltage curve when the forward bias voltage is higher than the threshold voltage of light-emitting diodes begin emitting light, and then the differential capacitance of pn junction fall down quickly for radiative-recombination, so apparent resistor is maximum. We obtain the expression of max apparent resistor. We analyze the extreme point of apparent resistor-voltage curve. The result of theory analysis agree with the experiment. That proved the accuracy of this theory model. The research on apparent resistance phenomenon will be valuable for study the electrical characteristics of GaN light-emitting diodes, and will be valuable for the knowledge improvement of the characteristic and parameter relevant the pn junction internal structure of GaN light-emitting diodes. The study of apparent resistance may open research for novel devices and applications.