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 characte-ristic 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.