Experimental Research On Negative Capacitance In LEDs

As one of the most fundamental and important semiconductor devices, it isself-evident that the semiconductor diodes are widely used in many fields. So theresearch on electrical characteristics of semiconductor diodes has been a valuablesubject since earlier time. Through a great deal of experimental studies and theoreticalanalysis, we gain more knowledge about the negative capacitance in LEDs(LightEmitting Diodes). Our work and achievement can be mainly summarized as follows:1.We set up experimental measurement devices including the low frequencymeasurement device and the voltage modulated electroluminescence (VMEL)device;and we also made development and improvement for TH2819Precision LCR Meter in connecting with a computer, which increased ourmeasurement efficiency greatly.2.By using the forward ac small signal method we tested the electricalcharacteristics of LEDs of different kinds and functions. The experimentalresults show that negative capacitance phenomenons exist in mostsemiconductor LEDs.3.We presented a judgement experiment that testified the existence of thenegative capacitance phenomenon in LEDs.4.The further voltage modulated electroluminescence experimental results showthat the negative capacitance in LEDs is related closely to the radiativerecombination of the injected carriers.5.For the first time the relationship between capacitance and the magnitude ofa.c. has been discussed, and the results indicate that both depletioncapacitance and negative capacitance are irrelative to the magnitude of a.c.,at the same time diffusion capacitance increases with the increasing of themagnitude of a.c., which is more obvious at lower testing frequencies.6.We studied the frequency characteristics of capacitance in LEDs at lowfrequency (less than 10Hz), and discovered that the negative capacitances inall kinds of LEDs would turn into positive ones if the frequency was lowenough.What's more, negative capacitance of LEDs can possibly cause some effects onmodulation and switch circuits etc. Therefore it is significant to understand correctlyand study further both in theoretical fields and in practical application.