Abstract: Using the forward alternating current (ac) small signal method to measure the capacitance-voltage characteristic of an LED, negative capacitance can be observed.We propose that this phenomenon is a surface phenomenon, and there is no real negative capacitance.We further propose that the capacitance of the p-n junction in an LED is equivalent to the variable capacitance in a certain range of forward voltage.By the corresponding analysis of variable capacitance to the alternating current small signal, we find that the variable capacitance for certain parameters can shift the phase of the current by π,leading to negative capacitance in measurement.Additionally, an expression for the negative capacitance in an LED that relates it to the rotational frequency is obtained for the first time.

Key words: light-emitting diode, negative capacitance, forward alternating current small signal method, variable capacitance, rotational frequency