Forward Electrical Characterization In Light-Emitting Diodes

Electrical and optical properties of semiconductor light emitting diodes (LEDs)have been paid much attention by most researchers. This paper uses self-builtcharacterization method to measure electrical characteristics of LEDs accurately, andcalculates the electrical characteristics of p-n junction in detail by using basic principlesof physics. The main work can be summarized as follows:First, we reviewed characteristics, classification and application as well as theresearch status of electrical characteristics of LEDs, and summarized the traditionalmethod and Self-built method based on forward AC small signal method together withDC I-V properties.Second, we use Precision Impedance Analyzer to accurately measure the forwardelectrical characteristics of LEDs. The experimental results showed that, under certainconditions (low frequency and high voltage) all tested LEDs appear obvious negativecapacitance (NC). After carefully analyzing the dependences of apparent capacitance onvoltage and frequency, we quantitatively derived the experience formula of NC varyingwith voltage and frequency. Moreover, apparent conductance of different LEDs hardlyvaries with frequency, but exponentially with voltages. Junction capacitance andconductance are larger than the corresponding apparent parameters, but both junctionand apparent parameters have the same dependences on voltage and frequency;especially, junction capacitance is more than one magnitude of apparent capacitance. Atthe same time we also analyze the junction voltage, the series resistance and theidealistic factor of the light emitting diodes in detail.Third, using basic physical principles, we calculated the electrical characteristics ofone-dimensional symmetry p-n junction. First of all, according to the equivalent circuitpresented by “advanced theory of semiconductor devices” which considers regionalvoltages of p-n junction, we find out the relationship between voltages in different areas.Secondly, combining with carrier continuity equations, Poisson’s equation and boundary conditions, we detailedly calculate the electrical characteristics (including current，conductance and capacitance) of completely symmetric p-n junction and find out thecause of NC. In addition, after investigating all items in the expression of current density,the contributions of combination, diffusion and boundary effect to current, conductanceand capacitance, as well as the main reason of NC were obtained. Finally, we alsodetailedly calculated relationship between the carriers’ recombination lifetime andcapacitance and conductance.Classic Schockley p-n junction theory only includes the diffusion capacitancewhich increases exponentially with voltage at large forward bias; therefore, NCphenomenon experimental results are hard to be explained in this framework. However,when considering all contributions to the current, we can get NC easily, so theexperimental and theoretical results have a very important research value which willcontributes to amend the classic Schockley p-n theory and promoting the development of“advanced theory of semiconductor devices”.