Determination of LED equivalent circuits using network analyser measurements

Abstract

We performed measurements on a number of pre-production DH LEDs biased from 1 to 10 mA and, for each case, we obtained a broad-band input impedance plot which gives both magnitude and phase as a function of frequency. We deduced from these plots the carrier lifetime, differential diode resistance, diffusion capacitance, parasitic resistance, and the ideality factor of each diode. The parameters were then used in the equivalent circuit to calculate the modelled input impedance of the device which was then compared with the measured result. There is good agreement over a wide frequency range, at least up to the frequency equal to 1/(carrier lifetime). The existence of a maximum phase lag previously noticed in single-port laser measurements is fully accounted for in our model.