Characteristics Of Blue Light-emitting Diodes Research Articles

LED Modulation Characteristics in a Visible-Light Communication System

This paper conducts a research on modulation characteristics of blue light-emitting diodes (LEDs) used in a visible-light communication (VLC) system. Through analysis of the modulation characteristics of LEDs with different emitting sizes, we find that there is a similar linear relationship between LED’s 3dB bandwidth and the operation current density. This experiment also shows that high series resistance is one major issue that limits our LED's modulation speed. To further improve the LED bandwidth, the resistance can be reduced by optimizing device layout as well as reducing material bulk resistance. Clearly, this study provides an approach to increase the modulation bandwidth of GaN-based LEDs for VLC systems.

Influence of a GaN interfacial layer between n+-GaN and active layer on the characteristics of blue light-emitting diodes

Influence of a GaN interfacial layer with graded growth rate and modulated Si-doping between n+-GaN and InGaN/GaN multiquantum well on device performance of blue light-emitting diodes (LEDs) was investigated. It was found that the introduction of a GaN interfacial layer leads to an improvement of current–voltage characteristics and also an enhancement of output power compared to a conventional LED. These could result from the removal of spiral growth hillocks and smoothened morphology at the interface, as confirmed by atomic force microscopy measurements, which might cause the leakage current to decrease and the current injection into the active layer to enhance.

A laboratory experiment with blue light-emitting diodes

We present a laboratory experiment designed to show the main characteristics of blue light-emitting diodes. The analyzed devices are based on III–V nitrides (in which the active layer is a direct band gap In0.06Ga0.94N alloy) and SiC (where the active layer is the 6H polytype of this semiconductor, which is of an indirect band gap class). From the measurements, an In relation could be established between light emission and device current, with n values related to both the physical structure and electroluminescence characteristics of these semiconductors. The spectral response measurements allow us to obtain the peak wavelength of emission with reasonable precision (449±4 nm in the former one and 470±4 nm in the last one). It is also possible to accurately obtain the band gap value of the active layer in the device based on III–V nitrides (3.18±0.03 eV) by analyzing its electroluminescence spectra. However, the same analysis is not possible in the SiC-based device due to its extremely low measured luminous intensity values. A comparison of power conversion efficiency of both devices is also given.

Influence of defects on the electrical and optical characteristics of blue light-emitting diodes based on III–V nitrides

We have measured the electrical and optical properties of blue light-emitting diodes (LEDs) based on III–V nitrides. The current–voltage characteristic is described by means of the relation I=I0 exp(αV). In this equation α is temperature independent, suggesting a process of conduction by tunneling, as was recently reported also for blue-green LEDs based on III–V nitrides [Appl. Phys. Lett. 68, 2867 (1996)]. We explain the differences between blue and blue-green devices taking into account the tunneling process across semiconductor interfaces, in which a great number of defects is present. The light output intensity of the LED as a function of junction–voltage data reveals a dependence on the junction–voltage of the type L=L0 exp(qV/1.4 KT), indicating that the radiative recombination path is via deep levels located at the forbidden gap. Furthermore, we find that the light output–current characteristic follows a power law like L∝Ip. From the analysis of data it appears that, contrary to expectations, the nonradiative centers are saturated at very low current values that are comparable to the values at which this saturation takes place in LEDs based on III–V arsenides with a low content of defects.