Design And Simulation Of GaN-based LED's Electrode Structure

At present, the conventional GaN-based light-emitting diodes (LEDs) are fabricated on sapphire substrates by epitaxial growth. The lateral current transport are inevitably exist in the mesa-structure GaN based LEDs, which causes the current to crowd near the edge of the n-contact pad and causes the light output are not uniform.In this paper , the current spreading effect is qualitatively analyzed . The applied current density ,the n-type doping concentration and the effective length of the lateral current transport are found to have a considerable effect on the uniform current spreading. Electrical properties of a LED is simulated with the software of SILVACO,by using two-dimensional finite element method. Three kinds of different sizes of LEDs'current distribution inside the device are compared, and the relationship between the current density and L is found. It is also found that current uniformity can be improved by reducing the current expansion length( L).Surface current spreading plays an important role in the characteristics for the LED device. Top surface currents spreading and the internal current density distribution in three-dimensional models are simulated with finite element analysis software ANSYS. Four different electrode structure types of LEDs with a chip size of 300um×300um are compared and analyzed. It is clearly shown that two types of four have a uniform current distribution.Then, three special shape LEDs with three different electrode structures are designed and simulated. The current distribution is compared and two good structures are choosed to use in commercialization. At last,four different structure LEDs with a chip size of 1mm×1mm are designed and simulated. It is found that reducing the current expansion length( L) is an effective method to get a uniform current distribution. In the same process parameters, using plug-finger electrode structure can decrease the current crowding. When the number of inserted fingers is five, current distribution is most uniform and this electrode structure can be used as high-power LED's electrodes graphics.