I. Introduction
The improving performances of light-emitting-diode (LED)-based application and the worldwide policy on environment care are the main factors involved in the impressive growth of gallium-nitride-based LEDs' market. In the last few years, great efforts have been spent in the improvements of materials and structure in terms of process and reliability. Novel technologies for low defect densities in active region allow a better efficiency, and an efficacy of 110 lm/W at 350 mA has been demonstrated [1]. In addition, the recent development of technologies for phosphor deposition, together with new alloy configurations, allows the design of LEDs with color render index of up to 90% [2]. Finally, the development of processes like flip-chip technology has permitted a better heat dissipation and an increasing external efficiency for chips grown on sapphire.