High Quality GaN Substrates for Modern LED Technology

Figure 3: AFM data illustrating the NI density for a) sample A and b) sample B. The overgrowth of sparse NIs can result in formation of new islands (c). Using increased reactor pressure the formation of new islands during HT overgrowth can be suppressed (d). All the scans are 10 μm × 10 μm. It is well known that modern III-nitride based technologies for light emitting diode (LED) production include (as an initial inevitable step) the preparation of GaN templates. Such GaN templates are usually delivered on foreign substrates possessing high lattice mismatch with respect to GaN layers, e.g., sapphire or silicon carbide substrates. The growth of GaN layers on highly mismatched substrates occurs via the formation of 3D islands with their following coalescence [1]. Such growth mode results in an extremely high density of threading dislocations (TDs) up to 1010–1011 cm–2 in the layer interior [1]. These TDs are known as main sources for the deleterious performance of LEDs and other GaN based electronic and optoelectronic devices [2]. Therefore there exists an ultimate necessity in the developing of effective and low-cost recipes for low TD density GaN template manufacturing. Read more »


Measurement of LEDs

Figure 5: NIST gonio-spectroradiometer for the total spectral radiant flux scale realization Various new types of light emitting diodes (LEDs) are being developed and introduced for general illumination and other applications, and there are increasing needs for accurate measurements of various optical parameters of LEDs. Traditional standard lamps do not meet the calibration needs for LED measurements as LEDs differ substantially from traditional lamps in terms of physical sizes, flux levels, spectra, and spatial intensity distributions. Read more »


Off-Line LED Control Circuit

Figure 1: Fluorescent dimming ballast block diagram Resonant mode topologies offer many benefits over traditional buck, boost and flyback solutions. These include soft-switching, higher operating frequencies, higher power density and higher efficiency. Electronic ballast designs for fluorescent lighting applications have already been taking advantage of these benefits for many decades. Much can be learned from electronic ballast circuits and applied to LED driver circuits. This article compares the load requirements for fluorescent lamps and LEDs, explains the functionality of a new dimming electronic ballast control IC, and describes a new resonant mode control circuit for LEDs that uses the new IC. Experimental results of the new circuit are also presented and summarized to show final performance. Read more »


Primary and Secondary Optic Materials - LED Luminaire Performance and Lifetime

Figure 1: LumiBright LE line of light engines for commercial applications and incorporating high efficiency primary optics, photosensor, and thermistor on a high thermal conductivity metal core PCB in one package. Numerous articles have been published regarding the lifetime of Light Emitting Diode (LED) luminaires, however, the primary focus has typically been on the lifetime of the LED die itself. Given that the LED luminaire is a system, it is important to recognize all aspects of the system that can affect or limit lifetime. Read more »