ESD characterization of multi-chip RGB LEDs S. Vaccari a , M. Meneghini a,⇑ , A. Griffoni b , D. Barbisan a , M. Barbato a , S. Carraro a , M. La Grassa a , G. Meneghesso a , E. Zanoni a a University of Padova, Department of Information Engineering, via Gradenigo 6/B, 35131 Padova, Italy b OSRAM SpA, via Castagnole 65, 31100 Treviso, Italy article info Article history: Received 24 May 2013 Received in revised form 29 June 2013 Accepted 12 July 2013 abstract In this paper we present an extensive analysis of the failure mechanisms of RGB (multi-chip) LEDs sub- mitted to ESD testing: the tests have been carried out on several commercially available LEDs of three different suppliers. In order to better understand the failure mechanisms, we have submitted LEDs to ESD tests under reverse and forward bias condition separately, by means of a Transmission Line Pulser (TLP). The experimental results indicate that: (i) red LEDs (based on AlInGaP) have an higher ESD robustness with respect to green and blue samples (based on InGaN), both under reverse and under forward bias test; (ii) TLP negative pulses with a current smaller than the failure threshold can induce a decrease of the leakage current in GaN-based LEDs, due to a partial annihilation of defective paths responsible for reverse conduction; (iii) typical failure mechanism of devices is represented by a catastrophic event, with short-circuiting of the junction. Moreover, some of the analyzed red LEDs had shown ‘‘soft’’ failure, with gradual increase of the leakage current and corresponding decrease of the optical power, even without a catastrophic damage. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Over the last decade, many efforts have been done by several research groups and companies in order to increase the perfor- mances of Light Emitting Diodes (LEDs). As result, LEDs with lumi- nous efficacy in excess of 100 lm/W have been recently demonstrated [1], thus clearing the way for a massive penetration of these devices in the lighting market. Despite this, GaN-based LEDs, employed for the realization of blue, green and white sources, still suffer by reliability issues, that limit their lifetime. In particular, several factors lead to a gradual degradation of opti- cal and electrical properties of these devices, such as increase in the nonradiative recombination rate induced by constant current stress, degradation of ohmic contacts due to high-temperature stress and degradation of optical properties of packages and phos- phors in white LEDs for high temperature operation [2–5]. Another reliability issue of LEDs is represented by Electrostatic Discharge (ESD) events: differently from degradation mechanisms mentioned above, ESD can induce the catastrophic failure of de- vices. Over the last years, different solutions have been proposed in order to improve the ESD robustness of LEDs, including: employ- ment of internal inverse-parallel protection diodes [6–9] or CMOS protection circuits [10], introduction of an Al film with an air gap on the bottom side of sapphire substrate [11] or a n À -GaN layer [12], realization of flip-chip structures with Metal–Oxide–Silicon submount [13] or insertion of a floating ring near the n-electrode [14]. Despite the importance of this topic, only few works can be found in literature concerning the investigation of physical mech- anisms that limit the ESD robustness of LEDs [15–20]. The purpose of this work is to reach a better understanding of the physical mechanisms responsible for the failure of RGB LEDs. To do this, for the first time we propose the results of ESD testing carried out on RGB devices of three different suppliers, describing a comparison between two different technologies: AlInGaP-based red LEDs and GaN-based green and blue devices. The analysis indi- cates that: (i) robustness of red LEDs (based on AlInGaP) is higher with respect to green and blue samples (based on InGaN alloys), both under reverse-bias and forward-bias ESD events; (ii) most of tested red devices do not fail under forward-bias test; (iii) ESD negative pulses with low-moderate current levels can induce a de- crease in the leakage current of green and blue LEDs, consequent to the annihilation of some of the defective paths responsible for re- verse-current conduction; (iv) some of the tested red samples un- der reverse bias show a no-catastrophic failure, resulting in a gradual worsening of electrical and optical characteristics. 2. Experimental details The analysis was carried out on commercially available RGB de- vices from three different suppliers, identified as ‘A’, ‘B’ and ‘C’ in the following. Each sample include a AlInGaP-based red LED with 0026-2714/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.microrel.2013.07.049 ⇑ Corresponding author. Tel.: +39 049 8277664; fax: +39 049 8277699. E-mail address: matteo.meneghini@dei.unipd.it (M. Meneghini). Microelectronics Reliability 53 (2013) 1510–1513 Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel