Experimental observation of TDDB-like behavior in reverse-biased green InGaN LEDs

Highlights

• Time-dependent failure of InGaN LEDs submitted to constant reverse bias was observed.

• Leakage current is correlated with reverse-bias electro-luminescence signal.

• The Time-To-Failure has an exponential dependence on stress voltage.

• The TTF was found to be Weibull distributed.

Abstract

This paper reports the outcome of a series of reverse-bias experiments performed on commercial GaN-based green LEDs. The experimental results showed that green LEDs submitted to reverse bias i) show a time-dependent failure when they are submitted to constant (reverse) voltage stress, at a bias point smaller than the BDV; ii) experience an increase of the reverse-bias electro-luminescence signal, well-correlated with the increase of the reverse leakage current; iii) the TTF (Time-To-Failure) related to the time-dependent breakdown process has an exponential dependence on stress voltage; iv) the TTF is Weibull distributed. This work provides the first experimental demonstration of time-dependent failure of GaN LEDs.

Introduction

Time-Dependent Dielectric Breakdown (TDDB) is a well-known statistical phenomenon that affects the reliability of dielectrics in semiconductor devices exposed to high internal electric fields. While extensive studies have proven the physical models and mechanisms involved in the breakdown of Silicon MOS devices [1], more recent observations of time-dependent breakdown in GaN-based devices have not been clarified yet. The classic dielectric breakdown model explains the failure of GaN-based devices with a dielectric, e.g. MIS HEMTs [2]; on the other hand, it does not fit with the Weibull-distributed TTF found for dielectric-free forward-biased p-GaN gate transistors [3] or for reverse-biased Schottky junctions [4].

Since such a TTF distribution is typical for a dielectric degradation-driven breakdown process, the above observations suggest that the failure is rather due to the highly-depleted GaN, which – when submitted to high voltages - may behave like a leaky dielectric.

The aim of this paper is to demonstrate the existence of a time-dependent breakdown process in GaN-based LEDs submitted to reverse bias stress. The observed TTF of the devices was found to be Weibull-distributed, indicating that also in those dielectric-free structures the semiconductor, when exposed to high electric-fields, experiences a time-dependent (dielectric-like) degradation process.

This work also gives a first insight into the reliability issues that LEDs directly driven by AC in driverless lighting solutions may face during their lifetime [5]. This family of inexpensive light engines features two LEDs stripes connected in an anti-parallel configuration. Depending on the polarity of the supply voltage wave, only one of the two stripes is alternatively forward biased during each semi-period of the power line. Since the devices belonging to the reverse-biased stripe may be exposed to high levels of reverse voltage, the effects of such a bias condition need to be investigated. In particular, while it is a well known fact that reverse-bias stress may have a negative impact on the optical performance of the device and on its long-term reliability [6], the bias and time dependencies of this kind of degradation phenomenon have not been deeply analyzed, yet.