1/f Noise in GaN HEMTs grown under Ga-rich, N-rich, and NH3-rich conditions

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Abstract

The magnitude of the low-frequency 1/f noise in GaN/AlGaN HEMTs grown under Ga-rich, N-rich, and NH3-rich conditions varies in response to hot-electron stress. Density-functional-theory (DFT) calculations show that the Ga vacancies that are responsible for the positive shift in pinch-off voltage due to electrical stress in Ga-rich and N-rich devices do not contribute significantly to the observed changes in 1/f noise with electrical stress. The N anti-sites that cause negative shifts in pinch-off voltage in ammonia-rich devices can cause an increase in the noise magnitude after stress. DFT calculations also show that singly hydrogenated and dehydrogenated Ga–N divacancies also can contribute to the noise before and after stress, respectively. A decrease in noise magnitude is also observed in some devices after stress.

Introduction

Low-frequency 1/f noise is a valuable tool in the reliability analysis of semiconductor devices [1], [2], [3], [4], [5]. Here we use 1/f noise to analyze hot-electron-induced degradation mechanisms in GaN/AlGaN high electron mobility transistors (HEMTs) that are grown by molecular beam epitaxy (MBE) under gallium-rich, nitrogen-rich, and ammonia-rich conditions. Gallium-rich MBE is used to ensure low surface roughness, while a nitrogen-rich environment results in low threading dislocation density [6], [7]. Ammonia-rich MBE growth, where ammonia is used as the nitrogen source, allows a faster growth rate and a larger growth window. Defect densities in GaN buffer layers can be reduced in ammonia-rich MBE since higher temperatures can be used because of the metal-free surface [8], [9].

We have performed low-frequency 1/f noise measurements on GaN/AlGaN HEMTs before and after the application of electrical stress. The gate voltage for noise measurements is chosen so that the noise originates from the channel. The difference in noise after stress is attributed to pre-existing defects that are activated in the channel by hot electrons. Density-functional-theory calculations of formation energies of native defects are used in combination with the noise measurements to understand the degradation mechanisms of the GaN/AlGaN HEMTs.

Section snippets

Experiments

AlGaN/GaN HEMTs were fabricated on AlGaN/GaN heterostructure layers grown by molecular beam epitaxy (MBE) on SiC substrates at the University of California, Santa Barbara. The MBE growth was performed under (i) Ga-rich, (ii) N-rich, and (iii) NH3-rich conditions. The gate length of these samples is 0.7 μm; LGD = 1.2 μm, and LGS = 0.7 μm. The devices are 150 μm wide. Fig. 1 shows the structure of one such HEMT. The devices were all electrically stressed with a drain voltage of 20 V and a gate voltage of

Gate voltage dependence of Svd

Low frequency noise of HEMTs originates in both the gated (G) and ungated (U) portions of the channel, as indicated in Fig. 1. The channel resistance Rtotal comprises the resistance of the gated and the ungated portions (access regions) [10], [11]:Rtotal=RG+RU=Lgate|Voff|Wqμnch|VGS-Voff|+RUHere μ is the channel mobility, and nch is the carrier density. From the empirical relationship given in [12], we have:SVV2=SII2=SRR2=αfNHere Sv is the noise power spectral density, N is the total number of

Conclusions

Noise originating in the channel increases after stress for some devices grown under Ga-rich, N-rich, and NH3-rich conditions, but decreases in others because of the dehydrogenation of different defects present in the AlGaN layer. The increase in 1/f noise with increasing leakage current confirms that defects in the AlGaN barrier contribute to the noise. The positive shift in pinch-off voltage in Ga-rich and N-rich devices after stress is caused by the dehydrogenation of hydrogenated Ga

Acknowledgements

This work was supported in part by the Office of Naval Research DRIFT MURI Grant N-00014-08-100655 and the McMinn Endowment at Vanderbilt University.

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