Reliability and degradation mechanism of AlGaN/GaN HEMTs for next generation mobile communication systems

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Abstract

Excellent reliability performance of AlGaN/GaN HEMTs on SiC substrates for next generation mobile communication systems has been demonstrated using DC and RF stress tests on 8 × 60 μm wide and 0.5 μm long AlGaN/GaN HEMTs at a drain voltage of Vd = 50 V. Drain current recovery measurements after stress indicate that the degradation is partly caused by slow traps generated in the SiN passivation or in the HEMT epitaxial layers. The traps in the SiN passivation layer were characterized using high and low frequency capacitance–voltage (CV) measurements of MIS test structures on thick lightly doped GaN layers.

Introduction

The reliability and degradation mechanism of AlGaN/GaN HEMTs for applications in future high-efficiency base station systems for next generation mobile communication has been investigated by long term DC and RF stress tests. The major degradation mechanism of AlGaN/GaN HEMTs is caused by hot electron induced charge trapping at the surface, the AlGaN barrier or in the bulk [1]. This degradation mode leads to an increase of dispersion of the pulsed measured output characteristics and a decrease of the maximum transconductance and maximum saturation drain current [2]. Another very important degradation mechanism, especially under pinch-off and low drain current conditions, is the inverse piezoelectric effect in the gate drain region [3]. Due to the high vertical electric field at the drain side, the inverse piezoelectric effect leads to an increase of the gate leakage current and an increase of the drain resistance by trap generation in the AlGaN barrier [4]. In order to study the degradation mechanism of our devices the gate leakage current, the input and the pulsed measured output characteristics were measured before and after stress. In addition the traps of the SiN passivation were characterized by comparing low and high frequency capacitance–voltage measurements of MIS test structures on thick lightly doped GaN layers with theoretical curves.

Section snippets

Experiment

The devices used in this work were grown by MOCVD on 3 in semi-insulating 4H–SiC(0 0 0 1) substrates. The epitaxial growth starts with a thin AlN layer, followed by a GaN buffer layer, the AlGaN barrier layer and a thin GaN cap layer. The AlGaN/GaN HEMT devices have Ni/Au gates defined by e-beam lithography and were passivated using PECVD SiN. The processing technology has been described by Waltereit et al. [5]. After processing the HEMT devices were diced and attached to copper heat sinks using an

DC and RF stress test results

Eight packaged devices from two different lots were stressed at a constant case temperature of 25 °C which results to a channel temperature of approximately 90 °C at Vd = 50 V. The gate voltage was adjusted to obtain an initial drain current density of 50 mA/mm, which is a typical class AB operation point for base station applications Fig. 1 shows the relative change of drain current and the absolute gate current during DC stress for two different lots. The degradation is proportional to the

Degradation mechanism

Fig. 3 shows the transfer characteristic before and after DC stress. The threshold voltage shift is approximately 100 mV and the decrease of the maximum transconductance is about −5%. The change of the drain resistance is less than 5%. The positive threshold voltage shift is possibly caused by electron trapping below the gate contact. Fig. 4 shows a decrease of the Gate diode leakage current after stress, which is not consistent with the inverse piezoelectric field effect [4]. Minor differences

Traps in SiN passivation

In order to obtain devices with high efficiency and good long term stability, it is important to reduce carrier trapping in the SiN passivation and at the SiN/GaN interface. For the electrical characterization of traps in the SiN passivation layer, MIS capacitors on thick lightly doped GaN without two dimensional electron gas are required, since too high leakage currents were observed when MIS capacitors on AlGaN/GaN HEMT structures are forced into accumulation. Therefore, MIS capacitors on 2 μm

Conclusion and outlook

Very good long term stability under DC and RF stress conditions at Vd = 50 V has been demonstrated. The extrapolated drain current degradation during DC stress at Vd = 50 V is less than 20% after 20 years of operation. The drain current degradation is probably caused by hot electron induced slow trap generation at the AlGaN/GaN interface or at the SiN/GaN cap interface. Drain current recovery experiments after stress and the positive threshold voltage shift indicate that the degradation is partly

Acknowledgements

The authors would like to thank A. Michailov for packaging of the devices and H. Konstanzer for setting up the DC and RF stress test experiment. The authors also acknowledge the financial support of NXP, the German Ministry of Education and Research (BMBF) and the German Ministry of Defence (BMVG).

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