Elsevier

Microelectronics Reliability

Volume 51, Issues 9–11, September–November 2011, Pages 1788-1791
Microelectronics Reliability

Thermal optimization of GaN-on-Si HEMTs with plastic package

https://doi.org/10.1016/j.microrel.2011.06.033Get rights and content

Abstract

In this paper, the degradation of a GaN-on-Si based RF power amplifier is investigated by means of electrical characterization. The reliability issues identified during this work are clearly related to the high thermal resistance between the device and the heat sink, which causes gate-leakage current and output power degradation. Moreover, we have demonstrated a low cost thermal optimization approach by increasing the thermal dissipation area and reducing the device carrier thickness. Measurement results show that the saturated output power can be increased from 1 W up to 5 W without device degradation at 3.8 GHz.

Highlights

► We examine the GaN-on-Si based RF power amplifier degradation. ► Improper thermal contact may cause the PA performance degradation. ► Reducing the heat spread distance can improve the PA performance. ► Increasing the thermal dissipation area can improve the PA performance.

Introduction

AlGaN/GaN based high electron mobility transistors (HEMTs) are promising devices for high power and high frequency applications [1]. Recent research efforts are focused on improving the GaN growth quality on large scale Si substrate as well as the reliability [2], [3]. Since the use of silicon as substrate is economical and well developed in semiconductor industry, the resulting GaN-on-Si HEMTs can offer a significant cost reduction as compared to their counterparts such as sapphire and silicon carbide (SiC) [4]. Up to now, a high power density (>10 W/mm) with high thermal stability at 325 °C has been reported [3]. Moreover, a plastic overmold packaging solution is being used for GaN-on-Si HEMTs, offering >10× cost reduction compared to a typical ceramic packaging solution [5].

However, most of the reported reliability studies on GaN-on-Si HEMTs are mainly done at wafer level [3], [6], [7], [8] and without taking the practical assembling and packaging effects into account. In this paper, we experimentally study the thermal effects on the GaN-on-Si power amplifier (PA) performance degradation by means of DC and RF characterizations. Moreover, we demonstrate a practical thermal optimization approach with a low cost plastic packaged GaN-on-Si PA module to further improve the RF performance.

Section snippets

Failure analysis

The employed GaN-on-Si HEMTs have been grown by metal–organic chemical vapor deposition (MOCVD) on a high resistive silicon substrate. The device die has a dimension of 0.5 mm × 0.6 mm, and is grounded using vias after wafer thinning down to 150 μm [2]. The total gate length is 2 mm. A previous reliability study shows that the processing technology has a MTTF > 107 hours at 150 °C and a high temperature operating life of 1000 h with 28 V drain voltage and 150 °C junction temperature [5]. In contrast to the

Horizontal optimization

In order to address the degradation of LTCC-PA, we employ the above GaN-on-Si HEMTs with a commercially available plastic package PSOP [9]. Fig. 3 illustrates the cross section of the fabricated PA module on a PCB. The device die is attached to a thin Cu flange with silver filled epoxy. In order to enhance the heat spread dissipation efficiently, the Cu flange area is fixed to 3 mm × 2 mm in this package which is around 20 times larger than the device die. Wire bondings are then encapsulated in a

Conclusion

We have investigated a failure mechanism which occurs in low cost GaN-on-Si HEMTs, and which is observed after attaching the HEMTs on LTCC substrate and PCBs. Failure analysis demonstrated that the performance degradation is mainly due to thermal effects. Practical thermal optimization is carried out by increasing the thermal dissipation area within a low cost plastic package, and by reducing the PCB thickness. The measured results at 3.8 GHz show that the maximum output power can be improved by

Acknowledgment

The authors like to thank imec assembly team for their help with die attachment and wirebonding.

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