Structure optimization of multi-finger power SiGe HBTs for thermal stability improvement

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Abstract

The two-dimensional temperature profile of a power SiGe HBT with traditional uniform emitter finger spacing is calculated, which shows that there is a higher temperature in the central region of the device. With the aid of the theoretical analysis, an optimized structure of the HBT with non-uniform emitter finger spacing is presented. The peak temperature is lowered by 23.82 K, and the thermal resistance is also improved by 15.09% compared with that of the uniform one. The improvements above are ascribed to the increasing the spacing between fingers, and hence suppressing the heat flow from adjacent fingers to the center finger. Based on the analytical results, two types of HBTs with uniform emitter finger spacing and non-uniform emitter finger spacing are fabricated and their temperature profiles and thermal resistance are measured. The measured results agree well with the calculated results, verifying the accuracy of the calculations. For the HBT with non-uniform emitter finger spacing, the peak temperature and the thermal resistance are improved markedly over a wide biasing range compared with that of the uniform one. Therefore, both the calculated results and the experimental results verify that the optimized structure of power HBT with non-uniform emitter finger spacing is superior to the uniform emitter spacing structure for enhancing the thermal stability of power devices over a wide biasing range.

Introduction

In recent years, SiGe HBTs are becoming of great interest for high power applications due to high thermal conductivity of the Si substrate and their technology compatibility with established silicon technology [1], [2]. However, the thermal effects have been shown to significantly affect HBTs’ performance, which are particularly important for multi-finger HBTs where the self-heating in each emitter finger and the thermal coupling among the fingers lead to a non-uniform temperature profile. Because of the positive temperature coefficient of emitter current, the non-uniform temperature profile will leads to a non-uniform current distribution and consequently the non-uniformity of the temperature profile increases considerably, which eventually gives rise to the thermal instability and possibly permanent device damage.

In order to improve the non-uniformity of the temperature profile, the technology of non-uniform emitter finger spacing in GaAs HBTs was studied and the simulated temperature profiles is presented assumed that the temperature in a given finger is uniform and each finger has only one heat-source cell [3], [4]. In fact, the temperature profile in each finger is non-uniform and experimental results by Gao and Wuchen [5] shows the non-uniformity of temperature profile in a transistor with one emitter finger. In this paper, the temperature profile of a 20-finger power SiGe HBT with non-uniform emitter finger spacing is simulated which is considered the non-uniformity of the temperature in a given finger and each finger is subdivided into several heat-source cells to account for the temperature variation. Furthermore, the simulated temperature profiles are verified using experimental results measured by infrared micro imager. Both the simulated results and experimental results exhibit that, the optimized structure of non-uniform emitter finger spacing is very effective for improving the non-uniformity of the temperature, and hence enhancing the power handling capability and the thermal stability of power HBTs.

Section snippets

Thermal model and analysis

Fig. 1 shows the three-dimensional structure of a 20-finger SiGe HBT in the study, where L, W, d are the chip sizes in the directions of x, y, z, respectively. The emitter finger length and width are defined by l and w, respectively. The SiGe HBT in this work is interdigitated and composed of 20 emitter stripes with area of 3 × 60 μm2 for each stripe. Considered that the temperature is non-uniform in a given emitter finger, each one is divided into Mk × Nk (20 × 1) heat-source cells, and hence, each

Fabrication and results

Based on the theoretical analysis mentioned above, two types of 20-finger power SiGe HBTs are fabricated. The schematic cross section of device fabrication in double-mesa process is shown in Fig. 3, where the specifications of all epitaxial layers are given in Table 2. For more details of the SiGe HBT technology, the reader can be referred to the previously publication [10]. Measurement results show that both of them have the similar electrical performance.

The temperature profiles of two types

Conclusions

Two-dimensional temperature profiles of two types of power SiGe HBTs with uniform emitter finger spacing and non-uniform emitter finger spacing (optimum structure) are calculated, which are considered the temperature difference in a given finger, not assumed that the temperature is uniform given in the published papers. The calculated results show that HBT with non-uniform emitter finger spacing could lower the peak temperature and improve the non-uniformity of the temperature profiles

Acknowledgments

We would like to express special thanks to the anonymous reviewers whose precious suggestions, qualified comments, and corrections contributed to the quality of this work. This work was supported in part by National Natural Science Foundation of China under Grant Nos. 60776051 and 60376033, Beijing Municipal Natural Science Foundation, China under Grant No. 4082007, State 973 project, Beijing Municipal Education Committee, China under Grant No. KM200710005015, Beijing Municipal trans-century

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