The First Wafer-fused AlGaAs-GaAs-GaN Heterojunction Bipolar Transistor
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The First Wafer-fused AlGaAs-GaAs-GaN Heterojunction Bipolar Transistor Sarah Estrada,1 Andreas Stonas, Andrew Huntington, Huili Xing, Larry Coldren, Steven DenBaars, Umesh Mishra, Evelyn Hu Departments of Materials and Electrical & Computer Engineering, University of California, Santa Barbara, California 93106-5050
ABSTRACT We describe the use of wafer fusion to form a heterojunction bipolar transistor (HBT), with an AlGaAs-GaAs emitter-base fused to a GaN collector. In this way, we hope to make use of both the high breakdown voltage of the GaN and the high mobility of the technologically more mature GaAs-based materials. This paper reports the first dc device characteristics of a wafer-fused transistor, and demonstrates the potential of wafer fusion for forming electronically active, lattice-mismatched heterojunctions. Devices utilized a thick base (0.15um) and exhibited limited common-emitter current gain (0.20.5) at an output current density of ~100A/cm2. Devices were operated to VCE greater than 20V, with a low VCE offset (1V). Improvements in both device structure and wafer fusion conditions should provide further improvements in HBT performance. The HBT was wafer-fused at 750oC for one hour. Current-voltage characteristics of wafer-fused pGaAs/n-GaN diodes suggest that the fusion temperature could be reduced to 500oC. Such a reduction in process temperature should mitigate detrimental diffusion effects in future HBTs.
INTRODUCTION The large breakdown field and anticipated saturation velocity of GaN make this novel material particularly promising for high-frequency, high-power devices. With this goal in mind, quite a few researchers are working to develop GaN-based heterojunction bipolar transistors (HBTs).1,2,3,4,5 Although results have been promising, there are still a number of outstanding materials issues. For example, AlGaN/GaN HBTs appear to be limited by large acceptor ionization energies and low hole mobilities.6 An HBT structure utilizing AlGaAs-GaAs for the emitter-base, with GaN as the collector, could potentially combine the high-breakdown voltage of GaN with the high mobility of the technologically mature AlGaAs-GaAs heterostructure. Because the high degree of lattice mismatch between GaAs (lattice constant of 5.65A) and GaN (3.19A) precludes an all-epitaxial formation of this device, we have formed the GaAs-GaN heterostructure via the novel technique of wafer fusion, also called direct wafer bonding. The HBT demonstrates a modest current output and a current gain less than unity; however, the common-emitter I-V characteristic and Gummel plot are promising for the first wafer-fused HBT. Optimization of device structure and fusion conditions should improve electrical performance.
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[email protected], http://sarah.optimism.us/engineering/
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(a)
(b)
~ 2 µm nGaN (1018 Si)
1 µm pGaAs (5x1018 C) 0.5 µm AlAs etch-stop layer 0.3
(001) sapphire substrate
(100) n+GaAs substrate
Figure 1. Starting materials for the diode structures: (a) the n-GaN structure grown by MO
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