GaN-based hetero-junction bipolar transistors (HBTs) are suitable for power amplifiers because they have inherent advantages such as high current and power density, high linearity and normally off operation. However, low conductivity in the p-type base layer and plasma-induced dry-etching damage slow down the improvement of GaN HBTs. Although GaN-based HBTs demonstrated large current gain (β) and high current and power density by using p-type InGaN base [1], N₂-incorporation dry-etching process, wet repairing and regrown base contact layer [2,3], etching damage also exists, which limits the maximum oscillation frequency (f_max) [4].

This essay reports an AlGaN/GaN heterojunction bipolar transistor (HBT) on sapphire with selective-area regrown (SAG) n-AlGaN emitter by metal-organic chemical vapor deposition (MOCVD). 1-μm n+-GaN, 0.5-μm undoped-GaN and 100-nm p-GaN are grown on sapphire in sequence by MOCVD. Then a 100-nm thick SiO₂ mask layer is deposited on the wafer and patterned for SAG emitter. Then the wafer is reloaded to reactor to grow an n-AlGaN emitter layer at 980 ℃ and 50 torr.

Hall measurements performed on p-GaN base layer indicate a hole concentration of 9.8×10¹⁷ /cm³ and mobility of 14.1 cm²/V.s. The SAG n-AlGaN emitter is 100-nm thick with the Al component of 7%. The electron concentration and mobility of the SAG n-AlGaN layer are 1.6×10¹⁸ /cm³ and 290 cm²/V s. Ohmic contact is achieved on the base contact with the calculated sheet resistance R_s of 238 KΩ/sq, and the specific contact resistance ρ_c of 2×10^-3 Ω.cm². As a result, the fabricated HBTs exhibit high β of 110, which is highest among reported values on GaN-based HBTs on sapphire substrates. The common-emitter I-V characteristics show a high J _C of 22 kA/cm² and power density of 220 kW/cm² with a low V_offset <1V and V_knee<5V. These voltage are the lowest among reported values on GaN-based HBTs with comparable emitter areas, owing to the realization of base ohmic contact that decreases base resistance. The open-base breakdown voltage is 98 V with the leakage current of 1 μA, as a consequence the breakdown field is about 2.2 MV/cm. we also demonstrated RF performance of GaN HBTs with f_T of 4 GHz at the collector voltage of 9 V, although the chip size is large. It is expected that high RF performance can be achieved by down scaling the device layout.

These results for GaN HBTs/sapphire show that high performance III-N HBTs can be achieved with regrowth technology. In this talk, we will demonstrate that a large performance improvement can be obtained for RF HBT devices by the use of regrowth emitter. The current gain is increased with the emitter area decreasing. It is favorable for RF HBT devices, which are expected to achieve higher current gain, benefiting from small recombination current. By down scaling the device layout, an RF HBT is expected to be used in PA in the next generation communication.

References

[1] T. Makimoto, K. Kumakura, and N. Kobayashi, Jpn. J. Appl. Phys., 43 (4B), 1922, (2004).

[2] H. Xing, P. M. Chavarkar, S. Keller, et. al., IEEE Electron Device Lett., 24 (3), 141–143, (2003).

[3] Y.-C. Lee et al., IEEE Trans. Electron Devices, vol. 57(11), 2964–2969, (2010).

[4] R. Dupuis, H. J. Kim, Y.-C. Lee, Z. Lochner, et. al., ECS Trans. 58(4), 261-267 (2013).