Normally-off high-voltage GaN transistors with carbon-doped buffer
Fig. 1: Processed 3" wafer with more than 200 normally-off p-GaN gate 45 A / 600 V GaN power transistors.
Fig. 2: Output characteristic for a 0.25 mm wide p-GaN gate transistor with 1000 V breakdown strength. The gate-drain distance is 6 µm. The gate bias is indicated.
Fig. 3: On-state resistance vs. breakdown strength at different gate-drain spacing (color) for the former AlGaN buffer and the new GaN:C buffer. 1000 V was the detection limit.
Normally-off AlGaN/GaN HEMTs are ideal switching transistors in power converters due to their low on-state resistance combined with high breakdown strength. The normally-off p-GaN gate technology developed at FBH combines low leakage currents for the unbiased gate (0 V) in the off-state with high currents and low resistances in the on-state.
For high-voltage GaN transistors blocking up to the kV range a special carbon-doped GaN buffer has been introduced to keep the electrons well confined in the transistor channel made of undoped GaN. Additionally, the carbon-doped GaN buffer is very resistive for any injected electrons and thus prevents parasitic current flow. Recently, the p-GaN gate technology for normally-off transistors and the carbon-doped buffer structures enabling high breakdown devices have been combined and the formerly used AlGaN buffer of the p-GaN gate transistors has been replaced by the new carbon-doped GaN buffer. This resulted in normally-off transistors blocking 1000 V and showing a threshold voltage of 1.1 V. Compared to the former AlGaN buffer, the breakdown strength has more than tripled from 50 V/µm gate-drain distance to 170 V/µm, while the on-state resistance RON maintained on its low level. The obtained VBr-to-RON ratio surpasses the performance of silicon-based devices and ranks among the best normally-off GaN devices world-wide.
Publications:
E. Bahat-Treidel, F. Brunner, O. Hilt, E. Cho, J. Würfl, G. Tränkle, "AlGaN/GaN/GaN:C back-barrier HFETs with breakdown voltage of over 1 kV and low RON×A", IEEE Transactions on Electron Devices, Vol. 57, no. 6, pp. 3050-3058 (2010).
O. Hilt, A. Knauer, F. Brunner, E. Bahat-Treidel, J. Würfl, "Normally-off AlGaN/GaN HFET with p-type GaN Gate and AlGaN Buffer", Proceedings of the 22nd International Symposium on Power Semiconductor Devices (ISPSD), Hiroshima, pp 347-350 (2010).
FBH research: 11.04.2011