GaN Power HEMT with Breakdown Voltage >800 V Grown by MBE

Abstract—We report on a GaN power HEMT with a breakdown voltage exceeding 800 V, grown by MBE on sapphire. The high breakdown voltage, surpassing 800 V, is achieved through the use of an ultra-thin AlN buffer layer grown by MBE and the implementation of a single-level gate field plate technique. The MBE technique facilitates the growth of a dopant-free buffer layer and pseudomorphic epilayers, which is advantageous for the high reliability of GaN power HEMTs. Additionally, the single-level gate field plate technique aids in reducing the overall fabrication cost and also enhances the reliability of GaN power HEMTs. These merits highlight the potential of the reported GaN power HEMT for applications in high-power electronics.

Source/drain ohmic contacts with a typical contact resistance of 1.2 Ω·mm were achieved using Ti/Al/Ni/Au (20/120/30/50 nm) deposited by e-beam evaporation and rapid thermal annealing at 800 °C in N2 for 30 seconds. Subsequently, 150 nm SiN was deposited onto the wafer using inductively coupled plasma chemical vapor deposition (ICP-CVD). The gate window (GS) was opened in the 150 nm SiN layer, with a recess depth of ~120 nm, leaving ~30 nm of SiN as the gate insulator. Ti/Al/Ti (30/500/30 nm) was deposited to serve as the gate field plate metal (GT). The gate GS and the gate field plate metal GT together formed a single-level gate field plate structure to achieve a high breakdown voltage. Following this, 1 µm SiN was deposited by ICP-CVD, and via holes (V0) were created using ICP-RIE. Interconnection M1 to the source/drain ohmic contacts was established using Ti/Al/Ti (30/1000/30 nm) through the via holes V0. Finally, 2 µm SiN was deposited by ICP-CVD as the top passivation layer. The fabricated GaN HEMT features a gate length (Lg) of ~2 µm, a gate field plate length of ~9 µm, a gate width of ~100 µm, a gate-source spacing (Lgs) of ~3.5 µm, and a gate-drain spacing (Lgd) of ~22 µm.

The characterization of device mesa isolation was conducted on two mesa isolation squares with a separation of ~10 µm and widths of ~100 µm. Fig. 2 illustrates the mesa isolation characteristics with a two-terminal bias ranging from 0 to 1100 V. The leakage current is <2×10⁻⁷ A, demonstrating the excellent insulating properties of the 1 µm thin AlN buffer layer grown on sapphire using MBE. Such superior insulating behavior is essential for ensuring sufficiently high breakdown voltages. Figs. 3 and 4 depict the typical DC output and transfer characteristics of the fabricated GaN HEMTs.

Fig. 5 Breakdown properties of GaN power HEMT grown by MBE

IV. CONCLUSION

A GaN power HEMT with a breakdown voltage greater than 800 V, grown by MBE on sapphire, has been reported. A breakdown voltage exceeding 800 V was achieved by employing an ultra-thin AlN buffer layer grown by MBE and utilizing a single-level gate field plate technique. The MBE growth process facilitates the creation of a dopant-free AlN buffer layer, which enhances the high reliability of the GaN power HEMT. The single-level gate field plate technique leads to decreased fabrication costs and potentially improves the reliability of the GaN power HEMT. These advantages demonstrate the potential of the fabricated GaN HEMT for applications in high-power electronics at a low cost. Future work will focus on reducing dislocations to demonstrate the breakdown properties of large devices and to decrease ON-resistance by optimizing the compositions of the back barrier and top barrier, as well as the channel thickness.

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