GaN HEMTs have a great potential to be well suited for the forthcoming 5G communication infrastructures and other millimeter wave applications, where high-power and high-efficient devices are required. For GaN HEMT heterostructures used in high-frequency applications (> 8 GHz), semi-insulating (SI) SiC is the best choice of substrates, owning to its high thermal conductivity and high resistivity, and the maturity in large-wafer mass production (up to 150 mm). Nevertheless, the typical thickness of a GaN layer grown on SiC substrates has to be at least 1.5~2.0 µm to obtain a structural quality manufacturable for device fabrication, due to the large in-plane lattice mismatch with GaN (3.4%). As a result, the thick GaN layer must be doped with acceptor-like impurities like carbon or iron [1] during the epitaxial growth to increase the resistivity, preventing a parallel conduction in the device. However, these impurities introduce deep traps, which capture electrons during the RF operation, rendering a depletion of the channel electrons thus reducing the device current/power density.

We report the realization of a revolutionary heterostructure of III-nitride materials grown on SiC substrates by a unique MOCVD method. Our success in the material growth enabled a new design of HEMT heterostructures requiring no intentional Fe- or C-doping. Excellent DC and RF performances of the highly-scaled GaN-based high electron mobility transistors (HEMTs) using the proposed new heterostructures are demonstrated. The new HEMT heterostructures also open up new opportunities in material growth schemes to further reduce the charge trapping effects that have been prevailing in the field over that last two decades. Consequently, a high RF output power of ~4 W/mm was obtained at a fundamental frequency of 30 GHz in the HEMT devices with a gate length (Lg) of 0.1 µm, biased at V_DS= 30 volt. We also found that the thermal resistance of the new HEMT devices exhibits a substantially lower thermal resistance (R_TH) of 4.1 ^oC mm/W, as compared with that of the conventional ones, which is typically around 9~10 ^oC mm/W [2] The details of the new heterostructures and device characteristics will be presented. Our breakthrough in the material growth has brought the GaN HEMT technology to a new height and will serve as a new route for further improvement.

IV.References

[1] J.-T. Chen et al, Appl. Phys. Lett. 102, 193506 (2013). Impact of residual carbon on two-dimensional electron gas properties in AlGaN/GaN heterostructure.

[2] J. Joh et al, IEEE Trans. Elec. Dev. 56, 2896 (2009) Measurement of Channel Temperature in GaN High-Electron Mobility Transistors.