Microwave Performance of ‘Buffer-Free’ GaN-on-SiC High Electron Mobility Transistors

High performance microwave GaN-on-SiC HEMTs are demonstrated on a heterostructure without a conventional thick doped buffer. The HEMT is fabricated on a high-quality $0.25~\boldsymbol {\mu }\text{m}$ unintentional doped GaN layer grown directly on a transmorphic epitaxially grown AlN nucleation layer. This approach allows the AlN-nucleation layer to act as a back-barrier, limiting short channel effects and removing buffer leakage. The devices with the ‘buffer-free’ heterostructure show competitive DC and RF characteristics, as benchmarked against the devices made on a commercial Fe-doped epi-wafer. Peak transconductances of 500 mS/mm and a maximum saturated drain current of ~1 A/mm are obtained. An extrinsic $\text{f}_{\sf T}$ of 70 GHz and $\text{f}_{\sf max}$ of 130 GHz are achieved for transistors with a gate length of 100 nm. Pulsed-IV measurements reveal a lower current slump and a smaller knee walkout. The dynamic IV performance translates to an output power of 4.1 W/mm, as measured with active load-pull at 3 GHz. These devices suggest that the ‘buffer-free’ concept may offer an alternative route for high frequency GaN HEMTs with less electron trapping effects.