By Josh Perry, Editor
[email protected]

Researchers from SweGaN AB, Chalmers University of Technology, and Linkoping University, all in Sweden, have developed thinner gallium-nitride (GaN) structures on silicon carbide (SiC), which could lead to high-power and high-frequency, high-electron-mobility transistors (T-HEMT) and other devices.

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According to a report from Semiconductor Today, “Rather than having a ~1-2μm-thick gallium nitride (GaN) buffer layer, the new structure uses a high-quality 60nm grain-boundary-free aluminium nitride (AlN) nucleation layer to avoid extended defects over large areas. The nucleation layer allows high-quality GaN to be grown within 0.2μm.”

The buffer layer is used to avoid errors from the mismatch between GaN and SiC but thicker layers can create problems for high-power devices. Researchers previously doped buffer layers with carbon or iron to increase resistance and confine the flow of a current in the channels, but doping also impacted performance.

“The III-nitride material was grown on silicon-face 4H silicon carbide,” the article continued. “Hot-wall metal-organic chemical vapor deposition (MOCVD) was used to create epitaxial structures with 60nm AlN nucleation, a 200nm GaN channel, an AlN interlayer of up to 1.5nm, a 10-14nm AlGaN barrier (~30% Al), and a 2nm GaN cap. The 60nm AlN was produced using a low thermal-boundary-resistance (low-TBR) technique enabled by the hot-wall growth.”

The article added, “The device achieved a high on-current density of 1.1A/mm with low normalized on-resistance of 1.3Ω-mm (Figure 2). The saturation current was maintained up to 30V drain bias. With 10V drain bias, the pinch-off was sharp, with transconductance reaching 500mS/mm. The subthreshold swing depended on gate length: 250mV/decade for 0.1μm and 130mV/decade for 0.2μm. The breakdown voltages were 70V and 140V for the 0.1μm and 0.2μm gates, respectively.”

The research was recently published in Applied Physics Letters. The abstract read:

“We demonstrate that 3.5% in-plane lattice mismatch between GaN (0001) epitaxial layers and SiC (0001) substrates can be accommodated without triggering extended defects over large areas using a grain-boundary-free AlN nucleation layer (NL). Defect formation in the initial epitaxial growth phase is thus significantly alleviated, confirmed by various characterization techniques.

“As a result, a high-quality 0.2-μm thin GaN layer can be grown on the AlN NL and directly serve as a channel layer in power devices, like high electron mobility transistors (HEMTs). The channel electrons exhibit a state-of-the-art mobility of >2000 cm²/V-s, in the AlGaN/GaN heterostructures without a conventional thick C- or Fe-doped buffer layer.

“The highly scaled transistor processed on the heterostructure with a nearly perfect GaN–SiC interface shows excellent DC and microwave performances. A peak RF power density of 5.8 W/mm was obtained at V_DSQ = 40 V and a fundamental frequency of 30 GHz.

“Moreover, an unpassivated 0.2-μm GaN/AlN/SiC stack shows lateral and vertical breakdowns at 1.5 kV. Perfecting the GaN–SiC interface enables a GaN–SiC hybrid material that combines the high-electron-velocity thin GaN with the high-breakdown bulk SiC, which promises further advances in a wide spectrum of high-frequency and power electronics.”