Low-resistance graded AlGaN contact to high Al-content AlGaN PolFET grown by ammonia molecular beam epitaxy

To realize ultra-wide bandgap AlGaN transistors for high-frequency, high power applications, it is imperative to minimize the access resistance to the conductive, high-mobility channel to achieve high-frequency operation. The relatively low electron affinity of AlGaN presents serious challenges for forming ohmic contacts. In this work, we report on low-resistance, compositionally graded AlGaN contacts to Al0.75Ga0.25N in a polarization-doped field effect transistor (PolFET). The samples were continuously grown by ammonia molecular beam epitaxy. The resistances of Si-doped graded contacts are compared with standard alloyed vanadium-based contacts on identical graded PolFET channels. By optimizing the donor density in the graded contacts to counteract the negative volume polarization charge induced by the compositional grade, we achieve one of the lowest reported specific contact resistances to high-Al-content AlGaN, ρc = 7.2 × 10−7 Ω-cm2. This result enables improved device performance by increasing drain current and reducing switching losses in the PolFET and helps pave the way for ultra-wide bandgap radio frequency electronics.