AlN-based polarization-doped field-effect transistors with downward-graded AlGaN underlayers

Abstract

We propose AlN-based polarization-doped field-effect transistors (PolFETs) with upward-graded AlxGa1−xN channel layers combined with downward-graded AlGaN underlayers (ULs). In a structure where the channel layer is directly formed on an AlN buffer layer without a UL, the large polarization charge of the AlN buffer layer depletes the bottom side of the channel layer, resulting in narrow width of three-dimensional electron gas (3DEG) and high resistance in electrical characterization. In contrast, the structures incorporating ULs exhibit wide 3DEG by leveraging polarization engineering. Hall-effect measurements revealed a sheet electron concentration (nsh) of 6.2×1012 cm−2 and electron mobility (μ) of 187 cm2 V−1 s−1 for a 100-nm-thick upward-graded AlxGa1−xN (x:0.77→1) with an undoped 300-nm-thick downward-graded AlyGa1−yN (y:1→0.77) UL. Silicon doping in the UL compensates for their negative polarization charge, further increasing nsh to 7.2×1012 cm−2 while maintaining μ of 200 cm2 V−1 s−1 at a Si doping concentration of 3.3×1017 cm−3. The PolFET with the Si-doped UL exhibits a drain current of 70 mA/mm. The off-state leakage current is as low as 4×10−10 A/mm, leading to large on/off ratio of 108. Thus, the incorporation of downward-graded AlGaN ULs with Si doping concentrations comparable with the negative polarization charge density in the ULs can boost the performance of AlN-based PolFETs.