Two-dimensional electron gas transport properties in AlGaN/GaN heterostructure field-effect transistors designed for high-power applications

Abstract Two-dimensional electron gas (2DEG) transport properties and device characteristics in AlGaN/GaN heterostructure field-effect transistors (HFETs) have been investigated keeping high-power applications in mind. The 2DEG mobility as the function of the 2DEG density at low and above room temperatures has systematically been examined for different Al compositions, revealing that the 2DEG mobility compared at the same 2DEG density is higher for higher Al compositions up to 0.3 above room temperature. This indicates that the high Al composition is favorable not only for high 2DEG densities but also for high 2DEG mobilities. For an HFET (Al=0.3) with a gate-length ( L g ) of 1.5 μm, excellent DC and RF characteristics have been obtained, i.e., a maximum transconductance ( g m ) of 180 mS/mm, a cutoff frequency ( f T ) of 16 GHz, and a maximum oscillation frequency ( f max ) of 50 GHz. The selective-area regrowth technique has been applied to reduce the source and drain resistance, and an improved g m of 215 mS/mm has been obtained in an HFET ( L g =1.5 μm) as the result of the reduced ohmic resistance (0.3 Ω mm). Moreover, a novel layer structure with the back-doping (BD) design has been proposed where an asymmetric double-heterostructure is employed and the doping is performed also in the backside AlGaN barrier layer. A very high 2DEG density of 2.8 × 10 13 cm −2 with a record value of the mobility–density product (2.4 × 10 16 (V s) −1 ) has been obtained in this structure, proving that the BD design is promising for high-power applications.