Delta-doped AlGaN/AlN/GaN microwave HFETs grown by metalorganic chemical vapour deposition

The performance of an innovative delta-doped AlGaN/AlN/GaN heterojunction field-effect transistor (HFET) structure is reported. The epitaxial heterostructures were grown on semi-insulating SiC substrates by low-pressure metalorganic chemical vapour deposition These structures exhibit a maximum carrier mobility of 1058 cm²/V s and a sheet carrier density of 2.35 × 10¹³ cm^-2 at room temperature, corresponding to a large n_sμ_n product of 2.49 × 10¹⁶ V s. HFET devices with 0.25 μm gate length were fabricated and exhibited a maximum current density as high as 1.5 A/mm (at V_G = + 1 V) and a peak transconductance of g_m = 240 mS/mm. High-frequency device measurements yielded a cutoff frequency of f_t ≃ 50 GHz and maximum oscillation frequency f_max ≃ 130 GHz.