Results, potential and challenges of high power GaN-based transistors

The fabrication of AlGaN/GaN high electron mobility transistors (HEMTs) emp loys strong spontaneous and piezoelectric polarization. The material was gr own by OMVPE at 1050 degrees C and 75 Torr. The Hall measurements yielded a pproximate to 1 x 10(13)/cm(2) electron sheet density, with 1200 to 1600 cm (2)/Vs electron mobility for 300 Angstrom Al0.3Ga0.7N/GaN on sapphire at 30 0 K. The process steps for mesa isolation, Ohmic and Schottky contacts are presented. The frequency response and drain-source breakdown voltage have b een determined experimentally. The average electron transit velocity, deter mined from cut-off frequency, is 1.2 x 10(7) cm/s, and the breakdown electr ic; field is estimated to be 3 MV/cm. On sapphire substrates, large periphe ry devices yielded 1.8 W/mm at 75% power-added efficiency and small periphe ry devices yielded 3.3 W/mm at 36% power-added efficiency at 4 GHz. Perform ance limitations due to electron trapping during HEMT operation are include d. Predictions:of >20 W/mm microwave output power are presented for these d evices on SiC substrates.