Undoped AlGaN/GaN HEMTs for microwave power amplification

Undoped AlGaN/GaN structures are used to fabricate high electron mobility transistors (HEMTs). Using the strong spontaneous and piezoelectric polarization inherent in this crystal structure a two-dimensional electron gas (2DEG) is induced, Three-dimensional (3-D) nonlinear thermal simulations are made to determine the temperature rise from heat dissipation in various geometries. Epitaxial growth by MBE and OMVPE are described, reaching electron mobilities of 1500 and 1700 cm(2)/Vs, respectively, for electron sheet density near 1 x 10(13)/cm(2). Device fabrication is described, including surface passivation used to sharply reduce the problematic current slump (dc to rf dispersion) in these HEMTs. The frequency response, reaching an intrinsic f(t) of 106 GHz for 0.15 mum gates, and drain-source breakdown voltage dependence on gate length are presented. Small periphery devices on sapphire substrates have normalized microwave output power of similar to4 W/mm, while large periphery devices have similar to2 W/mm, both thermally limited. Performance without and with Si3N4 passivation are presented. On SiC substrates, large periphery devices hare electrical limits of 4 W/mm, due in part to the limited development of the substrates.