Undoped AlGaN/GaN HEMTs for microwave power amplification

Undoped AlGaN/GaN structures are used to fabricate high electron mobility t ransistors (HEMTs). Using the strong spontaneous and piezoelectric polariza tion inherent in this crystal structure a two-dimensional electron gas (2DE G) is induced, Three-dimensional (3-D) nonlinear thermal simulations are ma de to determine the temperature rise from heat dissipation in various geome tries. Epitaxial growth by MBE and OMVPE are described, reaching electron m obilities of 1500 and 1700 cm(2)/Vs, respectively, for electron sheet densi ty near 1 x 10(13)/cm(2). Device fabrication is described, including surfac e passivation used to sharply reduce the problematic current slump (dc to r f 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 dep endence on gate length are presented. Small periphery devices on sapphire s ubstrates have normalized microwave output power of similar to4 W/mm, while large periphery devices have similar to2 W/mm, both thermally limited. Per formance without and with Si3N4 passivation are presented. On SiC substrate s, large periphery devices hare electrical limits of 4 W/mm, due in part to the limited development of the substrates.