N-face AlGaN (cap)/GaN (channel)/AlGaN (barrier)/GaN (buffer) high electron mobility transistors (HEMTs) provide a simple solution for strong confinement of the two-dimensional electron gas (2DEG) from the back since carriers are induced on top of the AlGaN barrier. To reduce the adverse effects of random alloy scattering from the AlGaN barrier, an N-face GaN-spacer HEMT was designed with an epi-structure consisting of an AlGaN (cap)/GaN (channel)/AlN/GaN (spacer)/AlGaN (barrier)/GaN (buffer). The 2DEG is confined at the GaN (channel)/AlN interface, while the GaN-spacer layer separates the 2DEG from the AlGaN barrier. The large polarization-induced electric field of the AlN and its large conduction band discontinuity with GaN provide strong confinement for good pinch-off and low output conductance under high electric fields. Up to 20% improvement in electron mobility was measured in these devices. The devices showed a current gain cut-off frequency (ft) of 24 GHz and power gain cut-off frequency (fmax) of 44 GHz for a nominal gate length of 0.7 μm. An output power density of 4.5 W/mm was measured at 4 GHz, with 34% power added efficiency and a gain of 10.3 dB at a drain bias of 40 V. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)