AlGaN/GaN high electron mobility transistors (HEMTs) are favored for the use in high-power and high-frequency applications. Normally-off operation has been desired for various applications, but proved to be difficult to achieve. Recently, a new approach was proposed by Mizutani et al. [Mizutani T, Ito M, Kishimoto S, Nakamura F. AlGaN/GaN HEMTs with thin InGaN cap layer for normally-off operation. IEEE Elec Dev Lett 2007;28(7):549-51]: a thin InGaN cap layer introduces a polarization field, which raises the conduction band of the AlGaN/GaN interface. As a result, the threshold voltage is shifted in positive direction. Relying on the experimental work of Mizutani et al. we conduct a simulation study of the proposed devices. Our device simulation tool is expanded by material models for InN and InGaN and also an improved high-field mobility model accounting for the specifics of the III-N materials. Using this setup, we further explore the device specific effects and conduct an analysis of the AC characteristics. (C) 2008 Elsevier Ltd. All rights reserved.