The AlGaN/GaN heterostructure without oxide (HFET) and with additional aluminium oxide underneath the gate (MOSHFET) were investigated with regards to RF performance. Both cutoff frequency and maximum frequency of oscillation were measured and compared by means of small signal analyses and equivalent circuit parameter extraction. The maximum oscillation frequency f max is an important figure of merit because it is a defining factor for oscillator design.The cutoff frequency f t of the HFET with gate length of 300 nm resulted in 28 GHz and showed an increase up to 39 GHz for the MOSHFET device. The maximum frequency of oscillation f max showed contrary behavior. The HFET showed an f max of 120 GHz, while the MOSHFET revealed a reducedd RF performance of 78 GHz. RF simulations based on measured S-parameters indicated an increased gate drain capacitance C gd for devices with a dielectric layer underneath the gate metallization.