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This paper presents high performance device results using an ultra-thin AlN/GaN structure on sapphire substrate with a 100-nm T-gate. Excellent dc and RF characteristics are reported, including an extrinsic transconductance of 500 mS/mm and an extrinsic f_t/f_max(U) ratio of 78/111-GHz which is among the highest reported for AlN/GaN HFETs. Low gate leakage results are also presented despite the small barrier thickness and absence of a gate dielectric. Modeling of the small signal parameters is also discussed to gain an understanding of the limiting and contributing performance factors.