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The submicrometer gate HEMT is fabricated with an ultrathin-barrier (UTB) AlGaN/gallium nitride (GaN) combined with in situ SiN passivation. The sheet resistance of the UTB Al0.2Ga0.8N (4 nm)/GaN heterostructure is effectively reduced by the SiNx passivation layer grown by metal organic chemical vapor deposition (MOCVD), from 6500 to $312~ \Omega $ /⬜. With the 20-nm stress-engineered in situ SiN, the device not only provides a large output current of 1.05 A/mm but also demonstrates promising potential on the RF applications, which gives AlGaN material two records high cutoff frequency ${f} _{\text {T}}/{f} _{\text {max}}$ of 157 GHz/334 GHz for 100-nm gated device and 211 GHz/379 GHz for 70-nm gated device. During the continuous wave (CW) power measurement at 30 GHz, the 70-nm devices exhibit a large output power of 4.6 W/mm associated with a peak power-added efficiency (PAE) of 48.1% and a gain of 11.6 dB (${V} _{\text {ds}} =20$ V), and a high PAE of 53.8% with an output power density of 1.9 W/mm and a gain of 10.8 dB (${V} _{\text {ds}} =10$ V), respectively. The huge potential of the UTB-AlGaN/GaN is demonstrated for high-frequency and large-output power applications when it is combined with the in situ SiN, which is necessary for future communication systems. [ABSTRACT FROM AUTHOR]