High power performance InAlGaN/GaN high electron mobility transistor (HEMT) as a candidate for high power and high frequency amplifiers has been demonstrated versus the conventional AlGaN/GaN HEMT by using the same device processes. Comparing with its conventional AlGaN/GaN counterpart, the InAlGaN/GaN device exhibits a much larger output current density of 1.94 A/mm due to its higher 2-D electron gas density of 2.0 × 10¹³ cm^-2 by using a thin quaternary InAlGaN barrier layer, and almost twice as large as f_T of 142 GHz and f_max of 203 GHz. Through measurements of large-signal characteristics at frequency of 34 GHz and biased at 10 V, the InAlGaN/GaN device shows a high output power density of 2.75 W/mm, which is about 87% increase in comparison with that of its AlGaN/GaN counterpart with an output power density of 1.47 W/mm.