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In this study, we investigate the enhancement of effective transconductance and gain linearity in submicrometer gate AlGaN-barrier-based transistors utilizing ScAlN/GaN coupling-channel structures. A novel asymmetric gate design, incorporating an AlGaN cap on ScAlN/GaN HEMT, is proposed. This configuration demonstrates a flat transconductance profile and significantly reduced transconductance derivative, thereby improving the linearity of GaN-based transistors. For transistors with a gate length of 0.5 μm, both the current gain cut-off frequency and power gain cut-off frequency are measured at 53.52 GHz and 72.98 GHz, respectively. These values represent a more than threefold increase from the corresponding frequencies of conventional devices. Moreover, the theoretical Output Third-Order Intercept value of the proposed structure has increased by 7.5 dBm compared to conventional HEMTs. This superior linear performance can be attributed to the combined effect of AlGaN cap layer and air cavity. Furthermore, the introduction of the ScAlN layer results in a substantial impact, including a decrease in electron concentration from 4.96 × 1019/cm3 to 1.98 × 1019/cm3 and an increase in electron velocity from 4.5 × 107 to 7.6 × 108 cm/s. These findings underscore the potential of the proposed device for high-frequency applications requiring superior linearity.