Abstract <p>This research analyzes the impact of an AlGaN back barrier layer (BBL) on the performance of high electron mobility transistor (HEMT) devices. We compared the performance of recessed gate structures with a back barrier against devices without one. Including the BBL raises the conduction band, reducing leakage beneath the buffer layer and confining the 2DEG to a narrower channel. In this study, we fixed the BBL thickness at 10 nm and varied the mole concentration of &#xa0;Al and Ga in the Al_<i>x</i>Ga_1– _<i>x</i>N BBL with <i>x</i> values of &#xa0;0.22, 0.24, and 0.28. Additionally, we altered the gate length to 0.8, 1, and 1.2 µm. The research investigates how changes in mole fraction and doping of the BBL influence device performance, aiming to optimize the Al and Ga mix for high-power, fast-switching applications while enhancing overall efficiency. Increasing the Al concentration in the BBL reduces carrier concentration in the channel due to the parasitic channel effect, resulting in decreased drain current and transconductance (gm). We also explored the impact of varying the material under the gate, including Al₂O₃, SiO₂, and Si₃N₄, to study their influence on threshold voltage (<i>V</i>_th) and the <i>I</i>_on/<i>I</i>_off &#xa0;ratio. The design of the BBL significantly impacts Vth engineering, with this device achieving a threshold voltage of approximately 1.89 V at a gate length of 1.2 µm and an <i>I</i>_on/I_off ratio ranging from ~10⁻¹¹ to ~10¹³. The choice of gate insulating material and the Ga and Al mole content in the BBL optimize <i>V</i>_th and <i>I</i>_on/I_off, enabling the device to operate efficiently at high power and fast switching speeds. The future of HEMT technology is promising, with continuous innovations meeting evolving operational demands.</p>