## 1 Introduction  
Gallium Nitride (GaN) is widely used for wireless telecommunications and radar systems because of its superior material and electronic properties, such as wide bandgap (3.4 eV) and high breakdown field (2-3.3 MV/cm) [1; 2; 3]. These characteristics make GaN more practicable for high-voltage and high-temperature applications than many other materials, such as silicon or gallium arsenide [4]. Similarly, high electron mobility transistors (HEMTs) based on the AlGaN/GaN heterostructure show excellent performance owing to the two-dimensional electron gas (2-DEG) in the channel region. As a consequence of spontaneous and piezoelectric polarization effects, the 2-DEG shows high carrier mobility and high saturation velocity, which have important roles in the output current and power amplification [5; 6]. Nevertheless, HEMTs need to be improved to fully cater to the market requirements [7]. For class A HEMT operation [8; 9; 10], the AC maximum power (P) is described in Equation (1),  
\[\mathrm{P}=\frac{(\mathrm{V}_{\mathrm{SWING}}\;\times\;\mathrm{I}_{\mathrm{ SWING}})}{8} \tag{1}\]where the gap between the knee and breakdown voltages determines V\({}_{\text{SWING}}\), and the maximum current defines I\({}_{\text{SWING}}\). Hence, low knee voltage, high off-state breakdown voltage, and high maximum current are very important features to improve the device performance of HEMTs. Generally, the field plate (FP) technology has been exploited to increase the breakdown voltage of AlGaN/GaN HEMTs and has been applied to millimeter wave frequencies for many purposes [11; 12; 13]. However, the FP gate structure produces additional gate capacitance and also increases the gate-to-source capacitance (C\({}_{\text{gs}}\)) and gate-to-drain capacitance (C\({}_{\text{gd}}\)). These increased parasitic capacitances degrade the radio frequency (RF) performance at higher frequency operations and also reduce the power efficiency in the operational frequency range. Some groups have already studied the gate-head design modeling to improve the direct current (DC) and RF performance of AlGaN/GaN HEMTs [14; 15; 16]. Particularly, gamma-shaped, camel-shaped, and T-shaped gates are a few examples where a metal FP is connected to the gate electrode for optimum results [17; 18; 19]. These gate-head structures have been reported to exploit the versatility and high performance of GaN-based FP HEMTs [20]. The device performs differently as the FP is moved from the source-side edge to the drain-side edge of the device. Hence, the most appropriate results for transconductance, output current, and breakdown voltage are achieved by the T-shaped gate structure for high-power applications. AlGaN/GaN HEMTs using a dual-gate structure are proposed to modulate the threshold voltage without employing any special fabrication techniques such as plasma treatment, recessed gate-head structure, and the addition of a p-GaN gate layer [21]. Also, a floating gate is embedded between the control gate and drain electrode to improve the DC characteristics with low leakage current, high on/off ratio, and breakdown voltage [22]. However, the systematic optimization of dual-gate structures has not been reported for high-power and high-frequency operations. In this study, to analyze the DC characteristics, conventional T-shaped gate AlGaN/GaN HEMTs were simulated, and the fabricated two-finger device was characterized for clarity. For more accuracy, the electrical parameters were fixed, and the simulation was performed. On the basis of the conventional T-shaped gate structure, the fabricated and simulated data were first compared and contrasted. Then, the optimized device structure, with best possible electrical characteristics obtained by changing the geometry of the FP structure, was investigated using technology computer aided design (TCAD) simulations. The drain-side FP structure was characterized to boost the breakdown voltage, and the optimum FP length point was identified for the highest breakdown voltage.