## 3 Results and Discussion  
On the basis of the conventional T-shaped gate structure, the DC characteristics of the device were analyzed by changing the geometry of the FP structure. Figure 4a shows the basic structure with the 3D modeling of the device. Generally, many gate FP structures are used to increase the breakdown voltage beyond the stipulated limit for power electronics. Figure 4b explains the schematics of a drain-side FP device. Except for the drain-side FP length, which was changed from 0 to 2.1 \(\upmu\)m to probe into the difference in the breakdown voltage, all the remaining structural variables of the device were fixed. Figure 4: T-shaped AlGaN/GaN HEMT structure: (**a**) conventional T-shaped gate structure; (**b**) schematic of the drain-side field-plate (FP) structure. Figure 3: (**a**) The two-dimensional electron gas (2-DEG) density at the AlGaN/GaN interface; (**b**) transfer current–voltage characteristics of the measured and simulated devices at V\({}_{\text{ds}}\) = 25 V; (**c**) subthreshold drain leakage current on log scale, and (**d**) saturation currents of the fabricated and simulated devices. It is well known that the impact ionization and electron punch-through are two of the most eminent physical mechanisms that breakdown the HEMT devices [27; 28]. Impact ionization is a direct result of sufficient energy acquisition by an electron in the channel region from a high electric field. The electron punch-through occurs when electrons penetrate through the GaN buffer layer. This leakage current can be prevented by doping the buffer with highly resistive acceptor traps or by employing back barrier in the GaN buffer layer to confine the electrons only in the channel region. This analysis focuses on the breakdown characteristics caused by the impact ionization and electron punch-through effects. To consider both mechanisms, the impact ionization model and the substrate acceptor doping were applied as shown in Figure 2. In AlGaN/GaN HEMTs, a common approach used to enhance the breakdown voltage is to redistribute the electric fields over the channel by employing an FP structure. The electric fields originate at the corners of the gate electrode. Premature impact ionization results in a soft breakdown in the source-side FP structure [29], whereas the drain-side FP undergoes a hard breakdown, owing to a delayed impact ionization following the avalanche process. In FP technology, there is a direct dependence of breakdown voltage on the correlation between the carrier density and the electric field [30; 31]. Therefore, T-shaped gate FP structures are exploited to account for both source-side and drain-side electric fields and, hence, carrier concentrations. The high electric fields beneath both gate corners are uniformly distributed along the channel by the T-shaped gate, which results in high breakdown voltage. Among all gate-head designs, the highest off-state breakdown voltage with low leakage current can be achieved by the T-shaped gate structure. Figure 5a presents the breakdown voltage characteristics, showing the leakage current for different drain-side FP lengths. To turn off the channel, a pinch-off voltage of \(-7\) V was provided for breakdown voltage measurement via the transient simulation method. Although an increase in FP length reduced the overall off-state drain leakage current, impact ionization-based breakdown characteristics were recorded in the 2-DEG channel layer near the drain-side FP edge. Hence, the drain-side FP edge is a critical factor to be considered for the reduction in the breakdown voltage of the device. Figure 5b shows the peak breakdown voltages at different FP lengths. The breakdown starting point was the spot where the leakage current was \(1\) mA/mm according to the development specification of radar systems.