## 3 Results and Discussion
The length of the IFP structure was increased from 0.2 to 0.7 \(\upmu\)m. Hence, six different IFP lengths, i.e., 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 \(\upmu\)m were analyzed. When the IFP had a length of 0.7 \(\upmu\)m, a 100 nm gap was deliberately left between the IFP and the T-shaped gate. On the basis of the gate edge of the optimized FP structure, the aggregate length of the IFP and the gap are always set to 0.8 \(\upmu\)m. Although the IFP structure and the gate were separated, they were assumed to be connected, so that the same gate bias was provided to both. The breakdown voltage of the IFP structure was analyzed accordingly. As shown in Figure 8a, some degradation was found in the breakdown voltage of the device. The extension of the IFP towards the drain-side gate head edge reduced the drain leakage current in the off-state condition. Figure 8b indicates the peak values of the breakdown voltage as a function of the IFP length which were 370, 400, 413, 419, 555, and 557 V for IFP lengths of 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 \(\upmu\)m, respectively. The breakdown voltage changed abruptly between 0.5 \(\upmu\)m and 0.6 \(\upmu\)m. Though there were some decreases, all values were much larger than the values for a normal T-shaped gate structure. Small signal RF analysis was performed on different IFP lengths for fixed DC parameters at V\({}_{\text{ds}}\) = 12 V and V\({}_{\text{gs}}\) = \(-\)2 V. The current gain decreased monotonically when the IFP was stretched from 0.2 to 0.6 \(\upmu\)m, and started to slightly increase with any further IFP stretching beyond 0.6 \(\upmu\)m. The f\({}_{\text{T}}\) of 0.2 \(\upmu\)m IFP structure was formed near 9.05 GHz, which was 35.1% better than for the connected structure, as shown in Figure 9a. The unilateral power gain was changed with different IFP lengths because of changes in the device capacitance, as shown in Figure 9b. At the IFP structure, when the IFP length was 0.3 \(\upmu\)m, f\({}_{\text{max}}\) was higher than for other IFP lengths. The highest f\({}_{\text{max}}\) of 25 GHz was achieved when the IFP was connected to the gate. Figure 8: (a) Drain leakage current characteristics of AlGaN/GaN HEMT in a log scale and (b) breakdown voltage of the device as a function of IFP lengths. Figure 7: T-shaped gate AlGaN/GaN HEMT structure: (a) optimized FP structure; (b) schematic of inner field-plate (IFP) structure. Furthermore, the capacitance was extracted to probe the relationship between frequency and capacitance. Figure 10 shows the capacitance variations for different IFP structure lengths. The C\({}_{\text{gs}}\), which is directly proportional to the IFP length, showed the smallest value for an IFP structure length of 0.2 \(\upmu\)m. Finally, C\({}_{\text{gs}}\) had a saturated value of about 1690 fF/mm. However, compared to C\({}_{\text{gs}}\), only a little change in C\({}_{\text{gd}}\) was observed for various IFP lengths. The changes in the geometrical length of the drain side can affect both C\({}_{\text{gs}}\) and C\({}_{\text{gd}}\)[14; 15]. Although the geometry was changed only in the drain side, the C\({}_{\text{gs}}\) variation was much larger than the C\({}_{\text{gd}}\) variation. The drain-side FP not only affected its drain-access capacitance but reflected an influence on the source-access capacitance, and vice versa for the source-side FP. The coexistence of the FP own extrinsic capacitance along with the intrinsic capacitance beneath the gate edges (gate-source and gate-drain) due to the depletion region is cited as the reason for the said phenomenon [17; 38]. The FPs regulate the depletion layer by a uniform distribution of the electric field beneath both gate edges. The electric field redistribution by FPs along the gate edges results in the suppression and extension of the channel depletion length, hence raising the capacitance [14]. In the device configuration, as depicted in Table 1, L\({}_{\text{SG}}\) was shorter than L\({}_{\text{GD}}\).