## 3 Result and Discussion  
Figure 6 shows the output characteristics of the proposed device with a different depth and length, respectively. As seen in Figure 6, LCCT does impose the device current by modulating the fixed charge in SiN which will repel the 2 DEG under LCCT. As a result, with the increasing depth and length, on-state resistance increases from around 9 \(\Omega\)-mm to around 15 \(\Omega\)-mm. However, it should be noted that in order to maintain the polarization charges introduced by LPCVD-SiN and avoid hurting the AlGaN lattice, the depth of LCCT should not exceed 75 nm.  
Figure 5: The calibration of the simulator with the experiments (inset figure is the structure used in the calibration).  
Figure 6: The output characteristics of the proposed LCCT-HEMT with different (**a**) LCCT depth and (**b**) LCCT length.  
Figure 7 shows the extracted on-resistance and corresponding BV, respectively. As shown in Figure 7a,b, the resistance and BV of LCCT-HEMT increase with longer and deeper LCCT. When the LCCT depth reaches 70 nm, which means that the thickness of the remaining LPCVD-SiN under LCCT is only 10 nm, the charge modulated by LCCT repels the 2 DEG heavily, making the resistance increase dramatically. Figure 7c shows the calculated BFOM of the LCCT-HEMT in comparison with that of UTB-HEMT. The BFOM firstly grows with the increasement of LCCT depth and then drops after the depth of LCCT is deeper than 40 nm. A peak BFOM of 281 kW/mm is achieved when the length reaches 9.2 \(\upmu\)m and the depth reaches 40 nm. Compared with the BFOM of the conventional UTB-HEMT, which is about 142 KV/mm, the proposed LCCT- HEMT performs almost two times better in terms of the BFOM.  
Such an improvement was achieved by the suppression of the high E-field under the gate, provided by LCCT. In the \"off\" state, owing to the charges introduced by LCCT, the E-field around the gate redistributes into a smooth form, as shown in Figure 8. As a result, the E-field targeting to the gate is suppressed, and hereby, the BV increases effectively. Specifically, when the drain voltage is at 800 V, the LCCT reduces the E-field concentration around gate (see red region in Figure 8). That is, when the LCCT depth is 30 nm in Figure 8b, the charge introduced by LCCT is not strong enough to redistribute the E-field. On the contrary, when the depth is 50 nm, the charge is effective enough to suppress the E-field around the gate, which is shown in Figure 8c,d. Simultaneously, the leakage current is constricted greatly by the long and deep LCCT when the drain voltage is at 800V, as shown in Figure 9.  
Figure 7: The extracted (**a**) on-resistance, (**b**) corresponding breakdown voltage (BV), and (**c**) Baliga Figure-of-Merits (BFOM) of proposed LCCT-HEMT with different LCCT configurations.  
Additionally, the E-field distribution along the channel is presented in Figure 10 to further demonstrate the LCCT effects. As seen in Figure 10, a maximum E-field reduction of 2.8 MV/cm was achieved by optimizing LCCT configuration. Apparently, the E-field distribution along the channel in LCCT-HEMT is smoother than that in UTB-HEMT, which is consistent with the mechanism demonstrated in Section 2. Therefore, by optimizing the LCCT length and the depth, the proposed HEMT could exhibit better performance in power applications. However, notably, the precise parameters of LCCT depends on the process, which suggests that exclusive experiments are required in future studies.  
Figure 8: The simulated E-field distribution in (**a**) conventional UTB-HEMT, and (**b–d**) LCCT-HEMT with different LCCT configurations when the drain voltage is 800 V.  
Figure 9: The simulated current density distribution in (**a**) conventional UTB-HEMT, and (**b–d**) LCCT-HEMT with different LCCT configurations when the drain voltage is 800 V.