Role of the AlGaN Cap Layer on the Trapping Behaviour of N-Polar GaN MISHEMTs

We investigate the static and dynamic (trapping) performance of N-polar Gallium Nitride MIS-HEMT devices as a function of the aluminum concentration in the top cap layer (22%, 34% and 46%). The analysis is based on combined dc characterization, double pulse measurements, and threshold voltage transient investigation. The de results demonstrate that the use of high aluminum concentrations in the cap layer results in a lower gate leakage current (around 9 µA/mm for % Al=46, compared to 70 µA/ mm for %Al=22, measured at VGs=-7 V and VDS=15 V). In addition, pulsed and transient investigation showed that the use of high Al concentration in the cap layer can substantially suppress the current collapse (slump ratio = 15 % for %Al=46, compared to 26 % for %AI=22). Trapping is ascribed to the presence of a defect state located at EC-0.5 eV, which is responsible for a threshold voltage shift. The results point out the key role of the AlGaN cap layer on the performance of AlGaN-based HEMTs, and give indication on how to optimize the performance of the devices.

I. Data/results and Discussion

Table I reports a summary of the dc characteristics of the analyzed samples. The devices with higher aluminum concentration in the cap were found to have comparable dc current (1.09 A/mm for %Al=46, compared to 0.83 A/mm for %Al=22) and, transconductance (0.54 S/mm for %Al=46, compared to 0.40 A/mm for %Al=22), but with a significantly lower gate leakage at high drain bias, i.e. in a bias point that can favor charge trapping. Median leakage was found to be 9 µA/mm for %Al=46, compared to nearly 70 for %Al=22, measured at and V. A clear view of the difference in gate leakage is shown in Figure 2. Figure 2:

Gate leakage current measured at and V for devices with different al concentrations in the cap layer. Median leakage was found to be 9µa/mm for %al=46, compared to nearly 70 µa/ mm for %al=22

Table I: Average value of drain saturation current and transconductance for devices with the three different percentages of aluminum.

Al %	IDs(A/mm) @ VDS= 7 V, VGS= -1 V (Mean Value)	gm_max(S/mm) @ VDS= 5 V (Mean Value)
46	1.09± 0.1	0.54± 0.05
34	0.88± 0.03	0.417± 0.007
22	0.83± 0.03	0.40± 0.02