# Impact of Silicon Substrate with Low Resistivity on Vertical Leakage Current in AlGaN/GaN HEMTs
## 3 Results and Discussion  
To investigate the effect of low-resistivity substrates on VLC of AlGaN/GaN epitaxial layers, vertical I-V measurements were performed on both samples, as shown in Figure 1b. Different I-V characteristics were investigated for the samples between the top contact (an  
Figure 1: Growth structure with p-Si (111) substrate (**a**). Measured vertical leakage current of grown structures on p-Si and n-Si (111) substrate (**b**). surface and bottom contact (cathode) on the Si substrate. The voltage was swept up from 0 V to higher values with a step of 1 V during the forward-bias measurement. According to the difference of the slope of log J-V, the curves were divided into region I at low biases (0-400 V), and region II at high biases (400-740 V). It was found that the VLC of the p-type samples was reduced at region I and region II, compared to n-type, which shows the substrate effect. At region II for the p-type samples, VLC increases more rapidly compared to that in region I, and the log J-V curve exhibits a steeper slope than that for n-type. While at higher biases, the vertical current for p-type is the same with n-type. In that case, the effect of substrates is ignorable, and the buffer limitation plays a key role for both samples. To investigate the role of different types of substrates on VLC conduction mechanisms, we conducted the I-V-T measurements for both samples, as shown in Figure 2a,b. For the p-type samples, a log J-V dependence can be observed at 0-400 V, which corresponds to the space-charge-limited current conduction mechanism [14]. The slope of log J-V starts to get larger from the kink at 400 V, as shown in Figure 2a, which is due to the Poole-Frenkel conduction at 400-600 V [15; 16]. However, the log J-V dependence for n-type samples can be also observed in the whole range of 0-600 V, corresponding to the space-charge-limited current conduction mechanism [14]. The effective energy barrier related to the VLC on different types of substrates has been investigated. The activation energy E\({}_{\text{a}}\) refers to the barrier that electrons have to overcome for injecting from the Si substrate into GaN layers by thermionic emission. It is evaluated from the slope of ln J-I/T, as reported in literature [17; 18]. In this work, E\({}_{\text{a}}\) at 200 V, 300 V, 500 V, 600 V is measured on p-type (Figure 3a) and on n-type (Figure 3b) in the whole temperature range. E\({}_{\text{a}}\) in p-type is larger than that in n-type at these biases. Further analysis of the E\({}_{\text{a}}\) values at different biases is shown in Figure 3c. It can be observed that the E\({}_{\text{a}}\) values of the p-type samples are always higher than that of n-type samples in the whole range (0-600 V). For p-type samples, E\({}_{\text{a}}\) changes a little at low biases via the increase of voltages (0-400 V), while E\({}_{\text{a}}\) decreases quickly at high biases (400-600 V). For n-type samples, the values of E\({}_{\text{a}}\) did not change much in the whole region. That can be attributed to the fact that another conduction process exists for p-type Si samples, which is different from the n-type case. It clearly demonstrated the substrate limits of the leakage in the whole region (0-600 V). According to the results above, the band diagrams of both samples are used to further analyze the impact of the low-resistivity Si substrates on the forward-bias leakage mechanisms. As shown in Figure 4a at 0-400 V, due to the generation of a space charge region at the AlN/p-Si interface, high E\({}_{\text{a}}\) would occur, preventing electron injection from the p-Si substrate into the AlN layer, and hence reducing the leakage. While for n-type samples, as shown in Figure 4b, many electrons in n-Si substrate could be transported into the AlN layer, and the electron injection is increased at the lower E\({}_{\text{a}}\) case, leading to larger VLC. Based on these mechanisms, phenomenon observed at low biases in Figure 1b can be well understood.