# Impact of Silicon Substrate with Low Resistivity on Vertical Leakage Current in AlGaN/GaN HEMTs
## 1 Introduction  
Gallium-nitride (GaN)-based devices have recently attracted increasing attention for optoelectronic applications, such as light-emitting diodes (LED) [1], laser diodes (LD) [2], ultraviolet (UV) detectors [3, 4], and power electronic devices [5]. Especially high-electron-mobility transistors (HEMTs) of GaN-based power devices on silicon (Si) substrates are considered promising candidates for high-power electronic applications because of their superior properties, such as high breakdown voltage, high current driving capability, and high thermal stability [6]. AlGaN/GaN heterostructures on Si substrate and their power devices have become frontier areas and hot spots in research in the international academia and industry, due to their unique advantages of low cost and capability with mainstream CMOS technology for the preparation of Si integrated circuits. However, several challenges remain that need to be resolved regarding Si substrate, in particular, stronger charge trapping effects caused by high-resistivity Si [7], degradation due to electron injection from Si [8], and breakdown related to the inversion channel at the AlN/Si interface in GaN-based HEMTs [9]. The breakdown of GaN-on-Si lateral devices is thought to be limited by top-to-bottom vertical leakage current (VLC) in the end [10]. Therefore, understanding the vertical leakage mechanisms related to Si substrate is an essential step toward potential power device applications.  
There are several reports about the role of substrate on the vertical leakage in AlGaN/GaN-on-Si. [PERSON] et al. concluded that the use of a highly resistive p-type silicon substrate could increase the vertical breakdown voltage of the transistors, as the fact that the voltage drop on the GaN buffer is mitigated by the partial depletion of the substrate [7]. [PERSON] et al. showed that the generation of Shockley-Read-Hall (SRH) in the silicon depletion region impacts electron supply and current conduction [11]. However, they have not investigated the details on the VLC mechanisms in the case of low-resistivity Si substrates. Although [PERSON] et al. investigated the carrier transport from n+, n, p+ and p-Si (111) substrates through AlN nucleation layer on Si structure [12], they have not yet discussed the VLC in the whole AlGaN/GaN HEMTs. Owing to the advantage of low-resistivity Si substrates for the low curvature growth of AlGaN/GaN epilayers [13], these kinds of substrates are usually used to grow thick AlGaN/GaN epilayers. It is thus desirable to investigate the VLC in AlGaN/GaN HEMTs grown on low-resistivity substrates.  
In this letter, we present an extensive analysis to reveal that the structures with low-resistivity p-Si substrates (named as p-type below) could reduce the electron injection from Si into the top layer and lead to lower VLC at low voltage values, in comparison to the case of that with low resistivity n-Si substrates (named as n-type below). The temperature-dependent current-voltage (I-V-T) measurements indicate that the substrate limitation greatly affects the VLC at low biases.