FIG. 10 is a diagram showing the relationship between the electric field and the position in the semiconductor devices according to embodiments and comparative embodiments of the present disclosure. According to one embodiment of the present disclosure, the vertical length Lv of the vertical extension portion 126 in the semiconductor device 100-1 shown in FIG. 1 may be adjusted, and the impact ionization rate at various depths of the semiconductor device may be measured. The “position” shown in the vertical axis of FIG. 10 refers to a “vertical position”, and the position of “0” substantially corresponds to the top surface of the first electrode of the semiconductor device. When the value in the vertical axis becomes larger, the corresponding position becomes more close to the substrate. Referring to FIG. 10, the horizontal length of the horizontal extension electrode may be fixed at 1.5 μm, and the vertical length Lv of the vertical extension portion of the first electrode 120 may be set at 1 μm, 1.5 μm, and 2 μm, respectively. The corresponding curves are represented by the curve C1, the curve C2, and the curve C3, respectively. Referring to FIG. 10, when the vertical length of the vertical extension portion is longer, the peak position of the electric field may become deeper. However, when the bottom surface of the vertical extension portion is located in the semiconductor buffer layer and adjacent to the semiconductor channel layer, the peak intensity of the electric field may be the smallest (corresponding to curve C2). Therefore, by providing the vertical extension portion, the distribution of electric field may be modified so as to reduce the peak value of the electric field. Accordingly, the semiconductor device is less likely to generate impact ionization.