PN junction and method for preparing same

The embodiment of the invention discloses a method for preparing a PN junction, comprising the following steps: obtaining a substrate material; forming a gallium nitride layer is on the substrate material; forming a first aluminum-gallium-nitride layer on the gallium nitride layer, wherein the content of the aluminum changes linearly from the part near to the gallium nitride layer to the part away from the gallium nitride layer; and forming a second a second aluminum-gallium-nitride layer on the first aluminum-gallium-nitride layer, wherein the content of the aluminum changes linearly from the part near to the first aluminum-gallium-nitride layer to the part away from the first aluminum-gallium-nitride layer according to the direction which is opposite to the linear change direction of the content of aluminum in the first aluminum-gallium-nitride layer. In the embodiment of the invention, because the aluminum-gallium-nitride layers with the content of aluminum changing linearly are formed on the gallium nitri