Epitaxial structure having diffusion barrier layer

An epitaxial structure, comprising:

• a substrate;
• a buffer layer disposed on the substrate;
• a channel layer disposed on the buffer layer;
• a barrier layer disposed on the channel layer;
• and
• a P-type gallium nitride layer disposed on the barrier layer, and the P-type gallium nitride layer having a first lattice constant;
• wherein the epitaxial structure further comprises a diffusion barrier layer disposed between the barrier layer and the P-type gallium nitride layer, the diffusion barrier layer includes a chemical composition of Inx1Aly1Gaz1N, where x1+y1+z1=1, 0≤x1≤0.3, 0≤y1≤1.0, and 0≤z1≤1.0;
• wherein the chemical composition of the diffusion barrier layer has a proportional relationship, so that the diffusion barrier layer has a second lattice constant that matches the first lattice constant, and the second lattice constant is between 80% and 120% of the first lattice constant;
• wherein the barrier layer includes a chemical composition of Aly2Gaz2N, where y2+z2=1, 0.1≤y2≤0.3, and 0<z2<1;
• wherein the diffusion barrier layer including the chemical composition of Inx1Aly1Gaz1N is directly disposed on and connected to the barrier layer including the chemical composition of Aly2Gaz2N;
• wherein at least a part of the aluminum (Al) content of the diffusion barrier layer is not less than 0.4 and is greater than the aluminum (Al) content of the barrier layer;
• wherein the diffusion barrier layer is a multi-layer structure that has four layers to ten layers, and the aluminum (Al) content of the diffusion barrier layer is decreased along a thickness growth direction of the diffusion barrier layer;
• wherein an initial content of the aluminum (Al) content along the thickness growth direction is between 60% and 80%, a final content of the aluminum (Al) content along the thickness growth direction is between 0% and 20%, and a step change rate of the aluminum (Al) content in the multi-layer structure is reduced by 5% to 25% per step.