Manufacturing SiC-based GaN HEMT devices is challenging due to limited process windows, which results in low process yields and high costs. In this paper, process improvement techniques have been proposed by using a composite algorithm to address the issue of wafer-level CD non-uniformity in the lithography process. This algorithm is composed of a compensation module and a classification module. The compensation module corrects the exposure conditions by analyzing the Focus-Exposure Matrix (FEM), and the classification module is utilized to minimize redundancy in global exposure conditions. The effectiveness of the composite algorithm has been verified by applying it to two representative photolithography processes in the fabrication of HEMT devices. From the experiments, the performance of wafer-level CD has been improved, reducing the uniformity from over 10 % to 2 %, while the CPK has increased from 0.2 to over 2.