Herein, we proposed a simple 3‐(aminopropyl) triethoxysilane/polyethylene glycol (APTES/PEG)‐AlGaN/GaN high electron mobility transistors (HEMT) biosensor that enabled direct detection in high ionic strength solutions. Unlike previous modification strategies, the PEG‐modified on the GaN surface established a stable screening system by forming a porous biopolymer layer, which can overcome Debye shielding. In our work, the APTES/PEG modification enabled immunoglobulin G (IgG) to be easily detected in solutions with concentrations of up to 30 mM (3x) of phosphate buffered saline (PBS), indicating that the APTES/PEG modification strategy had the potential for direct detection of biomolecules in physiological solutions, which provided a new method for bio‐surface modification on GaN and advanced the practical application of AlGaN/GaN HEMT biosensors.