The influence of MBE and device structure on the electrical properties of GaAs HEMT biosensors *Project supported by the National Key Research and Development Program of China (No. 2017YFB0405400), the Open Research Fund Program of the State Key Laboratory of Virology of China (No. 2017IOV002), the National Natural Science Foundation of China (Nos. 61274049, 61404130, 61574140), and the Shenzhen Science and Technology Innovation Commission (No. JSGG20160608100922614). (December 2018)