4.3 THz quantum-well photodetectors with high detection sensitivity *Project supported by the National Key R&D Program of China (Grant No. 2017YFF0106302), the National Basic Research Program of of China (Grant No. 2014CB339803), the National Natural Science Foundation of China (Grant Nos. 61404150, 61405233, and 61604161), and the Shanghai Municipal Commission of Science and Technology, China (Grant Nos. 15JC1403800, 17ZR1448300, and 17YF1429900). (March 2018)