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Semiconductor quantum dot detectors as well as semiconductor charge-sensitive infrared phototransistors are described. They are the only detectors that can count single photons in the terahertz region at present. In terms of the noise equivalent power (NEP), the detectors realize experimental values on the order of 10-21 W/Hz1/2, while theoretically expected values are even much lower, on the order of 10-24 W/Hz1/2. These NEP values are by several orders of magnitude lower than any other state-of-the-art highly sensitive detectors. In addition to the outstanding sensitivity, the detectors are featured by strong advantage of huge current responsivity (106-1010 A/W ) and extremely large dynamic range of response (106-108). The mechanism of detection as well as application of the detectors is discussed.