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Carbon nanotube (CNT) based infrared (IR) detectors have been reported and shown promising properties taking advantage of its one dimensional structure and unique electrical properties. The CNT photodiodes detect IR signal through Schottky barriers that are formed when metal and semiconductor CNT come into contact due to the difference of the energy levels. However, the generated photocurrent was small due to the structure of the CNT photodiodes, consisting of two reversely connected Schottky diodes. In addition, the mechanism of the Schottky barriers of these photodiodes were not well understood. In this paper, a CNT Field Effect Transistor (CNTFET) is designed and used as an IR detector, improving the performance by tuning the doping level of the CNT through the gate.By introducing the gate as in CNTFET to the photodiodes, the Fermi levels of the CNTs are able to be adjusted by the electrostatic doping, as a result the dark currents of CNT photodiodes can be screened. On the other hand, the photocurrent is determined by the capability of Schottky barriers to separated photogenerated electron and hole pairs, thus gate can shift the Fermi level of a CNT to the most sensitive level in order to produce a maximum photocurrent. The test results from the proposed CNTFET IR detectors provided a better understanding of its working principle in order to inspire optimal design of a CNT photodetector. The experimental results showed the photocurrent was proportional to the depletion width of Schottky barriers.