The imager consists of a planar semiconductor-gas discharge (SGD) cell allowing the ultra fast IR-to-visible conversion with response time on the microsecond scale. The semiconductor wafer is made of Si:Zn providing the spectral range of 1.1 - 3.5 micrometers . The 100 micrometers discharge gap is filled with Ar under the pressure of 100 hPa. The cell is cooled down approximately to 90 K. Among studied properties are noise, both in time and space domains, detectivity, noise equivalent irradiance and, when applying the imager in a thermal imaging system, noise equivalent temperature difference (NETD). Investigations of the spatial noise and NETD have been carried out by using a low-noise CCD camera capturing output images of the SGD cell. For measuring the temporal noise, a low-noise photomultiplier is used to detect gas discharge radiation from the area of about one resolved pixel. The own noise of the SGD cell is found by comparing signal-noise dependencies obtained at acquiring outgoing light of the cell, on the one hand, with those at observing a thermal radiation source with well describable photon noise, on the other hand. The results indicate that the imager has surprisingly low noise which is very close to the photon-noise limit.