  We study electrical transport properties in exfoliated molybdenum disulfide
(MoS2) back-gated field effect transistors at low drain bias and under
different illumination intensities. It is found that photoconductive and
photogating effect as well as space charge limited conduction can
simultaneously occur. We point out that the photoconductivity increases
logarithmically with the light intensity and can persist with a decay time
longer than 10^4 s, due to photo-charge trapping at the MoS2/SiO2 interface and
in MoS2 defects. The transfer characteristics present hysteresis that is
enhanced by illumination. At low drain bias, the devices feature low contact
resistance of 1.4 k{\Omega}/{\mu}m, ON current as high as 1.25 nA/{\mu}m, 10^5
ON-OFF ratio, mobility of 1 cm^2/Vs and photoresponsivity R=1 A/W.
