In this investigation, the carrier concentration gradient between channel and contact region is achieved to improve the Thin film Transistors (TFT) performance by employing annealing at 350 degrees C in forming gas (N-2 + 5% H-2). The contact region is covered with Mo metal and the channel region is only exposed to forming gas to facilitate the diffusion controlled reaction. The TFT using a-IGZO active layer is fabricated in ambient of Ar:O-2 in ratio 60:40 and the conductivity of the order of 10 (-3) S/cm is measured for as-deposited sample. The electrical conductivity of an annealed sample is of the order of 10(2) S/cm. The device performance is determined by measuring merit factors of TFT. The saturation mobility of magnitude 18.5 cm(2)V(-1) s(-1) has been determined for W/L (20/10) device at 15 V drain bias. The extrapolated field effect mobility for a device with channel width (W) 10 mm is 19.3 cm(2)V(-1) s(-1). The on/ off current ratio is 109 and threshold voltage is in the range between 2 and 3 V. The role of annealing on the electronic property of a-IGZO is carried out using X-ray photoelectron spectroscopy (XPS). The valance band cut-off has been approximately shifted to higher binding energy by 1 eV relative to as-deposited sample. (C) 2015 Elsevier B.V. All rights reserved.