In this paper, we present the current research efforts on the atomic layer deposition (ALD) ZnO based TFT devices carried out in our laboratory. ZnO thin film deposition was carried out by two different ALD processes; thermal ALD using water as a reactant and plasma-enhanced ALD using oxygen plasma as a reactant. The film properties were comparatively studied showing large difference in terms of electrical properties. For thermal ALD ZnO, carrier concentrations were too high to fabricate well-operated ZnO TFTs. To control the carrier concentration, nitrogen doping was utilized based on NH4OH reactant. Meanwhile, for PE-ALD, highly resistive films were obtained at low growth temperature below 200°C. To reduce the resistivity to a proper level for the fabrication of TFTs, UV-light exposure was used. At properly controlled conditions, high performance TFT devices were fabricated based on these processes. ZnO TFTs were also fabricated on flexible substrates and the initial research was carried out on the effects of device bending on device properties.