Thin film transistors (TFTs) have been fabricated with a zinc oxide (ZnO) channel layer and a hafnium dioxide (HfO2) gate dielectric layer. The oxide layers were deposited using an atomic layer deposition (ALD) system. The use of ALD for ZnO deposition allows subnanometer thickness control of the deposited layer, and thereby provides a means to vary TFT threshold voltage by controlling the carrier density in the ZnO channel: the carrier density is dependent on the layer thickness because band structure changes result in charge depletion in thinner layers. Enhancement-mode devices have been fabricated and have an on-off current ratio above 106. The enhancement-mode devices of the inverted (gate down) TFT structure were realized by decreasing the ZnO channel layer thickness to 15 nm and below, thereby reducing the carrier density of the as-deposited n-type ZnO layer. An important aspect of the fabrication of the inverted TFTs was the use of either an aluminum sacrificial layer or a thin HfO2 cap layer to eliminate the etching of the ZnO during the photolithography process. The results demonstrate that enhancement-mode TFTs with as-deposited n-type ZnO channels can be produced by tailoring the thickness of the ZnO channel by ALD and are the first reported TFTs in which both the ZnO channel and HfO2 gate dielectric are deposited by ALD to the authors’ knowledge.