In this article, enhancement-mode thin-film transistors (TFTs) with atomic layer deposition (ALD)-derived ultrathin (<inline-formula> <tex-math notation="LaTeX">\approx</tex-math> </inline-formula>3 nm) amorphous indium-zinc oxide (a-IZO) channel were demonstrated. Our devices showed improved device characteristics as benchmarked with thicker IZO thin-film channels. The ALD-deposited IZO channel TFT with an In/Zn ratio of <inline-formula> <tex-math notation="LaTeX">\approx</tex-math> </inline-formula>6:4 exhibited a high field-effect channel mobility (<inline-formula> <tex-math notation="LaTeX">\mu_{\text{FE}}\text{)}</tex-math> </inline-formula> of 53.6 cm<inline-formula> <tex-math notation="LaTeX">^{\text{2}}</tex-math> </inline-formula>/V-s, a threshold voltage (<inline-formula> <tex-math notation="LaTeX">\textit{V}_{\text{th}}\text{)}</tex-math> </inline-formula> of 0.28 V, a low subthreshold gate swing of 74 mV/decade, an <inline-formula> <tex-math notation="LaTeX">I_{\biosc{on}}/I_{\biosc{off}}</tex-math> </inline-formula> ratio of <inline-formula> <tex-math notation="LaTeX">></tex-math> </inline-formula>10<inline-formula> <tex-math notation="LaTeX">^{\text{9}}</tex-math> </inline-formula>, and a contact resistance of 0.18 k<inline-formula> <tex-math notation="LaTeX">\Omega </tex-math> </inline-formula>-<inline-formula> <tex-math notation="LaTeX">\mu </tex-math> </inline-formula>m after 300 <inline-formula> <tex-math notation="LaTeX">{^{\circ}}</tex-math> </inline-formula>C anneal in oxygen atmosphere. Physical analysis, including X-ray and ultraviolet (UV) photoelectron spectra of IZO films, was conducted to understand the mechanisms of enhancement in electrical performance after annealing. The threshold voltages of the TFT also exhibited high stability (<inline-formula> <tex-math notation="LaTeX">\Delta\textit{V}_{\text{th, PBS}}</tex-math> </inline-formula> <inline-formula> <tex-math notation="LaTeX"><</tex-math> </inline-formula> 16 mV and <inline-formula> <tex-math notation="LaTeX">\Delta\textit{V}_{\text{th, NBS}}</tex-math> </inline-formula> <inline-formula> <tex-math notation="LaTeX"><</tex-math> </inline-formula> 12 mV) after positive bias stress (PBS) and negative bias stress (NBS) test for 3600 s. To the best of our knowledge, we reported the TFT with thinnest IZO ternary oxide semiconductor (OS) channel exhibiting superior channel mobility and subthreshold characteristics.