We report ultrathin (

${\sim }4$

nm) channel indium tin oxide (ITO) transistors, comparing different precursors for atomic layer deposition (ALD) of the Al2O3 top-gate dielectric, and analyze the role of dielectric deposition on transistor performance and gate bias stress stability. Water-based ALD leads to very negative threshold voltage (

${V}_{\text {T}}$

), with devices remaining in the on-state. In contrast, both ozone and O2-plasma precursors yield devices that can turn off, but ozone-based ALD devices have less negative

${V}_{\text {T}}$

shift at short channel lengths, and relatively more positive

${V}_{\text {T}}$

at all channel lengths. We achieve maximum drive current,

${I}_{\text {max}} \approx 260 \mu \text{A}/\mu \text{m}$

${V}_{\text {DS}}=$

1 V, on/off current ratio of

$10^{{10}}$

(limited by the instrument’s noise floor) for

${L} \approx 700$

nm ozone-Al2O3 top-gated transistors. Across multiple devices, the effective mobility is

$\sim $

42 cm

$^{{2}}\text{V}^{-{1}}\text{s}^{-{1}}$

and contact resistance is

$\sim 376 \Omega \cdot \mu \text{m}$

. The transistors also show good gate bias stability with normalized

${V}_{\text {T}}$

shift of +0.12 V(MV/cm)

$^{-{1}}$

at gate stress field >3 MV/cm, a

$\sim 3\times $

improvement vs. our previous reports of uncapped ITO transistors.

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