# Optoelectronic Characterization of Trap Density of States in Indium Gallium Oxide Thin-Film Transistors and Their Impact on Bias Stability

## TL;DR

This paper introduces a new type of transistor using indium-gallium oxide that offers high performance and stability for display technology.

## Contribution

The study presents zinc-free crystalline indium–gallium oxide TFTs with high mobility and bias stability, suitable for single-material backplanes.

## Key findings

- Zinc-free crystalline IGO TFTs achieve high mobility (83.3 cm²/V·s) and low off-current.
- Crystalline IGO TFTs show minimal threshold-voltage shift (<0.5 V) under bias stress.
- Trap density of states analysis reveals stable electrical behavior due to oxygen-vacancy-related states near the conduction band.

## Abstract

Amorphous oxide semiconductors have become the industry
standard
for display backplanes, but their limited mobility necessitates complex
low-temperature polycrystalline oxide (LTPO) stacks, increasing cost
and reducing yield. To realize a practical oxide-only backplane that
can serve as both drive and switching transistors, a channel combining
high mobility, low off current, and robust stability is required.
Here, we report zinc-free crystalline indium–gallium oxide
(IGO) thin-film transistors (TFTs) that crystallize below 400 °C
with preferential (222) orientation. By tuning the In:Ga ratio, an
amorphous-to-crystalline transition is achieved, enhancing mobility
to 83.3 cm2 V–1 s–1 while maintaining an off-current of ≈10–13 A. The optimized crystalline IGO TFTs (In:Ga = 12:3) exhibit the
smallest threshold-voltage shift (<0.5 V) under bias stress and
reproducible electrical characteristics across multiple devices. Furthermore,
we provide experimental investigation of the observed stability by
analyzing the trap density of states (TDOS) near both the valence-band
maximum (VBM) and conduction-band minimum (CBM) using a combined optoelectronic
approach based on photoexcited charge collection spectroscopy (PECCS)
and photoresponse capacitance–voltage (C–V). The complementary
analysis reveals that crystalline IGO stabilizes oxygen-vacancy-related
states closer to the conduction band, with the extracted TDOS evolution
showing self-consistent correlation with the measured bias-stress-induced
electrical behavior. Circuit-level validation with a complementary
inverter confirms stable gain and noise margins. These results establish
crystalline IGO as a viable single-material oxide channel combining
high mobility, robust bias stability, and simplified processing for
next-generation oxide-only backplanes.