Shallow Trap States Control Electrical Performance of Amorphous Oxide Semiconductor Thin‐Film Transistors

ABSTRACT The performance of n ‐type amorphous oxide semiconductor thin‐film transistors (TFTs) is controlled by the electronic states near the conduction band mobility edge. Here, the full subgap density of states (DoS) of amorphous InGaZnO (a‐IGZO) TFTs, used in display panels and dynamic random‐access memory (DRAM) development, is measured by ultrabroadband photoconduction (UP‐DoS) microscopy to within 0.1 eV of the mobility edge. The measured subgap DoS for 25 TFT processing conditions accurately predicts each transfer curve, showing how shallow defect states are electron traps that rigidly tune subthreshold swing, threshold voltage, and drift mobility. For a set of TFTs, the subthreshold transfer characteristics can be simulated independently by the experimental shallow‐defect DoS, with no adjustable parameters. The full transfer curve is simulated by introducing a single parameter: the conduction band tail energy. Additionally, each simulation reveals how the shallow trap density controls subthreshold behavior and can be directly extracted from transfer curves. Finally, a systematic In‐enrichment study, combined with DFT+U DoS simulations, enables identification of vacancy‐cation coordination environments for experimentally observed subgap peaks. The dominant trap controlling conventional a‐IGZO TFT performance is centered at 0.32 eV below the conduction band mobility edge and is attributed to a Ga‐Ga‐In oxygen‐vacancy defect.