Mobility Enhancement of BCE-Type Amorphous InGaZnO TFTs Using Triple-Layer Channels

The back-channel-etch-type amorphous InGaZnO (a-IGZO) triple-layer thin-film transistor (TL-TFT) consists of a top barrier and a bottom barrier deposited with oxygen flow (OF) and an a-IGZO main channel deposited without OF in between. The TL-TFT has 1.7<inline-formula> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> and 1.3<inline-formula> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> electron mobilities for the bottom gate (BG) operation as compared to the single-layer and double-layer channel TFTs, respectively. The conduction band difference between the barrier layers and the main channel is high enough to confine the carriers in the main channel. The bottom barrier decreases the Coulomb scattering and the surface roughness scattering to increase the mobility, while the top barrier decreases the plasma-induced damage in the channel and the bottom barrier. However, both the hysteresis and subthreshold swing (S.S.) increase if only the bottom barrier is adopted double-layer TFT (DL-TFT) because the shallow states in the IGZO bottom barrier provide extra tunneling paths for the electrons to be trapped inside the BG oxide. With additional top barrier structure, plasma-induced damage in the main channel and the bottom barrier can be mitigated to reduce the degradation of hysteresis and S.S. Moreover, the TL-TFTs demonstrate better reliability under both positive bias stress and negative bias illumination stress than single-layer TFTs (SL-TFTs) and DL-TFTs.