High Field Temperature-Independent Field-Effect Mobility of Amorphous Indium–Gallium–Zinc Oxide Thin-Film Transistors: Understanding the Importance of Equivalent-Oxide-Thickness Downscaling

We investigate the temperature dependence of field-effect mobility (μ
_eff
) on amorphous indium-gallium- zinc oxide (α-IGZO) thin-film transistors (TFTs) having 10-nm-thick HfO
₂
. Thanks for the downscaling of the equivalent oxide thickness (EOT), we observe, for the first time, a temperature-independent μ
_eff
at a high field or high carrier concentration (N
_carrier
) regime and a strong temperature dependent μ
_eff
at a low field or low Ncarrier regime. The observation highlights the importance of EOT downscaling to enhance N
_carrier
and facilitate the Fermi level (EF) to approach and even exceed the potential barriers' peak (EM), leading to a high and temperature-independent μ
_eff
at high N
_carrier
. N
_carrier
at which E
_F
= E
_M
is extracted to be ~6 × 10
¹²
cm
^-2
. A detailed study of the relationship between EOT scaling and the gate bias voltage (V
_GS
) at which EF equals to E
_M
is performed. α-IGZO TFTs with an ultra scaled gate dielectric are capable of alleviating the negative effects from the potential barriers at low VGS and could enable low power applications with high performance. Besides, a weak temperature-dependent subthreshold swing (SS) of α-IGZO TFT was observed and explained.