Threshold-Voltage Extraction Methods for Atomically Deposited Disordered ZnO Thin-Film Transistors.

Area of Science:

• Materials Science
• Semiconductor Physics
• Electronics Engineering

Background:

• Zinc oxide (ZnO) thin-film transistors (TFTs) are widely used due to their n-type enhancement-mode characteristics.
• However, a gate-voltage-dependent and unreliable threshold voltage in ZnO TFTs hinders accurate performance assessment.
• Localized trap states within ZnO are identified as a primary cause for this threshold voltage instability.

Purpose of the Study:

• To develop and validate a robust method for extracting the threshold voltage in atomic-layer-deposited ZnO TFTs.
• To investigate the influence of localized trap states on the field-effect mobility and threshold voltage.
• To confirm the reliability of the extracted threshold voltage through temperature-dependent characterization.

Main Methods:

• Derivation of a current-voltage (I-V) relationship by dividing drain current with transconductance to eliminate gate-bias-dependent factors.
• Fabrication of bottom-gate atomic-layer-deposited ZnO TFTs.
• Analysis of temperature-dependent characteristics, including activation energy measurements.

Main Results:

• A reliable threshold voltage extraction method was successfully developed and implemented for ZnO TFTs.
• Field-effect mobility was accurately modeled using a gate-bias-dependent power law, accounting for trap states.
• Low-temperature measurements revealed an abrupt decrease in activation energy at the threshold voltage, confirming its validity and indicating a shift in conduction mechanisms.

Conclusions:

• The proposed method effectively extracts a reliable threshold voltage for accumulation-mode ZnO TFTs.
• The technique removes gate-bias-dependent factors, overcoming limitations of previous methods.
• Temperature-dependent analysis validates the extracted threshold voltage and provides insights into charge transport mechanisms in ZnO TFTs.