Tantalum (Ta)-doped hafnium dioxide (HfO₂, THO) gate dielectric layers were fabricated using pulsed laser deposition (PLD). The dielectric properties of these layers and the electrical characteristics of amorphous indium gallium zinc oxide (α-IGZO) thin-film transistors (TFTs) incorporating them were systematically investigated. Capacitance-voltage measurements of THO films deposited under varying oxygen partial pressures (0%, 10%, 25%, and 50%) revealed that the film prepared at 25% oxygen partial pressure exhibited the optimal dielectric performance, demonstrating the lowest capacitance equivalent thickness of 6.5 nm and the highest equivalent dielectric constant of 23.7. Ta doping significantly enhanced the dielectric properties compared to pristine HfO₂ gate dielectrics. Furthermore, α-IGZO TFTs employing this optimized THO (25%, O₂) gate dielectric demonstrated excellent device performance: a low threshold voltage of 0.57 V, a high saturation mobility of 22.5 cm²/(V·s), and a high on/off current ratio of 2.5 × 10⁹. These key performance metrics are significantly superior to those of control devices utilizing pure HfO₂ gate dielectrics.