We demonstrate that doping the semiconductor zinc tin oxide (ZTO) with yttrium leads to a high-voltage thin film transistor (HVTFT) with enhanced switching performance. Adding 5% yttrium leads to an increase in the on-off ratio from 40 to 1000 at an operating voltage of 500 V and to a drop of the subthreshold swing from 65 to 35 V/dec. The performance is improved because of the reduction of the saturation voltage and because of a decrease in the off-current from several μA for undoped ZTO HVTFTs to 100 nA for Y5%ZTO. The decrease in saturation voltage and off-current can be attributed to a lower trap concentration leading to enhanced space-charge limited current and to a decrease in the background charge carrier concentration. At a 500 V bias voltage, an inverter circuit with a yttrium-doped ZTO HVTFT can control the output voltage between 50 V and 500 V, while the undoped ZTO HVTFT can only control the output voltage between 150 V and 450 V. The improvement in high voltage performance of yttrium-doped ZTO HVTFTs is important for future work related to high voltage thin film transistors made of amorphous oxide semiconductors as it demonstrates that this technology enables HVTFTs with simultaneously high operation voltage, high on-current, and high on-off ratio.