Real high voltage transistors can be difficult to find. What I mean by "real" is that the transistor has characteristics similar to low voltage transistors, but just have higher operating voltage capability. For example, some HV transistors have very low beta, and very high leakage current, thereby making them unattractive in many applications, particularly low current applications. Thus, the application spectrum is narrowed somewhat.
Infineon's CoolMOS™ 900V power MOSFETs are the industry’s first 900 V superjunction MOSFETs specifically intended for high-efficiency applications. For example, quasi-resonant flyback designs for LCD TV power supplies can benefit from a higher flyback voltage, which provides a longer primary duty cycle with reduced peak current, true zero-voltage switching and significantly lower voltage stress on the secondary side. Other applications include crowbar circuits and high voltage switching.
Devices with ON resistance of 0.34ohms are available now, and soon to be released are parts with 0.12 ohm ON resistance.
As with many high voltage components, it is important to review the data sheets, since there are often limitations not found with their low voltage brethren. For example, in Infineon IPW90 family, drain-source breakdown voltage rating varies significantly with temperature. While it is 900V at 25C, it goes down to 850V at -20C. Also, drain-source leakage current is just 100nA, but it is specified with no D-S voltage applied. If you look at the breakdown spec, it specifies a maximum of 250 microamps at 900V, however.