Pn-junction type gas sensors, operated at 500degreesC, have been fabricated on 6H-SiC substrates with both ion implantation and laser processing techniques. The device structures are composed of catalytic metal layer/ohmic contact layer/SiC pn-junction/ohmic back contact layer. Ohmic contact layers play roles for adhesion and barrier layer to the catalytic metal material. Shallow p-type layers made with Al-doping in n-type SiC epi-layers show good responses to hydrogen, hydrocarbons, and ammonia dispersed in N-2 gas ambient. Device stability wholly depends on the contact stability at elevated temperature.