Planar silicon carbide waveguides are proposed to be fabricated on a silicon substrate with an buried SiO2 isolation layer. For a broad wavelength range, the waveguide structure exhibits low loss for fundamental modes and high loss for higher-order modes, which is extremely useful for waveguiding either over the communication wavelength range 1.3-1.6 mum or for light emission from porous silicon at 0.7 mum. Singlemode large cross-section rib waveguides based on the proposed planar structure are also analyzed. Such waveguides have potential application for silicon-based optoelectronic devices.