The impurity properties in n-type N doped single crystal 6H-SiC semiconductors have been studied by conductivity measurements. A slow decay of the dark current and persistent photoconductivity (PPC) have been observed at low temperatures. Different energy barriers have been obtained at different conditions. Measured donor binding energy, the electron capture barrier, and the emission barrier are 34.6±1.0, 14.9±0.3, and 45.6±1.2 meV, respectively. Our results suggest that PPC in SiC has a similar origin as that in AlxGa1-xAs. However the amount of lattice relaxation in SiC is much smaller compared with that in AlxGa1-xAs. Experimental results reported in this letter will help to understand the properties of lattice relaxed impurities. It also explains why different energy levels have been obtained previously by different methods.