We present investigation of carrier traps and their transport in 4H-SiC single crystals and high energy radiation detectors. SiC detectors have been produced from bulk vanadium-compensated semi-insulating 4H-SiC single crystal. They were supplied with a nickel ohmic contact on the back surface and titanium Schottky contact on the front surface. The prevailing defect levels were revealed by means of thermally stimulated current (TSC) and thermally stimulated depolarization (TSD) methods and their advanced modification ń multiple heating technique. From I-V measurements of the samples a barrier height of ~ 1.9 eV was found. In 4H-SiC:Va, the following thermal activation values were deduced: 0.18-0.19 eV, 0.20-0.22 eV, 0.33-0.41 eV, and 0.63 eV. The maximum with activation energy of 0.33-0.41 eV appears below 125 K and most probably is caused by the thermal carrier generation from defect levels. In contrast, the first two maxima with the lowest activation energies, which nevertheless appear at higher temperatures, are likely associated with material inhomogeneities causing potential fluctuations of the band gap. The existence of different polarization sources in different temperature ranges is also demonstrated by TSD.