We investigated the radiation mechanisms of THz radiation from semiconductor surfaces at high-density excitation under a magnetic field. Excitation density dependences of radiation intensity and the waveforms of the terahertz radiations from InAs and semi-insulating InP surfaces were investigated with and without magnetic fields (0, 2T, and - 2T). Substantial changes of the intensity and the waveforms including a polarity reversal were observed by changing the excitation densities. In InAs, the enhancement of the radiated energy is observed under a magnetic field of ±2 T and the radiated energy increases quadratically with increasing the excitation density below 0.1 μJ/cm2. The behavior of the dependence for ±2 T changes clearly above 1 μJ/cm2. The drastic change of the wave forms was observed at high density excitation and was explained by the polarity reversal of the THz wave induced by the magnetic field. The reversal originates from the crossover of the radiation mechanism of the magnetic induced component from the electrons in the accumulation layer to the diffusion current by the photogenerated electrons at high-density excitation under a magnetic field. In InP, the characteristic behavior including the polarity reversal of the angle independent component was observed in the crystal orientation angle dependence by changing the excitation density. These facts indicate that three different radiation mechanisms co-exist and that the dominant radiation mechanism changes with increasing the excitation density from the drift current for low-excitation density to the diffusion current and the optical rectification for high-excitation density.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.