The investigation results of the RTP enhanced diffusion of P in Si and Zn in GaAs, the mechanism, models and the role of quantum effects are presented in this paper. Shallow and ultra-shallow p + -n, n - -p junctions have been obtained and analyzed. The experimental concentration profiles were simulated based on the dissociative diffusion mechanism. The diffusion coefficients and activation energies of the RTP enhanced diffusion and conventional furnace annealing was analyzed. The activation energy of RTP diffusion is lower than the conventional furnace diffusion and diffusion coefficient is higher by 1-3 order of magnitude. The p-n junctions with depth of 0.02 - 0.4 mum have been obtained by RTP for 0.1 - 3min diffusion time.