Ionization of DX-centers in AlGaAs : Te has been investigated in strong terahertz electric fields of FIR-laser radiation with photon energies much smaller than the impurity binding energy. Detachment of electrons from DX-centers is caused by phonon-assisted tunneling being independent of the field frequency as long as the tunneling time is smaller than the field period. In the opposite case an enhancement of the emission probability with rising frequency has been observed. At very high-field strengths direct tunneling without involving phonons dominates and finally emission rates get frequency independent.