The tunnelling ionization of DX- centers was observed in THz radiation fields. Tunnelling times were measured for auto-localized and on-site deep impurities. It was shown that, in one case, the tunnelling time was shorter than the reciprocal temperature multiplied by a universal constant. In the other case, it was longer. This was attributed to a difference in tunnelling trajectories. This made it possible to distinguish, in a direct manner, between the 2 types of impurity configuration potential.

S.D.Ganichev, I.N.Yassievich, W.Prettl, J.Diener, B.K.Meyer, K.W.Benz: Physical Review Letters, 1995, 75[8], 1590-3