  We report on the observation of terahertz (THz) radiation induced
band-to-band impact ionization in \HgTe quantum well (QW) structures of
critical thickness, which are characterized by a nearly linear energy
dispersion. The THz electric field drives the carriers initializing
electron-hole pair generation. The carrier multiplication is observed for
photon energies less than the energy gap under the condition that the product
of the radiation angular frequency $\omega$ and momentum relaxation time
$\tau_{\text l}$ larger than unity. In this case, the charge carriers acquire
high energies solely because of collisions in the presence of a high-frequency
electric field. The developed microscopic theory shows that the probability of
the light impact ionization is proportional to $\exp(-E_0^2/E^2)$, with the
radiation electric field amplitude $E$ and the characteristic field parameter
$E_0$. As observed in experiment, it exhibits a strong frequency dependence for
$\omega \tau \gg 1$ characterized by the characteristic field $E_0$ linearly
increasing with the radiation frequency $\omega$.
