  A novel breakdown (BD) mechanism of shallow impurity under the electric field
at low temperatures is suggested for n-GaAs samples with the donor
concentrations $N_D=10^{14} \div 10^{16}cm^{-3}$ and the compensation degree
$K=\frac{N_A}{N_D}=0.3 \div 0.8$ with acceptors of concentration N_A in the
external magnetic fields up to H=6.5~T. Diagnosis of the BD mechanism was
performed by SI Zeeman and cyclotron resonance photoelectric spectroscopy
methods in the wide interval of the electric field including the BD region too.
The obtained results reveal that the BD electric field $\mathcal{E}_{BD}$ does
not correlate with $K$ and the carriers mobility $\mu$ of the samples, which
contradict to the well-known impact ionization mechanism (IIM). A serious
discrepancy with IIM is that, $\mathcal{E}_{BD}$ does not almost depend on the
magnetic field up to H=6.5~T when $\mathcal{E}\| H$, though the SI ionization
energy increases two times. The cyclotron resonance (CR) measurements show that
the line width does not depend on the electric field for
$\mathcal{E}<\mathcal{E}_{BD}$ indicating the lack of free carriers (FC)
heating in contradiction with IIM. A considerable decrease of the free
carriers' capture cross section (CCS) by ionized SI centers with a subsequent
increase in the FC concentration n is observed by means of PES investigation of
the $1s \to 2p_{+1}$ and CR lines in the electric fields $\mathcal{E} \leq
\mathcal{E}_{BD}$ and at different magnetic fields, applied along $H\|
\mathcal{E}$ or perpendicular $H\perp \mathcal{E}$ to the electric field.
Various effects determined in the PES measurements at $\mathcal{E} =
\mathcal{E}_{BD}$, such as a drastic narrowing the $1s \to 2p_{+1}$ and CR
lines, a shift of the CR line to higher magnetic fields and disappearing of the
lines to higher excited SI states, were clarified to be a result of screening
of SI Coulomb potential by free carriers.
