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A novel breakdown (BD) mechanism of shallow impurity under the electric fieldat low temperatures is suggested for n-GaAs samples with the donorconcentrations $N_D=10^{14} \div 10^{16}cm^{-3}$ and the compensation degree$K=\frac{N_A}{N_D}=0.3 \div 0.8$ with acceptors of concentration N_A in theexternal magnetic fields up to H=6.5~T. Diagnosis of the BD mechanism wasperformed by SI Zeeman and cyclotron resonance photoelectric spectroscopymethods in the wide interval of the electric field including the BD region too.The obtained results reveal that the BD electric field $\mathcal{E}_{BD}$ doesnot correlate with $K$ and the carriers mobility $\mu$ of the samples, whichcontradict to the well-known impact ionization mechanism (IIM). A seriousdiscrepancy with IIM is that, $\mathcal{E}_{BD}$ does not almost depend on themagnetic field up to H=6.5~T when $\mathcal{E}\| H$, though the SI ionizationenergy increases two times. The cyclotron resonance (CR) measurements show thatthe 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 freecarriers' capture cross section (CCS) by ionized SI centers with a subsequentincrease in the FC concentration n is observed by means of PES investigation ofthe $1s \to 2p_{+1}$ 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 CRlines, a shift of the CR line to higher magnetic fields and disappearing of thelines to higher excited SI states, were clarified to be a result of screeningof SI Coulomb potential by free carriers.

Free electron screening mechanism of the shallow impurity breakdown in  n-GaAs: evidences from the photoelectric Zeeman and cyclotron resonance  spectroscopies