A novel breakdown (BD) mechanism of shallow impurity (SI) under the electric field at low temperatures is suggested for n − GaAs samples with the donor concentrations N D = 10 14 ÷ 10 16 cm −3 and the compensation degree K = N A N D = 0.3 ÷ 0.8 with acceptors of concentration N A in the external magnetic fields up to H = 6.5 T , oriented toward parallel or perpendicular to the external electric field. Diagnosis of the BD mechanism was performed by SI Zeeman (mainly from the ground state 1s to the 2p +1 and other excitation states) and cyclotron resonance photoelectric spectroscopy (PES) methods in the wide interval of the electric field including the BD region too.The obtained results reveal that the BD electric field E BD does not correlate with K and the carriers mobility µ of the samples, which contradict to the well-known impact ionization mechanism (IIM).A serious discrepancy with IIM is that, E BD does not almost depend on the magnetic field up to H = 6.5 T when 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 E < 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 → 2p +1 and CR lines in the electric fields E ≤ E BD and at different magnetic fields, applied along H E or perpendicular H ⊥ E to the electric field. The slope of the 1s → 2p +1 line intensity on the electric field for E H does not depend on magnetic field, which is valid for E BD too. Various effects determined in the PES measurements at E = E BD , such as a drastic narrowing the 1s → 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. The FC screening at the BD reduces the potential fluctuation and its influence to the PES line-shape of 1s → 2p +1 and other excited states. It is shown that an increase in the FC concentration reduces the CCS, which can be assumed as the main factor along with the increase in the ionization coefficient for the SI breakdown in the electric field. The screening length r s of the SI Coulomb potential decreases with increasing the FC concentration, reducing the CCS; the latter seems to vanish completely at r s = a * B (a * B is the effective Bohr radius), when high screening results in vanishing of all the bound states of the Coulomb potential. Note that this limit is similar to the Mott transition. Many experimental facts and our calculation of the CCS support the suggested mechanism for the SI breakdown. The well-known IIM is valid for samples with SI concentrations V (r) = e/(ǫ 0 r) + ∆V at small distances from pure Coulomb form. The value of the CS for SI in n − GaAs can be estimated to be ∆E ∼ (a 0 /a * B ) 2 ǫ...
