The hot-electron effect on electron transport in nominally undoped ZnO epilayers is studied at electric fields up to $430\pm 50$ kV/cm applied with 3 ns voltage pulses. The transient measurements do not demonstrate any change in the electron density at the fields up to 320 kV/cm. The deviation from Ohm’s law can be approximated by the linear dependence of the current on the electric field valid at the moderate fields (from 50 to 250 kV/cm). The model calculations based on the Boltzmann kinetic equation are used to demonstrate that the differential mobility at the moderate fields is almost independent of the scattering on the charged point defects. The electron drift velocity is estimated from the experimental values on the differential conductivity and the current density together with the calculated differential mobility as a reference for calibration. The drift velocity reaches $2.7\pm 0.3\times {10}^7$ cm/s at the field of $\sim 320$ kV/cm and approaches the theoretical limit predicted by the known Monte Carlo simulations.

Current–voltage characteristics have been measured for ZnO:Ga and Zn:Sb epitaxial layers with electron densities ranging from cm⁻³ to cm⁻³. Two-terminal samples with coplanar electrodes demonstrate virtually ohmic behavior until thermal effects come into play. Soft damage of the samples takes place at high currents. The threshold power (per electron) for the damage is nearly inversely proportional to the electron density over a wide range of electron densities. Pulsed voltage is applied in order to minimize the thermal effects, and thus an average electric field of 150 kV cm⁻¹ is reached in some samples subjected to 2 ns voltage pulses. The results are treated in terms of electron drift velocity estimated from the data on current and electron density under the assumption of uniform electric field. The highest velocity of ~ cm s⁻¹ is found at an electric field of ~100 kV cm⁻¹ for the sample with an electron density of cm⁻³. The non-ohmic behavior due to hot-electron effects is weak, and the dependence of the electron drift velocity on the doping resembles the variation of mobility.

Unexpected irreversible damage appeared in samples while investigating pulsed S-N switching in YBaCuO thin films. Examination of a scanning electron microscope image of a damaged film containing a defect revealed a step-like motion of the N-zone. Restoration of crack movement dynamics points to the formation of coherent jets. This would mean that cumulative convergence of unknown incompressible media took place. The known model of an edge barrier, when the S-N border, characterized by width λ _eff , bounds a semi elliptical N-zone at the film edges, was applied for clarification of the cumulative convergence. In that model, counteracting forces cause a wave-like unstable form of the barrier S-N border, and further current growth increases the amplitude of wave-like distortions of the border. Then, current reconnection separates bulges from the N-zone and induces the appearance of the circular currents with a trapped magnetic flux. Heat release at the places with trapped flux cause an expansion in these locations and coherent jets originate from touch points due to cumulative convergence. We substantiate that specific properties of the S-N border at λ _eff , such as high tensile strength, very high flexibility and also incompressibility, are responsible for the appearance of the cumulative effect. Thus, the N-zone propagation into S state begins by motion of the wave-like border. The non-damaging step-like N-zone propagation into the films without defects is clarified. It is concluded that both the critical current and the damage current are determined by the same barrier with the unstable S-N border, and, therefore, the irreversible damage of the films comes quickly after the current reaches the critical value.