Hot-electron energy relaxation time is deduced for Ga-doped ZnO epitaxial layers from pulsed hot-electron noise measurements at room temperature. The relaxation time increases from ∼0.17 ps to ∼1.8 ps when the electron density increases from 1.4 × 10¹⁷cm⁻³ to 1.3 × 10²⁰cm⁻³. A local minimum is resolved near an electron density of 1.4 × 10¹⁹cm⁻³. The longest energy relaxation time (1.8 ps), observed at the highest electron density, is in good agreement with the published values obtained by optical time-resolved luminescence and absorption experiments. Monte Carlo simulations provide a qualitative interpretation of our observations if hot-phonon accumulation is taken into account. The local minimum of the electron energy relaxation time is explained by the ultrafast plasmon-assisted decay of hot phonons in the vicinity of the plasmon–LO-phonon resonance.

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.

Fluctuation technique is used to measure hot-phonon lifetime in dual channel GaN-based configuration proposed to support high-power operation at high frequencies. The channel is formed of a composite Al_0.1Ga_0.9N/GaN structure situated in an Al_0.82In_0.18N/AlN/Al_0.1Ga_0.9N/GaN heterostructure. According to capacitance–voltage measurements and simultaneous treatment of Schrödinger–Poisson equations, the mobile electrons in this dual channel configuration form a camelback density profile at elevated hot-electron temperatures. The hot-phonon lifetime was found to depend on the shape of the electron profile rather than solely on its sheet density. The camelback channel with an electron sheet density of 1.8 × 10¹³ cm⁻² demonstrates ultrafast decay of hot phonons at hot-electron temperatures above 600 K: the hot-phonon lifetime is below ∼60 fs in contrast to ∼600 fs at an electron sheet density of 1.2 × 10¹³ cm⁻² obtained in a reference Al_0.82In_0.18N/AlN/GaN structure at 600 K. The results suggest a suitable method to increase the electron sheet density without the deleterious effect caused by inefficient hot-phonon decay observed in a standard design at similar electron densities.

In this work, we investigated the self-heating effects of annealed Ti/Al/Ni/Au ohmic contacts and two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures under strong electric field by using the short pulse current–voltage and microwave noise measurement techniques. The experimental results demonstrated that the self-heating phenomena in ohmic contacts with the increase of current may dominate over the electrical performances of 2DEG channel. Moreover, the excess noise temperature of contact resistance was found to be more than four times higher than that of 2DEG channel at the same high current density values. The results reveal the importance of self-heating of the ohmic contacts on the device performance under short-duration (only 100 ns) and low-duty-cycle (only 10⁻⁵) pulse regime.

High-field electron transport in two-dimensional channels at ZnO/MgZnO heterointerfaces has been investigated experimentally. Pulsed current–voltage (I–V) and microwave noise measurements used voltage pulse widths down to 30 ns and electric fields up to 100 kV/cm. The samples investigated featured electron densities in the range of 4.2–6.5 × 10¹² cm⁻², and room temperature mobilities of 142–185 cm²/V s. The pulsed nature of the applied field ensured negligible, if any, change in the electron density, thereby allowing velocity extraction from current with confidence. The highest extracted electron drift velocity of ∼0.5 × 10⁷ cm/s is somewhat smaller than that estimated for bulk ZnO; this difference is explained in the framework of longitudinal optical phonon accumulation (hot-phonon effect). The microwave noise data allowed us to rule out the effect of excess acoustic phonon temperature caused by Joule heating. Real-space transfer of hot electrons into the wider bandgap MgZnO layer was observed to be a limiting factor in samples with a high Mg content (48%), due to phase segregation and the associated local lowering of the potential barrier.