Current–voltage characteristics of a nominally undoped AlGaN/GaN two-dimensional electron gas channel is measured at a room temperature, and electron drift velocity is deduced under assumption of uniform electric field and field-independent electron density. No velocity saturation is reached at fields up to 130 kV/cm, when the effect of Joule heating is minimized through application of nanosecond pulses of voltage. The estimated drift velocity is near 2×107 cm/s at 130 kV/cm. Monte Carlo simulation of the drift velocity is carried out with and without effects of channel self-heating for a many-subband model, with hot phonons and electron gas degeneracy taken into account.

Hot-electron transport was probed by nanosecond-pulsed measurements for a nominally undoped two-dimensional channel confined in a nearly lattice-matched Al 0.82 In 0.18 N / AlN/ GaN structure at room temperature. The electric field was applied parallel to the interface, the pulsed technique enabled minimization of Joule heating. No current saturation was reached at fields up to 180 kV/cm. The effect of the channel length on the current is considered. The electron drift velocity is deduced under the assumption of uniform electric field and field-independent electron density. The highest estimated drift velocity reaches ϳ3.2ϫ 10 7 cm/ s when the AlN spacer thickness is 1 nm. At high fields, a weak ͑if any͒ dependence of the drift velocity on the spacer thickness is found in the range from 1 to 2 nm. The measured drift velocity is low for heterostructures with thinner spacers ͑0.3 nm͒.

Microwave noise and electron transport are studied in silicon-doped GaN channels grown by molecular beam epitaxy and subjected to a high electric field. The drift velocity of 2.8 ϫ 10 7 cm/ s is reached at 290 kV/ cm for n ϳ 1 ϫ 10 18 cm −3 channel. No negative differential resistance is observed. The noise temperature exceeds ϳ5000 K at ϳ110 kV/ cm for n ϳ 3 ϫ 10 17 cm −3 channel. The hot-phonon effect on power dissipation in GaN:Si is 3-4 times weaker as compared with the effect in an undoped AlGaN / GaN two-dimensional channel. Monte Carlo simulation shows a weak effect of hot phonons on hot-electron energy distribution.

Nanosecond‐pulsed current–voltage measurements were used to study hot‐electron transport in undoped AlGaN/GaN and AlGaN/AlN/GaN two‐dimensional electron gas channels biased up to 200 kV/cm. Voltage pulses of 1 ns were applied in order to minimize channel heating. The current saturated at 100 kV/cm in the AlN‐interbarrier‐contained structures; the estimated saturated drift velocity was about 1.1 × 107 cm/s. No saturation of the current was observed in AlGaN/GaN channels in the field range below 200 kV/cm. Monte Carlo simulation of hot‐electron transport was carried out with electron gas degeneracy and hot phonons taken into account. A fairly good fit of the experimental results with the Monte Carlo simulation data was obtained in the electric field range up to 30 kV/cm. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A GaN/ultrathin AlN∕GaN heterojunction has been used to introduce a GaN spacer between the GaN channel and the AlGaN barrier in AlGaN∕GaN high electron mobility transistors (HEMTs). In conventional AlGaN∕GaN devices, the alloy scattering of the electrons with the AlGaN barrier degrades the electron velocity at high electric fields. This effect is significantly reduced in GaN-spacer transistors, which therefore have much better high field transport properties. While the dc performance of these transistors is similar to conventional AlGaN∕GaN HEMTs, a 20% increase in the electron velocity has been measured by two different techniques.