1. Introduction

Group III-nitride semiconductors and heterostructures have unique fundamental material properties which make them very promising for a number of optoelectronic and high-power, high-frequency applications. They have the energy bandgaps ranging from 0.8 eV (InN) to 6.2 eV (AlN), relatively small effective masses1 of 0.11 m ₀ (InN), 0.2 m ₀ (GaN), and 0.48 m ₀ (AlN) for the electrons and approximately (0.3 0.5) m ₀ for the holes ( m ₀ is the free electron mass), and large optical phonon energies of about (90 100) meV. Yet, electron-polar optical phonon coupling is much stronger in comparison to that of the III V compounds; the Fr lich phonon-coupling constant a is estimated to be 0.22, 0.41, and 0.74 for InN, GaN and AlN, respectively (for GaAs ?=0.075). This results in high-field carrier transport mainly determined by the interaction with the polar optical phonons. The energy splitting between the lowest ( ?) and upper (M L) conduction valleys is large ( ?2 eV for InN, ?1.5 2 e V for GaN, and ?0.7 eV for AlN). This leads to large peak velocities in the steady-state regime for the group III-nitrides. The velocities are estimated to be 4.3 10 ⁷ cm/s (InN), 3.1 10 ⁷ cm/s (GaN), and 1.7 10 ⁷ cm/s (AlN).1 The materials are also expected to demonstrate the negative differential resistance due to the inter valley electron transfer or occurrence of...