The room-temperature drift mobility of electrons in the channel of a AlxGa1
-xN/GaN field-effect transistor associated with scattering by optical and a
coustic phonons is calculated as a function of areal density taking into ac
count the two-mode nature of the alloy and half-space and interface modes.
It is found that the two-dimensional (2D) mobility is significantly greater
than the bulk mobility and this means that the mobility goes through a max
imum as a function of density corresponding to a transition from bulk to 2D
transport. A simple model is used to describe the transition from bulk to
2D transport as the electron density increased. It is shown that for the st
ructure considered the mobility at 300 K goes through a weak maximum at abo
ut 2000 cm(2)/V a at a density of about 2 x 10(12) cm(-2) dropping to about
1300 cm(2)/V s at 10(13) cm(-2). Results for the Hall factor and for the t
emperatures 77 and 600 K are also obtained. The contribution made by interf
ace-roughness scattering is also discussed.