We solve the linearized Boltzmann equation for a degenerate quasi-two-dimen
sional electron system confined to a triangular quantum well at a III nitri
de heterojunction and interacting only with the polar optical phonons. The
method of solution makes use of a ladder technique, and employs the Fang-Ho
ward approximation for the description of the confined electrons. The varia
tions of the effective-momentum relaxation time with the electron energy, a
nd of the mobility with electron density are presented for a GaN/AlN hetero
junction.