  The effective electron mass parameter in Si-doped Al$_{0.72}$Ga$_{0.28}$N is
determined to be $m^\ast=(0.336\pm0.020)\,m_0$ from mid-infrared optical Hall
effect measurements. No significant anisotropy of the effective electron mass
parameter is found supporting theoretical predictions. Assuming a linear change
of the effective electron mass with the Al content in AlGaN alloys and
$m^\ast=0.232\,m_0$ for GaN, an average effective electron mass of
$m^\ast=0.376\,m_0$ can be extrapolated for AlN. The analysis of mid-infrared
spectroscopic ellipsometry measurements further confirms the two phonon mode
behavior of the E$_1$(TO) and one phonon mode behavior of the A$_1$(LO) phonon
mode in high-Al-content AlGaN alloys as seen in previous Raman scattering
studies.
