  A theoretical model is presented for the phonon limited mobility of the
Dirac-Fermion gas in a three-dimensional (3D) Dirac semimetal Cd$_3$As$_2$
considering scattering from both the acoustic and the optical phonons.
Screening effects are taken into account and it is found that they lead to a
significant enhancement of the mobility. Simple analytical equations and power
laws are obtained in both the Bloch-Gruneisen and equipartition regimes. The
dependence of mobility on the temperature $T$ and the electron density $n_e$ is
investigated. It is found that optical phonon limited mobility $\mu_{op}$
dominates over the acoustic phonon limited mobility $\mu_{ap}$ in the higher
temperature region. There is a crossover of $\mu_{ap}$ and $\mu_{op}$ and the
crossover temperature $T_c$ shifts to higher value with increasing $n_e$. Our
calculations of the mobility are in good agreement with the recent experimental
data. Comparison is also made with the results in the conventional 3D electron
gas in a degenerate semiconductor.
