The intersubband relaxation time of an electron is calculated by consi
dering electron-electron and electron-phonon (bulk LO phonon) scatteri
ng in a GaAs quantum well. The relaxation time is derived and numerica
lly evaluated within the random-phase approximation with full multiple
subband and frequency-dependent screening. The electron scattering du
e to the coupled system of electrons and phonons is compared with the
decoupled scattering where both electron-electron and unscreened elect
ron-phonon scattering are considered separately. It is shown that the
intersubband relaxation time is heavily influenced by the electron den
sity in the well. It is also shown that at room temperature it is nece
ssary to use the finite-temperature dielectric function to accurately
determine the intersubband relaxation time.