We review the spectroscopic and dynamic properties of cavity polaritons in a λ-wide GaAs/GaAlAs microcavity (MC) structure having a single GaAs/AlAs quantum well in which a variable density two-dimensional electron gas (2DEG) is photoexcited, with a density in the range 0 < ne < 5 × 1011 cm-2. The main observed effects that will be discussed here are: (a) photoexcitation of negatively charged polaritons. These polaritons result of the coupling of the MC photon and the negatively charged excitons. They appear at low temperatures (T < 25 K), in the range of low 2DEG densities (ne < 5 × 1010 cm-2), and their coupling strength increases as √ne. (b) The effect of electron-polariton scattering on the polariton spectral linewidth, its complex dependence on the detuning energy, and the electron-induced transition from strong to weak coupling regimes, as ne increases. (c) The efficient polariton transfer across the relaxation bottleneck in the lower branch induced by electron-polariton scattering, and the resulting non-linear dependence of the polariton photoluminescence intensity on ne.