We propose and investigate a practical scheme to observe the polaritonic quantum phase transition (QPT). The polariton hopping energy is controlled by the optical evanescent field coupling between two adjacent photonic crystal microcavities, while the polariton-polariton interaction energy is provided by the ac Stark shift originated from the many-atom cavity QED effect. We show that our scheme can be implemented using a two-dimensional array of GaAs photonic crystal microcavities doped with substitutional donor impurities e.g. Si. Such a system is robust against the fluctuation of doping density per cavity, and requires only a moderate cavity Q factor when choosing an appropriate frequency detuning.