Motivated by its potential properties and applications, the energy band alignment of the amorphous-crystalline Ge2Sb2Te5 heterojunction in thermal equilibrium is explored. An approximate analytic model based on the exact solution to Poisson's equation is constructed to describe the electrostatics of the heterojunction between the amorphous phase and the face-centred cubic crystalline phase of Ge2Sb2Te5. The model captures the physics of accumulation in the crystalline layer, as well as that of depletion and inversion due to the deep defect distribution in the amorphous layer. Without introducing fitting parameters, the model approximates the influence of the density of states parameters of each phase on the electric potential distribution across the heterojunction. It is then validated against the exact solution obtained numerically using solar cell capacitance simulations. Apart from the small inaccuracy in modelling the electric potential distribution in the depletion region, simulation results reveal that the approximations are successful in modelling the electrostatics of the heterojunction.