This paper reviews and compares the main doubly-fed induction generator (DFIG) models for stability assessment in the literature by PSCAD/EMTDC simulation. The main contribution of the paper is a detailed study on small- signal admittance-based modeling of DFIGs for stability analysis. A complete DFIG model characterized from a 2x2 admittance-based transfer matrix is proposed which has frequency coupling between positive and negative sequences resulting from considering the PLL and the DC capacitor. In addition, two simplified DFIG models in the literature are derived from the complete model and discussed. The models are applied to stability analysis using the generalized Nyquist criterion and the positive-mode-damping stability criterion. The predictions are validated for several mechanical speeds and short-circuit ratios by PSCAD/EMTDC time-domain simulation. The study shows the need to apply the complete model to accurately assess subsynchronous stability, as significant differences from simplified models may occur at low frequencies. Peer Reviewed