A compact model of the biased dual-gate GaAs pHEMT device is proposed. The biased dual-gate pHEMT is considered as one macro unit to simplify the model and facilitate simulation. We derive the model based on analytical formulation and represent it with a simplified circuit containing only eight elements. The extrinsic elements are extracted using improved open-short method with a larger frequency range than traditional method. The simulated S-parameters based on the proposed model agree with the measured results well up to 40GHz for 0.25um dual-gate GaAs pHEMT devices. In addition, the large-signal model is constructed with a new empirical drain current model. A 2.4GHz power amplifier (PA) is designed using the proposed model, and the measurement results agree well with the simulation.