A triangular quantum well model is introduced to investigate a doped AlGaN/GaN high electron mobility transistor (HEMT) structure as a sensor for polar liquids. We calculate the drain current of the transistor as a function of the dipole moment of the polar liquid. The results show good agreement with experimental measurements for different polar liquids. It is also found that the device has large linear sensitivity by detecting the change of drain current with dipole moment and therefore it can distinguish molecules with slightly different dipole moments. The device can be extended to sense biomolecules (such as proteins) with very large dipole moments.