On the basis of a GaAs-based heterostructure field-effect transistor with a catalytic Pd gate film, an interesting hydrogen sensor was fabricated and studied. A study of the hydrogen-induced dipole effect on device performance, including output resistance (r(o)), Early voltage (V-A), and the drain saturation current (I-DS) operating regime, is reported. A significant change in the relative sensitivity ratio [Sr(%)] was observed in the cutoff region. In addition, a linear dependence between the logarithmic values of response time and hydrogen concentration is consistent with theoretical analysis. Experimentally, a high S-r(%) of 348% in 9970 ppm H-2/air was obtained in the cutoff region at 50 degrees C. The width of the IDS operating regime decreased from 115.3 to 108.2 mA/mm with an increase of the hydrogen concentration from air to 9970 ppm H-2/air. From the experimental results, it is speculated that the polarization of a dipole layer reduces the depletion region, which results in a substantial change in the two-dimensional electron gas (2DEG) and the effective channel length and shape. (C) 2008 The Electrochemical Society.