An interesting Pd-GaAs high electron mobility transistor (HEMT) hydrogen sensor is fabricated and studied. For the studied device, a 5 nm-thick undoped GaAs cap layer is grown to suppress the oxidation of the underneath AlGaAs layer. Comprehensive analysis on the electrical properties including transconductance (gm), channel conductance (gD), and gate-source voltage shift (ΔVGS) is presented. Experimentally the maximum variation of transconductance (Δgm) is about 46.8 mS/mm while the maximum transconductance (gm,max) is still maintained at about 176 mS/mm. A high channel conductance variation (ΔgD) of 25.1 mS/mm is obtained in 9970 ppm H2/air gas at 50°C. This indicates that, in hydrogen-containing ambience, the channel resistance reduces in the linear region of transistor operation. The negative ΔVGS value in hydrogen-containing ambiance means that a lower VGS value is sufficient to maintain the same drain current (ID) value than that in air.