The Pd gate MOS capacitor was fabricated and tested as hydrogen gas sensor. To investigate the effect of thin film properties on the hydrogen sensing performance of Pd gate MOS sensor, the deposition condition such as rf power, deposition temperature was varied. As RF power increases, the grain size and the tendency of (111) preferred orientation increase. The change of RF power makes little influence on sensitivity and initial response rate. Pd thin films deposited at ℃ have larger grain and larger tendency of (111) preferred orientation than those deposited at room temperature. The MOS sensor with Pd thin film deposited at 150℃ gives a decreased hydrogen sensitivity and a decreased initial response rate. The $Pd/Si_xN_y/SiO_2/Si$ structure was tried using sputtered $Si_xN_y$ on $SiO_2$ as insulator layer instead of $SiO_2$. The structure mentioned above shows response as if it has several adsorption sites with different site density, adsorption energy, activation energy and even polarity. It was demonstrated that the deposition temperature and the insulator on which Pd thin film was deposited have an effect on the sensing performance of Pd MOS sensor
