Drift effects in transition metal gate MOS and MISFETs

Abstract

The types of drift exhibited by palladium gate MOS and MISFET devices are examined and described in this paper. The drift prior to exposure of the devices to hydrogen (baseline drift) can be separated into two distinct types. The first of these is a field inversion effect caused by adsorption of moisture onto the device whilst it is cold creating an inversion layer under the field oxide, followed by the removal of this moisture during operation of the device at an elevated temperature. This drift effect was seen to degrade the gas sensitivity of the devices. The second drift effect is due to the drift of mobile ions in the gate oxide. This process causes the baseline to drift but does not alter the gas sensitivity of the device. Methods of reducing or circumventing these drift effects are described. The drift of signal of palladium gate MOS and MISFETs during exposure to hydrogen gas is also described. Palladium gate MOSFETs are found to have larger signals and greatly increased drift compared to palladium gate MISFET devices. Furthermore, this drift is seen to be greatly dependent on the operating temperature of the device with the minimum drift occurring at a temperature of 150 °C. It is concluded that a large portion of the signal observed for palladium gate MOSFET devices is due to hydrogen related species drifting in the gate oxide.