New method for achieving very high sensitivities of MOSFET gas sensors

It is known, that by integration of different devices, (like transistors with different cross-sensivities, or diodes for temperature measurement) it is possible to decrease the cross-sensitivity and temperature-sensitivity. Besides this by integration of several identical transistors one can achieve high sensitivties. Both serial and parallel connections of the devices are possible. In this paper only the serial integration of identical transistors will be discussed. MOS Gas-FETs that are driven in the constant current mode have characteristics similar to those of diodes. In the case of serial connection of these devices the sum of the threshold voltages will be created. The change of the threshold voltage and by this the gas-sensitivity will also be summated. In this publication the integration of carbonmonoxide- and ammonia-sensitive MOS transistors will be described. In this example ten devices were connected in series. Measurements show that the sensor signal as en times larger th an before. The problems that occured using this technology were also discussed.