550. Homogeneous semiconducting gas sensors

Fig. 1 Energy scheme of an n-type semiconductor.

Fig. 2 Schematic drawings demonstrating the formation of an accumulation layer on an n-type semiconductor by ionic chemisorption (a) and the parallel connection of the resulting surface conductance with the bulk conductance (b) For adsorption or desorption at constant temperature, gB = const.

G. Heiland, Sens. Actuators 2, 343 (1982)
https://doi.org/10.1016/0250-6874(81)80055-8

(1) A few words about a rough preliminary classiffication in the field of chemically-sensitive semiconductor devices, there are structured sensors like diodes and field-effect transistors.
(2) The shunting bulk conductance can be avoided by using thin films which are penetrated by the space-charge layer.
(3) In real sensors, a mixed situation may often prevail, depending on the conditions, with space-charge layers and also at high temperatures with diffusion processes, often without reaching a final state.
(4) High temperature may be desired for preparation, surface cleaning, short response times and discrimination between different gases.
(5) The response times are influenced by preparation, they decrease with increasing temperature and usually are found in the range between seconds and minutes.
(6) The shape of the calibration curve is sometimes described as proportional to the square root or to the logarithm of the concentration.
(7) With ZnO as well as SnO2, the sensitivity was found to increase with the humidity in the air.
(8) The humidity-induced conductivity does not vanish with the H2O molecules, but does with the OH groups.