Inorganic Semiconductor Chemiresistors

For an n-type semiconductor (e.g., ZnO), an increase of the surface concentration of the electron donor (e.g., hydrogen) will increase the conductivity and vice versa. On the other hand, adsorption of hydrogen on p-type oxide (e.g., CoO) will decrease its conductivity. 

The bulk conductance (Gb), which is not modulated by the surface reactions, is shown as a parallel conductor (Fig. 8.7). 

Subscript b refers to the bulk quantities and d is the total thickness of the oxide layer. Because the overall conductance is measured, it is advantageous to have the films as thin as possible. The relative change of conductance of the whole device is obtained from (8.12), (8.14), and (8.15) and by assuming that Mb Ms- 

For a high value of the relative conductance change (high sensitivity), it is necessary to have a low density of bulk carriers and a thin film. The space charge thickness is typically 10-100 nm, depending on the doping. 

As an example, let us consider a typical value of the excess surface state density ns = 1012 electrons cm 2 and the bulk density b = 1017 electrons cm 3. This means that for 100 jttm thick film, the sensitivity AGS/Gb = 10-3. However, for d = lOnm the sensitivity is 10 and for thin films ( 100nm) the space charge region extends throughout the whole film thickness. 

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The emphasis on the thickness of the selective layer is linked to the mode of interaction between the analyte and the selective layer. If this interaction takes place exclusively at the surface of the selective layer, then the bulk conductivity does not contribute and represents only a shunt which decreases the signal-to-noise ratio. This is a typical case of chemiresistors based on inorganic materials (Fig. 8.6). On the other hand in chemiresistors based on organic semiconductors, the signal usually originates in the bulk of the selective layer. In that case, the response time of the sensor is affected by its thickness. 

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