Measurement of changes in electrical conductivity

The simplest method of measuring conductivity is to determine the voltage drop across the sample and the current through the sample. The conductivity 

Bridge methods are also a standard technique for conductivity measurements. The basic Wheatstone bridge circuit is illustrated in Fig. 18. When the bridge is balanced, i.e. the detector D indicates zero current, then the unknown resistance is given by 

One important deficiency of the Wheatstone bridge is that it measures both sample and contact resistance. Thus it is particularly inadequate for measurements below 10 ohms. For measurements on metallic specimens with low resistances down to 

The term semiconductor covers a wide range of materials, including the elements silicon and germanium, and compounds such as oxides, sulphides, selenides, etc. A survey of the physics and chemistry of semiconductors is given in the book edited by Hannay and particular reference to semiconductivity and adsorption on oxide surfaces is to be found in an article by Gray . 

In semiconductors, the surface electrical double layer penetrates the crystal to a distance of about 10 cm, and the mobility, as well as the number, of current carriers may be altered by the adsorption process. For this reason, conductivity measurements alone are insufficient to quantitatively determine the extent of charge transfer across the surface. For a simple n-type semiconductor, adsorption 

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