Law of Mass Action

       If n is the mass concentration of free electrons and p is the concentration of holes then the law of mass action states that the product of concentrations of electrons and holes is always constant, at a fixed temperature.

       Mathematically it is expressed as,

           nP = n_i²                                              ................(1)

       where n_i is intrinsic concentration.

Important Observations

1. The law can be applied to both intrinsic and extrinsic semicoductors.

2. As n_i depends on temperature, the law is applicable at a fixed temperature.

3. In case of extrinsic semiconductors, n_i is the intrinsic concentration of the basic semiconductor material used.

Note : The law can be used to find the electron and hole densities in intrinsic semiconductors.

1.1 Law of Mass Action for Extrinsic Semiconductors

       In extrinsic materials,

1. In case of extrinsic semiconductors, n_i is the intrinsic concentration of the basic semiconductor material used.

2. For n-type material n = n_n while P = p_n  hence law can be stated as,

                        n_n p_n = n_i²                                        ..............(2)

3. For p-type material n = n_p while P =  p_p hence law can be stated as,

                       n_p p_p = n_i²                                            ..............(3)

4. The law is applicable irrespective of amount of doping.

5. The law can be used both majority and minority carrier concentrations in an extrinsic semiconductor.

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