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

_{i}^{2 }................(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_{i}^{2 }..............(2)
3. For p-type material n = n

_{p}while P = p_{p}hence law can be stated as,
n

_{p}p_{p}= n_{i}^{2 }..............(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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