It is known that the drift current density is proportional to the
mobility (μ) while diffusion current density is proportional to the
diffusion constant (D). There exists a fixed relation between these two
constant which is called Einsten's relation.

It states that, at a fixed temperature, the ratio of diffusion constant
to the mobility is constant. This is Einstein's relation.
Mathematically it is expressed as,

where T is the temperature in

^{o}K
And k is the Boltzmann's constant = 8.62 x 10

^{-5}eV/^{o}K__1.1 Voltage Equipment of Temperature__

In the equation (1), the product KT is called voltage equivalent of temperature.

The voltage equivalent of temperature is denoted by V

_{T}.
At room temperature i.e. at 27

^{o}C,
T = 273 + 27 = 300

^{o}K**.**V

^{.}._{T }= KT = 8.62 x 10

^{-5}x 300 = 0.02586 V = 26 mV at 300

^{o}K

**Note**: The value of V

_{T }= 26 mV at 27

^{o}C i.e. 300

^{o}K is very commonly used while solving the examples.

Substituting this in equation (1) we get,

D

_{n}/μ_{n }= D_{p}/μ_{p }= V_{T }= 0.02586 at room temperature**.**μ

^{.}._{n }= 39 D

_{n }and μ

_{p }= 39 D

_{p}at room temperature. ................... (3)

In general we can express the relation between mobility and diffusion constant as,

μ

_{ }= 39 D at room temperature .................. (4)

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