# In the ratio of the concentration of electrons that of holes in a semiconductor is 7/5 and the ratio of currents is 7/4 then what is the ratio of their drift velocities? Option 1) 4/7 Option 2) 5/8 Option 3) 4/5 Option 4) 5/4.

D Divya Saini

As we learnt in

Electrical Conductivity (σ) -

$\sigma =e \left (n_{e}\mu _{e}+n_{\lambda}\mu _{h} \right )$

- wherein

$n_{e}= electron \: density$

$n_{h}= hole \: density$

$\mu _{e}= mobility \: of \: electron$

$\mu _{h}= mobility \: of \: holes$

$I=nV_{d}\ eA$

$V_{d}\rightarrow$ drift velocity

$\frac{I_{e}}{I_{h}}=\frac{n_{e}}{n_{h}}.\frac{V_{d,e}}{V_{d,h}}$

$\Rightarrow\ \; \frac{7}{4}=\frac{7}{5}.\frac{V_{d,e}}{V_{d,h}}$

$\Rightarrow\ \; \frac{V_{d,e}}{V_{d,h}}=\frac{5}{4}$

Correct option is 4.

Option 1)

4/7

This is an incorrect option.

Option 2)

5/8

This is an incorrect option.

Option 3)

4/5

This is an incorrect option.

Option 4)

5/4.

This is an correct option.

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