JEE MAIN - Physics (2006 - No. 19)
If the ratio of the concentration of electrons to that of holes in a semiconductor is $${7 \over 5}$$ and the ratio of currents is $${7 \over 4},$$ then what is the ratio of their drift velocities?
$${5 \over 8}$$
$${4 \over 5}$$
$${5 \over 4}$$
$${4 \over 7}$$
Explanation
$${{{I_e}} \over {{I_h}}} = {{n{}_eeA{v_e}} \over {n{}_heA{v_h}}}$$
$$ \Rightarrow {7 \over 4} = {7 \over 5} \times {{{v_e}} \over {{v_h}}}$$
$$ \Rightarrow {{{v_e}} \over {{v_h}}} = {5 \over 4}$$
$$ \Rightarrow {7 \over 4} = {7 \over 5} \times {{{v_e}} \over {{v_h}}}$$
$$ \Rightarrow {{{v_e}} \over {{v_h}}} = {5 \over 4}$$
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